a. Recommend lung mechanics (bedside spirometry) tests to obtain additional data (Code: IC7) [Difficulty: ELE: R, Ap; WRE: An]

The bedside lung mechanics tests typically include the following:

Other common bedside tests are also presented in this section. Bedside spirometry tests are indicated in a patient who is showing signs of breathing difficulty and the physician and respiratory therapist are considering the need for mechanical ventilation. For example, a patient with a progressive neuromuscular disease or worsening cardiopulmonary condition would show a smaller vital capacity and MIP and MEP as he or she weakens. Conversely, as a mechanically ventilated patient improves, bedside spirometry is used to evaluate the patient’s spontaneous breathing for weaning.

The more advanced spirometry tests, such as flow-volume loop, timed expiratory volumes, and maximum voluntary ventilation, are included later in the chapter with the pulmonary function laboratory tests.

b. Perform any of the bedside spirometry tests (Code: IIIE7) [Difficulty: ELE: R, Ap; WRE: An]

Discussion of the steps to perform and interpretation of the tests follow.

a. Perform the procedure (Code: IB9d) [Difficulty: ELE: R; WRE: Ap, An]

The tidal volume (V_T) is the volume of gas breathed out in each respiratory cycle. Realize that individual tidal volumes are rarely identical. Figure 4-1 shows several different tidal volumes before and after a nonforced (slow) vital capacity (VC). For that reason, it is recommended that the tidal volumes be accumulated for 1 minute (thus providing a minute volume []) and the respiratory rate (f) counted. An average V_T is found by dividing the by the f. If this cannot be done, find the average volume of at least six breaths. The average predicted tidal volume for a resting, afebrile, alert adult should be about

Figure 4-1 Volume-time curve tracing of tidal volumes and nonforced vital capacity.

For example, the predicted tidal volume range of a 154-lb (70-kg) patient is calculated as follows:

The patient should be allowed to relax before the test is performed so that the measured volume is accurate and not enlarged because of any undue stress or excitement. Keeping the instructions and demonstration simple and easy to follow help reduce the patient’s anxiety. Some patients will not tolerate a full minute’s tidal volume measurement. In that case, measure the accumulated tidal volumes for as long as possible and divide by the number of respirations to obtain the average.

b. Interpret the results (Code: IB10d) [Difficulty: ELE: R, Ap; WRE: An]

A tidal volume that is larger or smaller than expected for the patient’s size requires further evaluation. A small tidal volume may be seen in patients who have a low metabolic rate, are asleep or in a coma, have a neuromuscular disease that make them unable to breathe deeply, or are alkalotic. A large tidal volume may be seen in patients with a high metabolic rate, fever, dead space–producing diseases such as a pulmonary embolism, increased intracranial pressure, or acidotic conditions.

c. Monitor the tidal volume and respiratory rate (Code: IA7c) [Difficulty: ELE: R; WRE: Ap]

A patient with a progressive neuromuscular disease or worsening cardiopulmonary disease should have the tidal volume, respiratory rate, and other bedside spirometry values monitored closely. If the patient’s condition worsens, mechanical ventilation may have to be started. The frequency of monitoring varies with the patient’s condition. For example, every shift initially and hourly if the patient is critical. Conversely, as a mechanically ventilated patient improves, bedside spirometry is used to evaluate the patient’s spontaneous breathing for weaning.

d. Monitor inspiratory/expiratory ratio (WRE code: IA7c) [Difficulty: ELE: R; WRE: Ap]

The I : E ratio is the ratio of the patient’s inspiratory time to the expiratory time. It can be simply measured at the bedside with a stopwatch. Again, make sure the patient is relaxed and breathing in the normal pattern to get an accurate timing. Measure several of the patient’s inspiratory times and expiratory times to figure an average for each. A spirometer that gives a printout is needed if a more complete analysis of the patient’s breathing pattern is necessary.

A normal, spontaneously breathing patient has an I : E ratio of 1 : 2 to 1 : 4. A prolonged inspiratory time often is seen in patients with an upper airway obstruction. A prolonged expiratory time often is seen in patients with asthma or chronic obstructive pulmonary disease (COPD). Any abnormal I : E ratio should be investigated. For example, patients with Kussmaul’s respiration, Cheyne-Stokes respiration, or Biot’s respiration will have unusual I : E ratios.

Exam Hint 4-1 (ELE)

The National Board of Respiratory Care (NBRC) is known to test the examinee’s ability to calculate (1) the T_I and T_E from a given I : E ratio and f, and (2) the I : E ratio from a given T_I and T_E. These examples should help.

1. Calculate the patient’s T_I and T_E when the I : E ratio is 1: 2 and f is 12/min.

a.

b.

c. T_I = 1 part = 1.66 sec

d. T_E = 2 parts = 3.32 sec

2. Calculate the neonatal patient’s I : E ratio when the T_I is 0.3 sec and the T_E is 0.9 sec.

a. I : E = I/E = 0.3/0.9 sec

b. I/E = 1/3 (The I : E ratio is 1 : 3.)

a. Perform the procedure (Code: IB9d) [Difficulty: ELE: R; WRE: Ap, An]

The minute volume () is the volume of gas exhaled in 1 minute. It usually is a more stable value than are individual tidal volumes. The minute volume is found by adding the accumulated tidal volumes for 1 minute. A simple handheld spirometer often is used to accumulate the tidal volume breaths. If the patient cannot perform the test for 1 minute, do it for 30 seconds and double the value.

b. Interpret the results (Code: IB10d) [Difficulty: ELE: R, Ap; WRE: An]

The predicted range for a minute volume in a resting, afebrile, alert adult should be 5 to 10 L/min. The wide range is found in part because the minute volume is a product of two factors: tidal volume and respiratory rate. It is possible for either one or both of these factors to be normal, abnormally high, or abnormally low. For these reasons, the minute volume must be evaluated along with the tidal volume and respiratory rate to reach any conclusion about the patient’s condition. The same factors that have an impact on the tidal volume affect the patient’s minute ventilation.

a. Perform the procedure (Code: IB9d) [Difficulty: ELE: R; WRE: Ap, An]

Alveolar ventilation (V_A) is the amount of tidal volume that reaches the alveoli. It is calculated by subtracting the physiologic dead space (anatomic plus alveolar dead space) from the measured exhaled tidal volume. For a bedside test, it is possible to subtract only the estimated anatomic dead space. It is estimated at 1 mL/lb, or 2. 2 mL/kg of ideal body weight. The alveolar dead space measurement requires sophisticated equipment, which usually is available only in the pulmonary function testing laboratory. Clinically normal people have very little alveolar dead space.

b. Interpret the results (Code: IB10d) [Difficulty: ELE: R, Ap; WRE: An]

The following examples show how the patient’s alveolar ventilation can vary considerably because of changes in the respiratory rate and tidal volume, even though the minute volume remains unchanged. These examples are included to show the importance of alveolar ventilation on the patient’s arterial partial pressure of carbon dioxide (PaCO₂) values.

a. Perform the procedure (Code: IB79n [Difficulty: ELE: R, Ap; WRE: An]

The maximum inspiratory pressure (MIP) is the greatest amount of negative pressure the patient can create when inspiring against an occluded airway. It also is known as negative inspiratory force (NIF). The following factors affect the test results: strength of the diaphragm and accessory muscles of inspiration, lung volume when the airway is occluded, ventilatory drive, and the length of time the airway is occluded. MIP is most commonly used to determine the weanability of mechanically ventilated patients. In addition, it is used to help monitor the strength of patients with a neuromuscular disease.

A study of the literature reveals that a number of measurement devices have been assembled and that different bedside techniques have been used to determine the effort of a patient breathing naturally, an intubated patient, and a patient breathing with assistance from a mechanical ventilator. Branson and colleagues (1989) and Kacmarek and colleagues (1989) make a strong case for the use of a double one-way valve to connect the intubated patient to the manometer (Figure 4-2). Use of the one-way valve lets the patient exhale but prevents an inhalation when the practitioner occludes the opening. This forces the patient to inhale from closer to residual volume with each breathing effort. These researchers also recommend that the patient make inspiratory efforts for 15 to 20 seconds.

Figure 4-2 Two systems for measuring maximal inspiratory pressure (MIP) in a patient with an artificial airway. System A, Simple occlusion. A, Pressure manometer; B, connecting tubing; C, port to be occluded during MIP; D, connection of the adapter to the manometer; E, port to connect to the patient’s airway. System B, One-way valve. A, Pressure manometer; B, connecting tubing; C, inspiratory port to be occluded during the MIP effort; D, expiratory port; E, port to be connected to the patient’s artificial airway.

(From Kacmarek RM, Cycyk-Chapman MC, Young-Palazzo PJ, et al: Determination of maximal inspiratory pressure: a clinical study and literature review, Respir Care 34:868, 1989.)

Steps in the MIP procedure for a normally breathing patient include the following:

b. Interpret the results (Code: IB10n) [Difficulty: ELE: R, Ap; WRE: An]

Patients of either gender and of any age should be able to generate at least −60 cm H₂O. This is enough to offer assurance that the patient has enough strength and coordination to protect the airway, take a deep breath, and cough effectively. Patients with neuromuscular diseases, diseases of the respiratory muscles, thoracic injury or abnormality, and chronic obstructive lung diseases tend to have decreased strength. The patient who cannot generate at least −20 cm H₂O is at risk. This patient probably does not have the strength to cough effectively. Depending on the blood gas values and other physical parameters, the patient may need to be intubated and maintained on a mechanical ventilator.

Black and Hyatt (1969) published MIP prediction formulas for spontaneous breathing in nonintubated adults 20 to 86 years old who are breathing from residual volume; these formulas are presented in Table 4-1. The values are in centimeters of water pressure (cm H₂O). As can be seen, the older the patient, the lower the predicted negative inspiratory force.

TABLE 4-1 Age and Gender in Predicting Negative Inspiratory Force

c. Monitor the patient’s MIP (Code: IA7b) [Difficulty: ELE: R; WRE: Ap]

A patient being weaned from mechanical ventilation or with a deteriorating neuromuscular condition should have the MIP measured on a regular, frequent basis. If the patient’s pressure is only −20 cm H₂O, the physician should be notified. Mechanical ventilation is probably needed.

It is important to monitor any patient for signs of undue stress and hypoxemia, such as tachycardia, bradycardia, ventricular dysrhythmias, hypertension, hypotension, and decreasing saturation on pulse oximetry. If any of these are seen, the procedure should be stopped and the patient reoxygenated and ventilated. Some patients achieve their best effort on the first or second inspiration and have decreasing effort as they continue trying. This is probably because of fatigue. Stop the procedure and record the best effort.

a. Perform the procedure (Code: IB9n) [Difficulty: ELE: R, Ap; WRE: An]

The maximum expiratory pressure (MEP) is the greatest amount of positive pressure the patient can create when expiring from total lung capacity (TLC) against an occluded airway. It also is known as a maximum expiratory force (MEF). The following factors affect the test results: patient cooperation and effort, strength of the expiratory muscles, lung volume when the airway is occluded, ventilatory drive, and the length of time the airway is occluded. The MEP is used to determine the weanability of mechanically ventilated patients and to monitor the strength of patients with a neuromuscular disease.

As with the MIP test, a study of the literature reveals that a number of measurement devices have been assembled and that different bedside techniques have been used to determine the effort of a patient breathing naturally and the effort of one who is intubated and breathing by way of a mechanical ventilator. A strong case can be made for the use of a double one-way valve to connect the intubated patient to the manometer (see Figure 4-2). Use of the one-way valves lets the patient inhale but prevents an exhalation when the practitioner occludes the expiratory opening. This forces the patient to exhale from closer to TLC with each breathing effort. However, the expiratory efforts should not be held for longer than 3 seconds. This test is similar to the Valsalva maneuver and can cause a reduction in cardiac output because of the high intrathoracic pressure.

Steps in the MEP procedure for a normally breathing patient include the following:

It is important to monitor any patient for signs of undue stress and hypoxemia, such as tachycardia, bradycardia, ventricular dysrhythmias, hypotension, and decreasing saturation on pulse oximetry. If any of these is seen, the procedure should be stopped and the patient reoxygenated and ventilated.

b. Interpret the results (Code: IB10d) [Difficulty: ELE: R, Ap; WRE: An]

Clinically normal people of either gender and of any age should be able to generate at least 80 cm H₂O. Patients with neuromuscular diseases, thoracic injury or abnormality, and COPD tend to have decreased strength. An MEP of +40 cm H₂O is probably enough to offer assurance that the patient has enough strength and coordination to cough effectively to clear secretions. However, depending on the blood gas values and other physical parameters, the patient may need to be intubated and maintained on a mechanical ventilator.

Black and Hyatt published MEP prediction formulas for spontaneously breathing, nonintubated adults 20 to 86 years old who are breathing from TLC; these formulas are presented in Table 4-2. The values are in centimeters of water pressure. As can be seen, the older the patient, the lower the predicted maximum expiratory force.

TABLE 4-2 Age and Gender in Predicting Maximum Expiratory Force

a. Perform the test (Code: IB9d) [Difficulty: ELE: R; WRE: Ap, An]

The nonforced (slow) vital capacity (VC or SVC) is the greatest volume of gas the patient can exhale after the lungs have been completely filled. The therapist should demonstrate the procedure to the patient. He or she must understand that there is no need to blow out fast while emptying the lungs. The measurement device can be a simple handheld spirometer if a printout of the result is not needed. A portable, computer-based spirometer can be used, if needed, to generate a printout of the results or a graphic tracing. Normally at least three efforts are made, and the largest is recorded.

b. Interpret the results (Code: IB10d) [Difficulty: ELE: R, Ap; WRE: An]

See Figure 4-1 for a graphic tracing of a nonforced VC. Compare it with the tracing on Figure 4-3, which shows a forced vital capacity (FVC). In a patient without obstructive lung disease, the same volume should be found in a nonforced VC and an FVC. A patient with asthma or chronic obstructive lung disease may have a smaller FVC than nonforced VC because of small airway collapse during the maximum effort. The following discussion on the FVC includes predicted values for male and female patients and guidelines on the interpretation.

Figure 4-3 Tracing of tidal volumes and forced vital capacity.

c. Monitor the vital capacity (Code: IA7b) [Difficulty: ELE: R; WRE: Ap]

The VC and other simple spirometry values are commonly measured at the bedside in patients with neuromuscular disease and those who are being weaned from mechanical ventilation. A VC that is increasing toward the normal, predicted patient value is a good sign, because it indicates that the patient is recovering. If the VC is decreasing, the patient likely is becoming weaker and will not be able to cough out secretions effectively. Mechanical ventilation may be needed. See Module C for more discussion on the vital capacity. See Chapter 15 for more discussion on how the VC is used to evaluate a patient’s need for mechanical ventilation.

a. Perform the test (Code: IB9e) [Difficulty: ELE: R; WRE: Ap, An]

The forced expiratory volume in one second (FEV₁) is the volume exhaled in the first second from a forced vital capacity. It can be measured at the bedside on some portable pulmonary function testing units, but this is most commonly done in the pulmonary function laboratory. The patient is instructed to take in as deep a breath as possible and blow it out as hard as possible. The therapist should demonstrate the procedure. It is not necessary for the patient to push out all air for several seconds until completely empty. The patient should be wearing nose clips and tightly seal the lips around the mouthpiece to prevent leaks. A good patient effort must be repeated three times. The highest recorded value is recorded in liters.

b. Interpret the results (Code: IB10e) [Difficulty: ELE: R, Ap; WRE: An]

The FEV₁ and peak flow are the bedside measurements most commonly used to evaluate the response of a patient with asthma or COPD to inhaled bronchodilator medications. If the medication dose is effective, the patient’s expiratory flow will increase significantly from the premedication value. See Module C for more discussion on the FEV₁.

a. Perform the test (Code: IB9d) [Difficulty: ELE: R; WRE: Ap, An]

The peak flow (PF) or peak expiratory flow rate (PEFR) is the highest flow rate seen in a patient’s forced expiratory effort. The PF usually is found at the start of an FVC effort. It can be measured at the bedside on some portable pulmonary function testing units, but it is most commonly done on a portable, disposable peak flowmeter. Select a unit that is appropriate for the patient. An adult peak flowmeter should be able to measure a flow of up to 850 L/min (about 14 L/sec), and a pediatric unit should be able to measure a flow of up to 400 L/min (about 7 L/sec). The patient is instructed to take in as deep a breath as possible and blow it out as hard as possible. The therapist should demonstrate the procedure. It is not necessary for the patient to push out all air for several seconds until completely empty. The patient should tightly seal the lips around the mouthpiece to prevent leaks. Nose clips are recommended but not required. A good patient effort must be repeated three times. Record the highest recorded value in liters per minute (or liters per second, depending on the unit).

b. Interpret the results (Code: IB10e) [Difficulty: ELE: R, Ap; WRE: An]

The peak flow and FEV₁ are the bedside measurements most commonly used to evaluate the response of a patient with asthma or COPD to inhaled bronchodilator medications. If the medication dose is effective, the patient’s peak flow will increase significantly from the premedication value. Many peak flowmeters come with adjustable markers for the National Asthma Education Program’s “color zone” scheme:

Current asthma guidelines state that if an asthma patient’s peak flow is in the green zone, the medications are adequately controlling the asthma. If the peak flow is in the yellow zone, the patient’s medications are not adequately controlling the asthma. Increased doses are indicated if ordered by the physician. If the peak flow is in the red zone, the patient’s medications are not adequately controlling the asthma. The patient needs more bronchodilator medication. The home care patient should get medical help as soon as possible. Each patient should have his or her individually calculated color zones marked on the peak flowmeter to guide medication usage. The patient should be instructed on the meaning of the zones and the proper use of medications. See Module C for more discussion of the peak flow.

a. Perform the test (WRE Code: IB9k) [WRE Difficulty: R, Ap]

Exhaled nitric oxide (eNO) is measured in patients with known or suspected eosinophilic (airway) inflammation. The test has been shown to be most useful for evaluating patients with asthma but may also be of use in patients with COPD. The patient should be told not to eat, drink, or smoke for at least 1 hour before the test. If other pulmonary function tests are scheduled, the exhaled nitric oxide test should be performed first. This is because the other tests, such as spirometry, exercise testing, and methacholine challenge testing, can change the eNo results.

The patient’s exhaled breath is analyzed through a chemoluminescent analyzer for the level of nitric oxide in parts per billion (ppb). The patient is instructed to exhale completely (to residual volume), insert the mouthpiece, and inhale as much as possible (to total lung capacity) through the eNO device. The inhaled gas is scrubbed of any NO that may be found in room air. The patient then slowly exhales through the eNO device at a controlled rate of 0.05 L/sec. This results in an exhaled NO plateau of several seconds that is analyzed for the patient’s value. The patient must perform at least two acceptable eNO efforts that are within 10% of each other. The two acceptable values are averaged for the final reported value.

b. Interpret the results (WRE code: IB10k) [WRE Difficulty: R, Ap, An]

The following normal ranges have been established for exhalation of a breath at the controlled rate of 0.05 L/sec:

Actual patient values greater than these indicate increased eosinophilic activity. In other words, a patient with asthma is having more airway inflammation and is not under control. The physician would likely consider increasing the dose of corticosteroids or other controlling medications. In addition, the following conditions have been shown to increase exhaled nitric oxide during an exacerbation: chronic bronchitis (COPD), pneumonia, alveolitis, bronchiolitis obliterans, bronchiectasis, sarcoidosis, or a condition with a chronic cough. Conversely, reduced eNO is found in smokers and patients with cystic fibrosis.

a. Perform the test (Code: IB9e and IIIE7a) [Difficulty: ELE: R, Ap; WRE: An]

The FVC is the greatest volume of gas that the patient can exhale as rapidly as possible after the lungs have been completely filled. Normally, the FVC is the same volume as that found in a slow or nonforced VC. Careful instructions, demonstrations, and coaching are needed to ensure that the patient’s efforts are the best possible. At least three proper efforts must be obtained.

If the measurement instrument does not give a printout, simply record the patient’s efforts in the chart. If the measurement instrument does give a printout, include copies of the efforts. See Figure 4-3 for the tracing of a properly performed FVC. The tracing allows comparison of the volumes exhaled in a series of 1-second intervals. Because of this, the tracing often is referred to as a volume-time curve. Note that the start of the effort is smooth and without interruption. The initial fast flow of gas from the upper airway is seen as the nearly vertical part of the tracing. The rest of the tracing is smooth without any coughing or other interruptions in the patient’s effort. The tracing becomes progressively more horizontal as the end of the effort is reached. Encourage the patient to try to push out as much air as possible as the end approaches. To provide an acceptable FVC, the patient must show maximum effort without coughing or closing the glottis, and the expiratory effort must last at least 6 seconds. The patient’s final 2 seconds of expiratory effort should show no appreciable airflow. Each of the three acceptable FVCs must show these traits and close similarity of the patient’s efforts.

b. Interpret FVC and spirometry graphics

Figure 4-3 was made on a chain-compensated, water-seal spirometry system. Note how the tracing progresses from the right to the left. The Stead-Wells system shows the same tracing “upside down” compared with the chain-compensated system. The tracing starts on the left and moves to the right (Figures 4-4 and 4-5). Other tracings may show either the chain-compensated or the Stead-Wells tracings in a mirror image or opposite shape.

Figure 4-4 Forced vital capacity (FVC) tracings showing an obstructed flow pattern and a restrictive flow pattern. Note that the patient with the obstructed pattern has a slower than normal exhalation, whereas the patient with the restrictive pattern has a faster than normal exhalation.

(From Ruppel G: Manual of pulmonary function testing, ed 6, St Louis, 1994, Mosby.)

Figure 4-5 Spirometry tracings of normal, obstructed, and restricted patients. Note how the obstructed and restricted patients’ volumes and capacities are out of proportion compared with those of the normal patient. VC, Vital capacity; RV, residual volume; FRC, functional residual capacity.

(From Cherniack RM: Pulmonary function testing, Philadelphia, 1977, WB Saunders.)

c. Interpret the results (Code: IB10e and IIIE7a) [Difficulty: ELE: R, Ap; WRE: An]

Normal racial differences in the FVC must be taken into consideration. Most modern pulmonary function systems automatically adjust the measured values for racial differences when so programmed by the operator. If not, the predicted values should be mathematically adjusted by the therapist. The predicted normal values, in liters, for the FVC in Caucasian patients were reported by Morris, Koski, and Johnson (1971) as:^*

African Americans are known to have a smaller lung capacity than Caucasians of the same height. For this reason, a 10% to 15% adjustment should be made for the predicted FVC and TLC of an African American patient. In other words, the predicted values for this patient are 85% to 90% of those of a comparable Caucasian patient.

Adjustments for Hispanic and Asian populations are not so well documented. It has been reported that the predicted FVC values should be adjusted downward by 20% to 25% for Asians.

It has been commonly accepted that a measured FVC at least 80% of the predicted FVC is considered to be within normal limits for adults of all races. In addition, the FEV₁ and TLC measurements have been included in this 80% of predicted rule. More recent studies by Knudson, Kaltenborn, Knudson, and associates (1987) and Paoletti, Viegi, Pistelli, and associates (1985) suggest that normal values for most tests should be determined by finding the percentage of predicted value above which 95% of the population would be seen (the so-called normal 95th percentile). Even though this method finds 5% (1 in 20) of healthy nonsmokers to be abnormal, it offers more realistic predicted values. It is normal to see a decline in the FVC with age.

Restrictive problems, such as advanced pregnancy, obesity, ascites, neuromuscular disease, sarcoidosis, and chest wall or spinal deformity, can result in a small FVC. Patients with chronic obstructive lung diseases, such as emphysema, bronchitis, asthma, cystic fibrosis, and bronchiectasis, commonly have a small FVC. (Figure 4-5 shows a comparison of the spirometry tracings of a normal, an obstructed, and a restricted patient.)

The limitations of this text prevent a discussion of back-extrapolation to find the start of a less than perfect effort or the calculations for correcting volumes and flows from atmospheric temperature, pressure, saturated (ATPS) to body temperature, pressure, saturated (BTPS). However, most textbooks on pulmonary function testing discuss these topics. Because the BTPS correction reflects the patient’s true effort, it is standard practice to report all flows and volumes in BTPS.

a. Perform the procedure (Code: IB9d) [Difficulty: ELE: R; WRE: Ap, An]

The peak flow is the highest flow rate seen in a patient’s forced expiratory effort. Some authors refer to the PF as the peak expiratory flow rate. It usually is seen at the beginning of the FVC effort. The instructions for the test must emphasize that the patient must “blast” the air out as hard and fast as possible. It is not necessary to encourage the patient to empty the lungs completely to residual volume.

For bedside spirometry or home care, the patient’s effort is easily measured directly with a handheld peak flowmeter. Usually at least three efforts are required to find two that are acceptably close. PF values that are consistent and low despite variable patient efforts probably indicate a malfunctioning unit that should not be used. It is reasonable to record the patient’s effort in liters per second, because the effort takes place in about that much time. However, do not be confused by some measurement instruments and other prediction equations giving the value in liters per minute. Simply multiply or divide by 60 to convert your patient’s effort from one time frame to the other. For example, a young man’s PF might be recorded as 10 L/sec or 600 L/min. Cherniack and Raber (1972) published the following formulas for predicting PF in liters per second^*:

b. Interpret the results (Code: IB10d) [Difficulty: ELE: R, Ap; WRE: An]

The PF is directly related to height and indirectly related to age. Therefore, the taller the patient, the greater the PF. The PF decreases with age. The PF is a rather nonspecific measurement of airway obstruction. It measures flow through the upper airways and is reduced in patients with an upper airway problem, such as a tumor, vocal cord paralysis, or laryngeal edema.

The PF test is most often given to patients having an asthma attack as a quick and easy measurement of small airway obstruction. Current asthma guidelines state that if the peak flow of a patient with asthma is 80% to 100% of predicted or personal best, he or she is in the green zone. This means that the patient’s medications are adequately controlling the asthma. If the peak flow is 50% to 79% of predicted or personal best, he or she is in the yellow zone. This means that the patient’s medications are not adequately controlling the asthma. Increased doses are indicated if ordered by the physician. If the peak flow is less than 50% of predicted or personal best, the patient is in the red zone. This means that the patient’s medications are not adequately controlling the asthma. The patient should get medical help as soon as possible.

It is the respiratory therapist’s responsibility to calculate the patient’s color zones and mark them on the patient’s personal peak flowmeter. The therapist must instruct the patient in the meaning of the color zones and the appropriate use of the prescribed inhaled bronchodilator and/or corticosteroid medications.

a. Perform the procedure (Code: IB9u) [Difficulty: ELE: R, Ap; WRE: An]

The FEF_25%-75% is the mean forced expiratory flow during the middle half of an acceptable FVC (Figure 4-6). The FVC effort to use for this test is the one that has the greatest combination of FVC volume and FEV₁. As mentioned earlier, the patient must give his or her best effort. The measurement usually is recorded in liters per second but may be recorded in liters per minute.

Figure 4-6 One method of determining the 25% to 75% forced expiratory flow (FEF_25%-75%) value from a forced vital capacity (FVC) tracing. First, mark the 25% and 75% points from the start of the effort. These are found by multiplying the FVC value by 0.25 and 0.75, respectively, and measuring from the start of the effort. Second, draw a line through these two points to intersect the dashed time lines at points A and B. Horizontal dashed lines are added from A and B to cross the volume scale. The FEF_25%-75% value is read as the distance between A and C, or about 2 L/sec. BTPS correct this measurement.

(From Ruppel G: Manual of pulmonary function testing, ed 9, St Louis, 2009, Mosby.)

b. Interpret the results (Code: IB10u) [Difficulty: ELE: R, Ap; WRE: An]

The results are normally less than in the PF test, because the flow being measured comes from medium-size and small airways (smaller than 2 mm in diameter). The results should decline with age and should be lower in women than in men. A patient with a restrictive lung disease may have a normal or increased value, whereas a low value is seen in a patient with obstructive lung disease. A small FEF_25%-75% value when the FVC and FEV₁ values are normal often is taken to indicate early small airway disease. A normal, 68-kg (150-lb) young man should have values of 4 to 5 L/sec or 240 to 300 L/min. The following formulas were developed by Morris, Koski, and Johnson (1971) and can be used to calculate the predicted values in liters per second:

a. Perform the procedure (Code: IB9u) [Difficulty: ELE: R, Ap; WRE: An]

The FEV_T is the volume of air exhaled from an acceptable FVC in the specified time. The time increments are 0.5, 1, 2, and 3 seconds or more and are listed as FEV_0.5, FEV₁, FEV₂, FEV₃, and so on. It is important that the FVC have a good start and a maximum effort to the end.

The FEV₁ is the measurement most commonly used, along with the FVC, to judge the patient’s response to inhaled bronchodilators, for bronchoprovocation testing to screen for asthmatic tendencies, to detect exercise-induced asthma, and for simple screening. BTPS correct all the measured values.

The timed forced expiratory volumes (FEV_0.5, FEV₁, FEV₂, FEV₃) effectively “cut” the FVC into sections based on how much volume the patient forcibly exhales in 0.5, 1, 2, and 3 seconds. Some patients with severe obstructive lung disease require several more seconds to exhale completely. In these cases, simply keep measuring the volume exhaled in each additional second. Figure 4-7 shows an FVC tracing that is subdivided at 0.5-, 1-, 2-, and 3-second intervals. Some bedside units give a numeric value for some or all of the timed intervals; however, it is best to have a spirometer that produces a printed copy of the patient’s FVC effort. The individual volumes can be determined by marking the vertical distance on the volume scale from the baseline (total lung capacity) to the respective arrow tips.

Figure 4-7 Forced vital capacity divided into FEV_0.5, FEV_1.0, FEV_2.0, and FEV_3.0.

(From Ruppel G: Manual of pulmonary function testing, ed 4, St Louis, 1986, Mosby.)

b. Interpret the results (Code: IB10u) [Difficulty: ELE: R, Ap; WRE: An]

The FEV_T values often are reduced in both restrictive and obstructive lung diseases. Patients with a severe restrictive lung disease exhale almost all of their small FVC within the first second. Patients with severe obstructive lung disease show low values at all time intervals, with the volumes at FEV₂, FEV₃, and so on, becoming progressively smaller. The most commonly evaluated values are the FEV₁ and the FEV_1%. The following formulas^* were developed by Morris, Koski, and Johnson (1971) and can be used to calculate the predicted values for FEV₁ in liters:

a. Perform the procedure (Code: IB9u) [Difficulty: ELE: R, Ap; WRE: An]

The FEV_T to FVC ratio compares, by division, the volume exhaled at 0.5, 1, 2, and 3 (or more) seconds (see Figure 4-7) with the FVC. This results in a series of decimal fractions. These are multiplied by 100 to convert the answers to percentages.

Because each person’s FVC is unique, the FEV time interval volumes taken from the FVC also are unique. The division process described previously standardizes the FEV time interval results, regardless of the patient’s FVC. Therefore, these percentage values can be standardized for all individuals despite different FVCs. The predicted values for normal patients are as follows:

b. Interpret the results (Code: IB10u) [Difficulty: ELE: R, Ap; WRE: An]

These values normally decrease slightly in elderly patients. Most patients with normal lungs and airways are still able to completely exhale the FVC within 4 seconds.

Patients with restrictive lung diseases often exhale the FVC more quickly than expected. This happens because these patients have a smaller than normal FVC and stiff lungs, which recoil more quickly than expected to their resting volume. See Figure 4-4 to compare the FVC curves of a patient with restrictive lung disease with the FVC curves of a patient with obstructive lung disease.

Patients with obstructive lung disease take longer than expected to exhale the FVC. As a result, the percentages of the FVC exhaled in the timed intervals listed are lower than normal. Obviously, the lower the percentage exhaled for any timed interval, the worse the obstruction to exhalation. A patient with restrictive lung disease exhales the FVC too quickly, and all of the derived values are higher than expected.

a. Perform the procedure (Code: IB9u) [Difficulty: ELE: R, Ap; WRE: An]

The following are common indications for the procedure:

The most commonly administered tests are the PF and FEV_1% from an FVC. Before starting the test, make sure that the patient has not taken a bronchodilating drug in the previous 4 to 6 hours. If no bronchodilating drugs have been administered, the test can be performed. A PF or FEV_1% test is performed and the value or values are measured, and then a fast-onset, sympathomimetic-type drug is given. The medication can be given by intermittent positive-pressure breathing (IPPB), handheld nebulizer, or metered-dose inhaler (MDI), as long as the method is done properly. Wait about 10 to 15 minutes for the medication to take effect and the patient’s blood gas values to return to normal. Then repeat the PF or FEV_1% test. The percentage of improvement is calculated using this formula:

b. Interpret the results (Code: IB10u) [Difficulty: ELE: R, Ap; WRE: An]

To prove that the medication is effective, according to the current standard, the patient must have at least a 12% improvement in PF or FEV_1% or both and a 200-mL increase in exhaled volume. (The old standard required a 15% to 20% improvement in flow.) It is not uncommon to see patients with asthma improve much more than this. Other patients may not have this much improvement but do show increases in airflow and FVC and say that they feel better. In these cases, the physician may decide to continue the medication.

a. Perform the procedure (Code: IIIE7f) [Difficulty: ELE: R, Ap; WRE: An]

A bronchoprovocation study (also known as bronchial provocation) is indicated when a patient has a history indicating asthma or hyperactive airways yet has normal spirometry testing. This means that the patient’s FVC and FEV₁ from the forced vital capacity are within normal limits. Other indications for a bronchoprovocation study include assessing the severity of airway hyperresponsiveness; evaluating the risk of asthma developing, including occupational asthma; assessing the effectiveness of medications to control the patient’s asthma; and excluding asthma during a diagnostic workup.

The most common testing regimen involves having the patient inhale increasingly higher doses of nebulized methacholine (a cholinergic [parasympathetic] drug) and measuring spirometry results. Because of this, the procedure may be referred to as a methacholine challenge test. Occasionally, other factors, such as cold air exercising; inhaling an histamine; or inhaling a targeted antigen, such as animal dander, are substituted for methacholine. In addition, to evaluate the effectiveness of asthmacontrolling medications, the physician may order that they be withheld for a specified period. The following discussion is limited to inhaled methacholine.

Testing should not be performed on an at-risk patient, such as someone who (1) has an FEV₁ of less than 50% of predicted, (2) has had a stroke or heart attack within the past 3 months, (3) has uncontrolled hypertension, or (4) has a known aortic or cerebral aneurysm. Remember, above all, that a positive test result indicates that the patient has some level of bronchospasm.

The basic procedure is as follows:

If the patient is normal, continue as follows:

b. Interpret the results (Code: IIIE7f) [Difficulty: ELE: R, Ap; WRE: An]

If the patient has the above-specified drop in FEV₁, the physician would interpret that to mean the patient had a positive test and has a bronchospasm-inducing condition. Inhaling a small amount of methacholine with a large drop in FEV₁ indicates a severe reaction. A negative test result is seen if the patient inhales the maximum dose of methacholine (16 mg/mL) without a significant drop in FEV₁.

a. Perform the procedure (Code: IB9u) [Difficulty: ELE: R, Ap; WRE: An]

The flow-volume loop is a graphic display of the flow and volume generated during a forced expiratory vital capacity (FEVC) that is immediately followed by a forced inspiratory vital capacity (FIVC). It is used to identify inspiratory or expiratory flow at any lung volume.

As with the FVC test, the patient should be coached to inhale completely and blast the air out until he or she is completely empty. When you are sure that the patient has exhaled to residual volume, coach him or her to inhale as quickly as possible until the lungs are completely full. No hesitation at the start, leaks, glottis closing, or coughing should occur throughout the entire procedure.

The expiratory half of the curve is called the maximum expiratory flow-volume (MEFV) curve. It begins at TLC and ends at residual volume. The inspiratory half of the curve is called the maximum inspiratory flow-volume (MIFV) curve. It begins at residual volume and ends at TLC. Ideally, the two halves of the loop meet at the TLC. Flow is recorded in liters per second and graphed on the vertical (ordinate, or y) axis. Volume is recorded in liters and graphed on the horizontal (abscissa, or x) axis. Both flow and volume should be BTPS adjusted.

b. Interpret the results (Code: IB10u) [Difficulty: ELE: R, Ap; WRE: An]

Flow-volume loops have gained great popularity, because the shape of the curve is diagnostic of the patient’s condition. In addition, the peak inspiratory and peak expiratory flows can be determined. If the effort can be timed, all the parameters found on the previously discussed volume-time curves can be found on the flow-volume loop. The following examples show a normal flow-volume loop and representative abnormal loops.

1. Normal

A normal flow-volume loop is shown in Figures 4-8 and 4-9. First look at Figure 4-8, in which the various volumes are measured on the horizontal scale. The tidal volume (V_T) of 500 mL is the small loop within the larger VC loop. The ERV and IRV are shown on both sides of the tidal volume. The FVC is shown as the total of all three volumes. Finally, TLC and residual volume (RV) are marked.

Figure 4-8 Flow-volume loop tracing of a normal adult showing the positions and values of the lung volumes. ERV, Expiratory reserve volume; FVC, forced vital capacity; IRV, inspiratory reserve volume; RV, residual volume; TLC, total lung capacity.

Figure 4-9 Flow-volume loop tracing of a normal adult showing the positions and values of the various inspiratory and expiratory flows. FEF, Forced expiratory flow; FIF, forced inspiratory flow; PEFR, peak expiratory flow rate; PIFR, peak inspiratory flow rate.

Figure 4-9 shows the same normal flow-volume loop in which the various flows are measured on the vertical scale. Starting from TLC with the FEVC, the peak expiratory flow rate is seen as the greatest flow generated; it is about 9 L/sec. Starting from RV with the FIVC, the peak inspiratory flow rate is seen as the greatest flow that is generated; it is about 7 L/sec. It is normal for the PEFR to be greater than the PIFR.

To find the instantaneous flow at any FVC lung volume, the FVC must be divided by 4 to find the 25th, 50th, and 75th percentile points. In Figure 4-8, the FVC is 4,800 mL. Dividing by 4 gives 1,200 mL per quarter of the FVC. These points are marked on the horizontal volume scale. If a vertical (dashed) line is drawn through these three points to the flow-volume tracing, the instantaneous flows at these volumes can be found. Expiratory flows are reported as follows:

• Flow at 75% of the FEVC = (maximum flow with 75% of the FVC remaining), or FEF_25% (forced expiratory flow with 25% of the FVC exhaled)

• Flow at 50% of the FEVC = (maximum flow with 50% of the FVC remaining), or FEF_50% (forced expiratory flow with 50% of the FVC exhaled)

• Flow at 25% of the FEVC = (maximum flow with 25% of the FVC remaining), or FEF_75% (forced expiratory flow with 75% of the FVC exhaled)

Inspiratory flows are reported in this way:

• Flow at 25% of the FIVC = FIF_25% (forced inspiratory flow with 25% of the FVC inhaled)

• Flow at 50% of the FIVC = FIF_50% (forced inspiratory flow with 50% of the FVC inhaled)

• Flow at 75% of the FIVC = FIF_75% (forced inspiratory flow with 75% of the FVC inhaled)

The PEFR and FEF₂₅ or values should be about the same, because they all measure flow through the large upper airways. Either test is a good gauge of the patient’s effort, because it will be low if the patient is not trying hard. The FEF₅₀ or values should approximate the FEF_25%-75% values, because they both show flow through the medium to small airways in the middle half of the FVC effort. It is normal for the FIF₅₀ to be greater than the FEF₅₀. The FEF₇₅ or values are the best indicator of early small airway disease because both show flow through the small airways as the patient approaches the residual volume. Note that the tracing from the FEF₂₅ or point to the residual volume is close to a straight line. In normal patients, the flow decreases in proportion to the decreasing lung volume, resulting in the straight-line tracing. Cherniack and Raber (1972) published formulas for predicting adult MEFV flows in liters per second; see the bibliography.

2. Small airway disease

Examples of conditions that result in disease in the small airways (i.e., those less than 2 mm in diameter) include asthma, chronic bronchitis, bronchiectasis, cystic fibrosis, and emphysema. The obstruction can be from bronchospasm, mucus plugging, or damage to the alveoli and small airways, leading to their collapse on expiration.

Figure 4-10 shows representative flow-volume loops for patients with asthma and emphysema superimposed over a normal flow-volume loop. Note that both loops are shifted to the left, toward the TLC, because the residual volumes are increased. Also note that the flows are decreased more than normal as the patient exhales closer to the residual volume. This “scooped out” appearance is very characteristic of small airway disease. Having the patient inhale a bronchodilator and repeating the flow-volume loop shows the degree of reversibility. Some computer-based systems allow the before and after bronchodilator loops to be superimposed to show further the amount of improvement.

Figure 4-10 A series of abnormal flow-volume loop tracings superimposed over a dashed-line tracing of a normal loop.

(From Ruppel G: Manual of pulmonary function testing, ed 9, St Louis, 2009, Mosby.)

a. Perform the procedure (Code: IB9u) [Difficulty: ELE: R, Ap; WRE: An]

The maximum voluntary ventilation (MVV) is the volume of air exhaled in a specified period during a repetitive maximum respiratory effort. It is most commonly done to evaluate a patient’s ability to perform a stress test. It also may be used as a preoperative screening test to help determine the patient’s chance of pulmonary complications.

The patient should breathe at a volume that is greater than the tidal volume but less than the VC, at a rate of 70 to 120 per minute. The minimum time for the test is 12 seconds. Figure 4-11 shows two different tracings of the MVV effort. The total volume exhaled in the given period is mathematically adjusted for 1 minute so that the derived value is in liters per minute. This is done by multiplying a 5-second effort by 12 or a 12-second effort by 5. The derived value then is BTPS corrected to give the final value.

Figure 4-11 Two tracings of the same maximum voluntary ventilation (MVV) effort. The saw-toothed tracing shows each individual volume effort and the respiratory rate. The stair-stepped tracing shows the cumulative volume during the effort.

(From Ruppel G: Manual of pulmonary function testing, ed 6, St Louis, 1994, Mosby.)

b. Interpret the results (Code: IB10u) [Difficulty: ELE: R, Ap; WRE: An]

The results of the MVV test are among the most difficult to evaluate. This is because the patient’s effort, the condition of the respiratory muscles, lung and thoracic compliance, neurologic control over the drive to breathe, and airway and tissue resistance all have an influence. Abnormalities in any of these can cause the MVV to decrease. Because more than one problem can exist, a decreased MVV does not point out the exact difficulty. A healthy young man can have an MVV of 150 to 200 L/min. Women tend to have smaller values, and the values of both genders decrease with age. Because of the many factors involved in the MVV, normal predicted values may vary by as much as ± 30%. Therefore, unless the MVV is less than 70% of predicted, the patient cannot really be considered abnormal.

Cherniack and Raber (1972) published the following equations^* for predicting the MVV in liters per minute:

The following considerations are important in the evaluation of an abnormally low MVV value:

Despite these difficulties in determining the cause of a decreased MVV value, doing so has proven helpful in preoperative evaluation and cardiopulmonary stress testing. Any patient with a lower than normal MVV value is at an increased risk of postoperative atelectasis and pneumonia. The risks of pulmonary complications related to MVV are low when the patient reaches 75% to 50% of predicted, moderate when the patient reaches 50% to 33% of predicted, and high when the patient reaches less than 33% of predicted. Patients with known moderate to severe COPD usually have to stop exercise testing because of their inability to breathe. An MVV value of less than 50 L/min is a good predictor of this.

a. Perform the procedure (Code: IB9u) [Difficulty: ELE: R, Ap; WRE: An]

The single-breath nitrogen washout (SBN₂) is used to measure two things: (1) the evenness of the distribution of ventilation into the lungs during inspiration and (2) the emptying rates of the lungs during exhalation. It is a helpful diagnostic test in any adult patient who is known to have or suspected of having obstructive airway disease. The NBRC uses the phrase nitrogen washout distribution test for its examinations.

The closing volume is one part of the SBN₂ test. It marks the lung volume when small airway closure begins and is used as an early indicator of small airway disease.

The SBN₂ test is done by analyzing the nitrogen (N₂) percentage exhaled after an inspiratory vital capacity (IVC) of 100% oxygen. It is beyond the scope of this book to go into detail on all the steps of the procedure; however, the general steps include instructing the patient to perform an IVC test while inhaling oxygen. The patient is told to exhale slowly and evenly until the lungs are empty again, without any breath holding. The exhaled gases are sent through a rapid N₂ analyzer to measure the percentage, a spirometer to measure the volume, and a graphing device.

b. Interpret the results (Code: IB10u) [Difficulty: ELE: R, Ap; WRE: An]

Figure 4-12 shows a normal tracing, which shows these phases:

Figure 4-12 Tracing of a normal single-breath nitrogen washout (SBN₂) test showing the four phases and other features. CC, Closing capacity; CV, closing volume; ΔN_{2 750-1250}, nitrogen percentage found in the 500 mL of gas exhaled between 750 and 1250 mL of the vital capacity; RV, residual volume; TLC, total lung capacity; VC, vital capacity.

(From Ruppel G: Manual of pulmonary function testing, ed 9, St Louis, 2009, Mosby.)

The start of phase IV is called the closing volume (CV). It marks the lung volume when small airway closure begins and is an early indicator of small airway disease. CV does not occur in healthy young adults until after about 80% to 90% of the VC has been exhaled. The closing capacity (CC) is found by adding the closing volume to the residual volume (found by another test). Healthy young adults have a CC that is about 30% of their TLC.

Problems can be indicated as follows: increases in the ΔN_{2 750-1250}, the slope of phase III, and especially early onset of phase IV; increased closing volume; and increased closing capacity. The diseases or conditions that can cause these abnormal test results are small airway disease, congestive heart failure with pulmonary edema, and obesity.

a. Perform the procedure (Code: IIIE7c) [Difficulty: ELE: R, Ap; WRE: An]

The functional residual capacity (FRC) is the volume of gas left in the lungs at the end of a normal expiration. It cannot be measured through spirometry. The FRC is needed to calculate a patient’s residual volume (RV) and TLC. It is necessary to know a patient’s RV, FRC, and TLC to diagnose and determine the severity of obstructive lung disease and restrictive lung disease.

The helium (He) dilution method basically involves diluting the resident gases in the lungs (mainly nitrogen and oxygen) with helium to mathematically determine the FRC. This also is called the closed-circuit method, because the patient and circuit are sealed off. Figure 4-13 shows a schematic drawing of the components that make up the circuit. These include a two-way valve to switch the patient from breathing room air to the helium mix, soda lime to absorb the patient’s exhaled carbon dioxide from the circuit, a combined carbon dioxide (CO₂) and water vapor absorber to prevent these gases from entering the helium analyzer, the helium analyzer, a variable speed blower to move the gases through the circuit, a spirometer for monitoring tidal volumes, an attached kymograph to trace the patient’s breathing pattern, and an oxygen supply to meet the patient’s needs. The helium supply is not shown.

Figure 4-13 Schematic drawing of the key components of a helium dilution system for measuring functional residual capacity.

(From Beauchamp RK: Pulmonary function testing procedures. In Barnes TA, editor: Respiratory care practice, St Louis, 1988, Mosby.)

It is beyond the scope of this text to cover all the steps in the helium dilution test; however, the following features of the procedure are important to know. Add enough helium to the room air in the circuit to create a 10% to 15% He mix. At the end of a normal exhalation, the patient is switched to breathing the mix so that the FRC can be determined. The patient breathes the gas mix until the helium is evenly distributed throughout the lungs and the helium percentage is stable. Typically, the test is performed for up to 7 minutes, if needed, to reach an equilibrium point. Extending the test longer may help to reach a stable equilibrium point in abnormal patients. The calculation of RV is rather complex and usually is done through the computer built into the pulmonary function system. Spirometry also must be performed, because the ERV is subtracted from the FRC to find the RV. Commonly the test is repeated. The patient should be allowed to breathe room air for 5 minutes between tests to clear the helium from the lungs. Patients with severe obstructive lung disease may need more time. Bates, Macklem, and Christie (1971) published the following equations^* for calculating the normal FRC in liters:

Goldman and Becklake (1959) published the following equations^* for calculating the normal RV in liters:

b. Interpret the results (Code: IIIE7c) [Difficulty: ELE: R, Ap; WRE: An]

A normal young man has an FRC volume of about 2,400 mL. As shown in Figure 4-14, it is composed of the ERV and the RV. The FRC and RV values are invaluable for diagnosing obstructive and restrictive lung diseases. See Figure 4-6 for the relative volumes and capacities for a normal patient, a patient with an obstructive pattern, and a patient with a restrictive pattern. Note that the obstructive patient has a disproportionate increase in the RV, with a resulting decrease in the FVC. The TLC may be normal, as shown, or, more commonly, lung capacity may be increased. The patient with restrictive disease has a proportionate decrease in all the lung volumes and capacities. It is commonly accepted that the normal limits of TLC are about ±20% of the predicted value. This ±20% of the normal limit applies to the FRC and RV values as well. In other words, obstructive lung disease can be diagnosed by an RV, FRC, or TLC that is more than 120% of the predicted value. Common examples of obstructive diseases include asthma, bronchitis, and emphysema. Restrictive lung disease can be diagnosed by an RV, FRC, or TLC that is less than 80% of the predicted value. Examples of restrictive diseases include fibrotic lung disease, air or fluid in the pleural space, obesity, kyphoscoliosis, pectus excavatum, and neuromuscular weakness or paralysis.

Figure 4-14 Lung volumes and capacities for a clinically normal young man.

The following factors are important to ensure that the measured values are accurate:

a. Perform the procedure (Code: IIIE7c) [Difficulty: ELE: R, Ap; WRE: An]

As discussed in the helium dilution method, the nitrogen (N₂) washout method is used to find the FRC so that the RV can be derived from it and TLC calculated. It is necessary to know a patient’s RV, FRC, and TLC to diagnose and determine the severity of obstructive lung disease and restrictive lung disease.

The nitrogen washout method basically involves having the patient breathe in 100% oxygen until all the resident nitrogen is removed from the lungs. It also is called the open-circuit method, because the patient inspires as much oxygen as needed to displace the nitrogen to a reservoir for measurement.

Figure 4-15 shows a schematic drawing of the components that make up the automated nitrogen washout system and circuit. These include a solenoid valve to switch the patient from breathing room air to breathing pure oxygen, an oxygen source with demand valve, a nitrogen analyzer with recorder, a pneumotachometer, and a microprocessor that directs all the necessary activities for the test. It is beyond the scope of this text to go into the complete procedure for the test; however, the following features should be known. The circuit is filled with pure oxygen. The patient is switched from room air to oxygen at the end of a normal exhalation so that the nitrogen in the FRC can be determined. Typically, the test is performed for up to 7 minutes or until the nitrogen percentage falls below a target level. This target percentage has been reported by various authors as 1% (best results) up to 3%. Extending the test longer may help to reach a target level in patients with increased airway resistance or an increased lung volume. As before, the calculation of RV is rather complex and is usually done through the computer built into the pulmonary function system. Spirometry also must be performed, because the ERV is subtracted from the FRC to find the RV. Commonly the test is repeated. The patient should be allowed to breathe room air for at least 15 minutes between tests to clear the oxygen from the lungs. Patients with severe obstructive lung disease may need more time.

Figure 4-15 Schematic drawing of the key components of the automated nitrogen washout system for measuring functional residual capacity.

(From Beauchamp RK: Pulmonary function testing procedures. In Barnes TA, editor: Respiratory care practice, St Louis, 1988, Mosby.)

Predicted patient values for the FRC and RV can be determined with the same equations listed in the previous discussion of the helium dilution method of determining FRC.

b. Interpret the results (Code: IIIE7c) [Difficulty: ELE: R, Ap; WRE: An]

As discussed earlier in the helium dilution test, the FRC and RV values are invaluable for diagnosing obstructive and restrictive lung diseases. When done properly, the helium dilution and nitrogen washout tests reveal similar patient values. If the nitrogen percentage is graphed over time, the shape of the washout curve also can be helpful for evaluating the degree of airway obstruction. Both the normal and abnormal tracings show rapid nitrogen washout from the upper airway dead space. However, as the test continues, a patient with obstructive lung disease shows a progressive slowing of the nitrogen washout rate.

The following considerations are important to ensure that the measured values are accurate:

a. Perform the procedure (Code: IIIE7c) [Difficulty: ELE: R, Ap; WRE: An]

The TLC is the volume of air in the lungs after inhalation of a VC. As discussed earlier, the patient’s TLC must be determined to diagnose obstructive or restrictive lung disease.

See Figure 4-14 for an example of the relationships between lung volumes and capacities. As can be seen, the TLC can be found by adding several combinations of volumes and capacities. Most commonly, it is calculated by adding the FRC to the inspiratory capacity (IC) found through spirometry. However, be prepared to add or subtract various combinations of volumes and capacities to find the TLC.

b. Interpret the results (Code: IIIE7c) [Difficulty: ELE: R, Ap; WRE: An]

The TLC results cannot be interpreted without looking at the volumes and capacities that compose it. Figure 4-5 shows representative TLC patterns for a normal patient and a patient with an obstructive pattern and a restrictive pattern. Note that with the abnormal patterns, the FRC and its components, the ERV and RV, are out of proportion. Patients with emphysema, bronchitis, or asthma often show the obstructive pattern with its large FRC of trapped gas. Patients with fibrotic lung disease, thoracic deformities, or obesity often show the restrictive pattern, with its decreased FRC and other lung volumes.

Some practitioners use the general rule that a TLC more than 120% of predicted indicates an obstructive pattern, and a TLC less than 80% of predicted indicates a restrictive pattern. However, this may be an oversimplification. It is more reliable to calculate the RV to TLC (RV/TLC) ratio. This takes into account the interrelation of the two. Normal healthy adults have an RV/TLC ratio of 0.20 (20%) to 0.35 (35%). An increased RV/TLC ratio is commonly seen in patients with emphysema and an increased RV. However, if the patient’s TLC is increased in proportion to the RV, the ratio may be within normal limits. A decreased RV/TLC ratio is commonly seen in patients with fibrotic lung disease. The ratio will be normal, however, if the patient’s TLC is decreased in proportion to the RV. Table 4-3 shows the relationship of the lung volumes and capacities, TLC, and the RV/TLC ratio found in a number of conditions.

a. Thoracic gas volume

Note: This test is not listed in the NBRC’s detailed content outlines. However, questions about interpretation of the results of lung volume tests have been included in recent versions of the Written Registry Examination.

1. Perform the procedure (Code: IIIE7c) [Difficulty: ELE: R, Ap; WRE: An]

The volume of gas measured in the lungs at the end of exhalation by a plethysmograph is called the thoracic gas volume (TGV or VTG). When the unit has been accurately calibrated and the test properly performed, the plethysmograph provides a more accurate FRC volume measurement than either the helium dilution or nitrogen washout methods. Once TGV is determined, the RV can be derived from it and the TLC calculated. It is necessary to know a patient’s RV, FRC, and TLC to diagnose and determine the severity of obstructive lung disease and restrictive lung disease.

It is not possible to go into a complete discussion of the procedure; however, the following steps are important. The unit must be sealed so that it is airtight during the patient’s breathing. The patient is instructed to breathe a normal tidal volume through the pneumotachometer. At the end of exhalation (FRC), a shutter is closed on the pneumotachometer so that no air leaks. The patient is instructed to continue to make tidal volume breathing efforts. The computer integrates the following two pressure changes: (1) a decrease in mouth pressure as the patient attempts to inhale, and (2) an increase in plethysmograph chamber pressure as the patient’s chest expands. The patient’s TGV is then determined at FRC. Figure 4-17, A, shows a normal TGV loop on the oscilloscope. Through spirometry, the patient’s ERV, RV, and TLC can be calculated.

Figure 4-17 Examples of body plethysmography tracings. A, A normal thoracic gas volume loop. B, A normal inspiratory and expiratory loop for airway resistance (Raw). Note that it is symmetrical. This patient has an Raw of 2 cm H₂O/L/sec at the standard flow of 0.5 L/sec. As the flow increases, the Raw increases as a result of the increased turbulence. The Raw of 4 cm H₂O/L/sec seen at the flow of 1 L/sec should not be recorded as the patient’s value. C, A patient whose expiratory resistance is greater than the inspiratory resistance. In this case, either both resistances should be recorded in the chart or just the expiratory resistance if only one can be recorded. D, A significant difference between early and late expiratory resistance. This is commonly seen in patients with obstructive airway disease, such as emphysema. Record the late resistance, because it better represents the patient’s disease condition.

(From Zarins LP, Clausen JL: Body plethysmography. In Clausen JL, editor: Pulmonary function testing guidelines and controversies, Orlando, 1984, Grune & Stratton.)

b. Lung compliance

2. Perform the procedure (Code: IB9n and IIIE7b) [Difficulty: ELE: R, Ap; WRE: An]

The patient must swallow a balloon 10 cm long to the midthoracic level. A catheter connects the proximal end of the balloon to a pressure transducer outside the patient. Air is injected into the balloon, and the transducer is calibrated accurately to measure changes in intrathoracic pressure as the patient breathes. The patient is then placed into a body plethysmograph that is sealed. He or she is told to breathe through the differential pressure pneumotachometer to measure lung volumes. The patient is then instructed to inhale slowly from the resting level (FRC) to TLC. As this is done, the pneumotachometer shutter is periodically closed to measure the intrathoracic pressure decrease at the increasing volumes (Figure 4-18). As the patient slowly exhales from TLC, the shutter is again periodically closed to measure the increasing intrathoracic pressure as the patient returns to FRC volume. Lung compliance is usually calculated from the pressure and volume points of FRC and FRC plus 500 mL (for a tidal volume).

Figure 4-18 Measurement of lung compliance (CL) with the esophageal balloon technique. A balloon is swallowed to the midthoracic level, filled with air, and connected to a pressure transducer to measure intrapleural pressure. The patient sits within a body plethysmograph and breathes through a pneumotachometer to measure mouth pressure and gas flow. The graph shows a tracing for a patient with normal compliance. In contrast, the patient with emphysema has an increased total lung capacity (TLC) and increased compliance; the patient with pulmonary fibrosis has a decreased TLC and decreased compliance.

(From Ruppel G: Manual of pulmonary function testing, ed 9, St Louis, 2009, Mosby.)

c. Airway resistance

2. Perform the procedure (Code: IB9n and IIIE7b) [Difficulty: ELE: R, Ap; WRE: An]

The patient is placed into a plethysmograph that is sealed. He or she is instructed to breathe through a differential pressure pneumotachometer. With the pneumotachometer shutter open, the patient is told to pant several tidal volumes of about 500 mL at a rate of 1 breath per second. Data on flow rate, tidal volume, mouth pressure changes, and chamber pressure changes are recorded and graphed (Figure 4-19). Then the shutter is closed at the patient’s resting FRC volume. The patient is told to continue panting at the same volume and rate. Again, flow rate, tidal volume, mouth pressure changes, and chamber pressures are recorded and graphed. The computer integrates the data to calculate the patient’s airway resistance during tidal volume breathing.

Figure 4-19 Measurement of airway resistance (Raw). A, The body plethysmograph in which the patient sits during the test. Tidal volume panting against an open and then closed pneumotachometer shutter is displayed on the storage oscilloscope. B, A printout of the pressure changes as the patient pants a tidal volume of 500 mL/sec.

(From Ruppel G: Manual of pulmonary function testing, ed 6, St Louis, 1994, Mosby.)

a. Perform the procedure (Code: IIIE7d) [Difficulty: ELE: R, Ap; WRE: An]

The diffusing capacity (DL or DLCO) tests look at the capacity for carbon monoxide to diffuse through the lungs into the blood. Carbon monoxide is used, because its high affinity for hemoglobin virtually eliminates blood as a barrier to diffusion. The measured value can then be correlated to the ability of oxygen to diffuse through the lungs. This test is indicated when it is important to know the extent of lung disability causing hypoxemia. This is most common with patients having emphysema, but it also is important in patients with fibrotic lung disease.

At the time of this writing, the single-breath carbon monoxide diffusing capacity test (DLCO-SB) is the only version that has a widely adopted standard technique for administration. It is recorded in milliliters of carbon monoxide (CO) per minute per millimeter of mercury at 0° C, 760 mm Hg, and dry standard temperature and pressure (STPD).

The following are key steps in the procedure. A reservoir or spirometer is filled with a mix of 0.3% CO, 10% He, 21% O₂, and the balance of N₂ (Figure 4-20). The patient is connected to the apparatus and breathes room air while being instructed in the test. After the patient is told to exhale completely (to RV), the practitioner switches the patient to the gas mix. The patient is instructed to rapidly inhale an IVC. A shutter automatically closes so that the patient cannot exhale for 10 seconds. This allows time for some of the carbon monoxide to diffuse into the patient’s bloodstream.

Figure 4-20 Schematic drawing of the components and breathing circuit used to perform a single-breath lung-diffusion test (DLCO-SB). Analysis of the patient’s exhaled helium (He) and carbon monoxide (CO) percentages is critical to the test.

(From Ruppel G: Manual of pulmonary function testing, ed 6, St Louis, 1994, Mosby.)

After the breath hold, the shutter opens, and the patient is told to exhale to resting volume. The equipment is designed to automatically let 750 to 1,000 mL of exhaled gas pass through to the spirometer. The next 500 mL of gas is diverted into the end-tidal gas sampler. This sample is then analyzed for He% and CO%. The remainder of the patient’s exhaled volume is passed through into the spirometer. The various measured parameters are integrated into the equations in the computer to give the patient’s DLCO-SB value.

This test is done only after the patient has been measured for both RV and TLC. That is because the patient’s lung volume directly affects the diffusibility of carbon monoxide.

b. Interpret the results (Code: IIIE7d) [Difficulty: ELE: R, Ap; WRE: An]

Interpretation is limited to the results of the DLCO-SB test. Ruppel (2009) reported that the average resting normal adult DLCO-SB is 25 mL CO/min/mm Hg STPD (standard temperature, pressure, dry, conditions). Gaensler and Wright (1966) reported the following DLCO-SB prediction equations, with values in mL/CO/min/mm Hg STPD:

It should be noted that a number of other authors have developed their own prediction equations. In general, patients who show DLCO-SB results within ± 20% of the predicted values (80% to 120% of predicted) are considered to be within normal limits. A patient who has actual results that are significantly below the predicted values (<80% of predicted) has a problem with lung diffusion. Figure 4-21 shows a number of common conditions that can lead to poor lung diffusion. The following factors also should be taken into consideration when interpreting the measured values:

Figure 4-21 Six factors that can cause decreased lung diffusion (DL). Factors I and II result in a decrease in DL that is in proportion to the decrease in alveolar volume. Factors III, IV, V, and VI result in a decrease in DL that is greater than the decrease in alveolar volume. Therefore these later conditions are more harmful to the patient’s pulmonary function and well-being. SBN₂, Single-breath nitrogen washout; TLC, total lung capacity; V_A, alveolar ventilation.

(From Ayers LN, Whipp BJ, Ziment I: A guide to the interpretation of pulmonary function tests, ed 2, New York, 1978, Roerig.)

a. Get the necessary equipment for the procedure

Water (or mercury) manometers have a vertical column of the liquid and are commonly used in a blood pressure sphygmomanometer. The pressure units depend on the type of liquid. A water manometer will have units of centimeters of water (cm H₂O) pressure; mercury manometers will have units of millimeters of mercury (mm Hg). The fluid column must be kept vertical for accuracy. An aneroid (spring-loaded) unit should be selected if the pressure gauge cannot be kept in a vertical position. Aneroid manometers can be calibrated in either mm Hg or cm H₂O pressure and look like a Bourdon gauge. Newer, digital manometers will display the measured pressure on a light emitting diode (LED) or other display. (Note: The NBRC recently substituted digital for mercury manometers in this section. Mercury manometers are being phased out because of environmental concerns over mercury spills.)

b. Put the equipment together and make sure that it works properly

These pressure gauges come preassembled by the manufacturer. It is necessary to attach the pressure source only to the inlet port on the unit to measure a pressure change. This connection must be airtight, or a leak will occur, and the measured pressure will be less than actually exists. The accuracy of the unit can be checked by opening the inlet port to room air and reading the pressure. A reading of zero should be seen (indicating no pressure change from atmospheric). Next, a known pressure is applied to the gauge. Often this is done by attaching it to a sphygmomanometer and pumping up the pressure to a known level, such as 50 mm Hg. The pressure gauge should show the same.

c. Troubleshoot any problems with the equipment

If, during the calibration procedure, the set pressure does not match the pressure in the gauge, a leak may be present in the system, or the pressure gauge is miscalibrated. Tighten all connections and attempt to calibrate again. Do not use a pressure gauge that cannot be calibrated.

a. Get the necessary equipment for the procedure

The maximum inspiratory pressure and maximum expiratory pressure tests are usually recorded in centimeters of water pressure. However, millimeters of mercury can be used. If a centimeters of water pressure gauge is used, it should be able to record a negative and positive pressure of at least 100 cm H₂O. However, a unit that can record ± 60 cm H₂O is probably adequate.

b. Put the equipment together and make sure that it works properly

No standard setup exists for these devices. See Figure 4-2 for two possible assemblies. The system can be sealed and pressure checked with a known force to make sure that the pressure manometer is accurate and all the connections are airtight.

c. Troubleshoot any problems with the equipment

If, during the calibration procedure, the set pressure does not match the pressure in the gauge, a leak may be present in the system, or the pressure gauge is miscalibrated. Tighten all connections and attempt to calibrate again. Do not use a pressure gauge that cannot be calibrated. The one-way valves must function so that the patient can exhale or inhale only as needed for the test.

a. Get the necessary equipment for the procedure

Pressure transducers are used in the body plethysmograph for measuring mouth pressure and chamber pressure. They come with the unit and are supplied by the manufacturer. Another type of transducer is used with an esophageal balloon when performing a lung compliance measurement. Follow the balloon manufacturer’s recommendations for the proper transducer.

b. Put the equipment together and make sure that it works properly

c. Troubleshoot any problems with the equipment

Each type of pressure transducer should be assembled as described by the manufacturer. As seen in Figure 4-22, the differential pressure transducer has small-bore tubing connecting it before and after a resistive element. These small tubes transmit the pressures before and after the resistance element to the transducer as gas flows through the large, main tube. Make sure that the tubing is properly connected to the main tube and transducer. If a tube were to disconnect, the transducer would give a reading of zero.

Figure 4-22 Cutaway view of a differential pressure pneumotachometer. Flow is measured as the pressure decreases between A and B, which are ports leading to a differential pressure transducer. The resistive element may consist of a mesh screen, a network of parallel capillary tubes, or other devices.

(From Beauchamp RK: Pulmonary function testing procedures. In Barnes TA, editor: Respiratory care practice, St Louis, 1988, Mosby.)

a. Get the necessary equipment for the procedure

b. Put the equipment together and make sure that it works properly

c. Troubleshoot any problems with the equipment

All pneumotachometers convert one type of physical information or signal to another (e.g., converting an airflow or pressure signal to an electrical signal). Two common types are discussed next. Either type should be acceptable for performing bedside spirometry. Make sure that the unit you select is capable of performing the ordered test. Be sure that you select a unit capable of printing out a hard copy of the patient’s test results and spirometry tracings if they are required for the chart.

d. Differential-pressure (flow sensing) pneumotachometer

Some articles refer to a differential pressure pneumotachometer as a Fleisch-type device. These units have a resistive element (tubes or mesh screen) in the flow tube. The faster the flow of gas through the flow tube, the greater the pressure difference before and after the resistance. Hoses connect the flow tubes before and after the resistive element to the differential pressure transducer. The transducer converts this pressure difference into an electrical signal. A microprocessor calculates the various patient values from this information (see Figure 4-22).

Assembly requires the addition of the patient’s mouthpiece to the inspiratory port so that no air leak occurs. The expiratory port should be kept completely open so that the only obstruction to the patient’s airflow is from the resistive element. A volume calibration check is performed by forcing a known amount of air from a super syringe (certified volume standard syringe) through the pneumotachometer. Minimally, several repetitions of a known 3-L volume should reveal identical measured volumes. As long as the measured volumes are within ± 3% or 50 mL (whichever is less), the unit is acceptably accurate.

Common problems with accuracy include an air leak around the mouthpiece; cracked, disconnected, or obstructed pressure-relaying hoses; water condensation or mucus on the resistive element; or obstructed upstream or downstream port. The resistive element is usually heated to minimize any condensation.

e. Heat-transfer pneumotachometer

Some articles refer to a heat-transfer pneumotachometer as a thermistor-type device or hot-wire anemometer. These units have a heated thermistor that is cooled as the gas flows past it. The temperature transducer automatically increases and measures the flow of electricity to the thermistor to keep it at the required temperature. A microprocessor calculates the various patient values from this information. The earlier discussion on assembly, calibration, and troubleshooting applies to the heat-transfer pneumotachometers, except that no pressure-relaying hoses are present (Figure 4-23).

Figure 4-23 Cutaway view of a heat-transfer pneumotachometer.

(From Ruppel G: Manual of pulmonary function testing, ed 6, St Louis, 1994, Mosby.)

a. Get the necessary equipment for the procedure

Following are typical pulmonary function tests performed at the patient’s bedside:

The tidal volume and VC volumes can be measured on a handheld, turbine-type flow sensor, such as the Wright respirometer. The patient’s peak flow is measured on a dedicated, handheld PF meter. A pressure gauge capable of measuring both positive and negative pressure is used for the MEP and MIP (see Figure 4-2). In addition, there are electrically powered, portable PFT units that contain a micropressor and can measure all of the standard spirometry volumes and flow.

b. Put the equipment together and make sure that it works properly

All of these separate pieces of equipment require additional items, such as patient nose clips and a mouthpiece. A double one-way valve system can also be added to direct gas flow properly through the equipment and to minimize cross-contamination between patients.

c. Troubleshoot any problems with the equipment

All units must work properly and have all ancillary equipment properly connected to be airtight. Any leaks will result in a loss of volume, flow, or pressure respectively. Tighten any loose connections and repeat the procedure if necessary.

a. Get the necessary equipment for the procedure

These tests are needed for a variety of special purposes, as described earlier, and require the appropriate equipment:

b. Put the equipment together and make sure that it works properly

Check the label on the tank to be sure it is the correct gas at the correct percentage. Review Table 6-1 for color codings for tanks, if needed. Each specialty gas has its own diameter index safety system (DISS) reduction valve to connect to the tank. A high-pressure hose connects the reduction valve outlet to the pulmonary function testing machine inlet.

c. Troubleshoot any problems with the equipment

The problems that can occur with specialty gas cylinders, pressure-reducing valves, and connecting high-pressure hoses are the same as can occur with oxygen cylinders. Review the discussion in Chapter 6 if needed.

a. Obstructive airway disease

The patient with severe obstructive lung disease shows low gas flow at all time intervals. However, a decreased FEV₃ and the FEF_25%-75% are early markers of small airway disease. Quite commonly, the RV is increased as a result of air trapping. This increases the FRC and often the TLC. Despite the increased lung volume, the patient’s diffusing ability is decreased. Examples of conditions that cause obstructive lung disease include asthma, emphysema, bronchitis, and bronchiolitis. Excessive mucus, foreign bodies, and airway tumors also cause bronchospasm and air trapping.

b. Restrictive lung disease

In restrictive lung disease, all lung volumes and capacities are reduced, and lung diffusion is reduced. Expiratory flows such as FEV₁ are increased. Examples of conditions that cause restrictive lung disease include fibrosis, pulmonary edema, hemothorax or pneumothorax, acute or infant respiratory distress syndrome, chest wall deformities, obesity, and various neuromuscular disorders.

a. Water-seal spirometers

Water-seal spirometers are the commonly found chain-compensated and Stead-Wells units made by Collins Medical (see Figure 4-25 for the cutaway appearance of the chain-compensated type). The bell falls and rises as the patient breathes in and out. A pulley system attached to the bell and the marking pens record the patient’s efforts on the rotating kymograph paper. Note that the tracing is inverted from the patient’s actual breathing effort. The Stead-Wells units have the marking pen attached directly to the bell, and the tracing directly shows the patient’s breathing efforts (Figure 4-26). The newer chain-compensated and Stead-Wells units also have microprocessors for calculating patient information.

Figure 4-25 Cutaway view of a chain-compensated water-seal spirometer. CO, Carbon monoxide; CO₂, carbon dioxide; He, helium; N₂, nitrogen.

(From Ruppel G: Manual of pulmonary function testing, ed 6, St Louis, 1994, Mosby.)

Figure 4-26 Cutaway view of a Stead-Wells water-seal spirometer. CO, Carbon monoxide; CO₂, carbon dioxide; He, helium; N₂, nitrogen.

(Courtesy Warren E. Collins, Inc, Braintree, Mass.)

Gas analyzers for helium, nitrogen, carbon monoxide, and other extra equipment can be added for RV and lung diffusion measurements. The carbon dioxide absorber should be left out of the breathing circuit for FVC tests because it interferes with the fast flow of gas; however, it must be put in line for any testing that will last more than 15 seconds and involves breathing repeatedly into the closed system. Supplemental oxygen must be added to the circuit for any tests that require the patient to breathe repeatedly from the closed system. The following is a checklist for setup and where to look when problem solving:

b. Dry rolling-seal spirometers

Dry rolling-seal spirometers are sometimes called piston spirometers. As can be seen in Figure 4-27, these units use a flexible silicone rubber (Silastic) or Teflon-coated rubber seal instead of water. The large volume piston moves in and out as the patient breathes. The patient’s efforts can be directly recorded by pen on paper.

Figure 4-27 Outside (A) and cutaway (B) views of a dry rolling-seal or piston spirometer.

(From Beauchamp RK: Pulmonary function testing procedures. In Barnes TA, editor: Respiratory care practice, St Louis, 1988, Mosby.)

Newer units also have microprocessors for calculating the information. They are still rather large and are best suited for the pulmonary function laboratory. These units also can be outfitted with a carbon dioxide absorber and various gas analyzers, so they, too, can be used for the full range of lung function tests. The previous checklists for setup and problem-solving ideas apply in this case except for items specific to the chain-compensated and Stead-Wells systems.

c. Wedge-type spirometers

Wedge-type spirometers are sometimes called bellows spirometers (Figure 4-28). Note that these units have plastic or rubber bellows that are fixed on one side and flexible like an accordion on other sides. The bellows move in and out as the patient breathes. The patient’s efforts can be directly recorded by pen on paper. Newer units also have microprocessors for calculating the information.

Figure 4-28 Outside (A) and cutaway (B) views of a wedge-type spirometer.

(From Beauchamp RK: Pulmonary function testing procedures. In Barnes TA, editor: Respiratory care practice, St Louis, 1988, Mosby.)

All the earlier information on the dry rolling-seal spirometers applies here except what is specific to those units. The same setup and problem-solving ideas apply here also.

d. Spirometry equipment

Volume-displacement respirometers should have the following quality control procedures performed on a regular basis:

a. Nitrogen washout equipment for measuring functional residual capacity

(See Figure 4-15 for the basic setup and breathing circuit of a nitrogen washout system.) Review the troubleshooting of volume-displacement and pneumotachometer spirometers; they are used with the nitrogen washout procedure to find the FRC. The nitrogen washout–type RV test uses an emission spectroscopy ionization chamber analyzer for nitrogen. It is more commonly called a Giesler tube ionizer. It uses a vacuum pump to draw a gas sample into the ionization chamber. The intensity of the light spectrum given off by the ionized nitrogen directly relates to its percentage in the sample. A two-point calibration check should be performed at least every 6 months to check for linearity. It involves the following steps:

Linearity (three-point calibration) can be checked by introducing 5% to 10% nitrogen into the unit to see whether the nitrogen meter measures that value. If all three points match, there is even greater confidence in the accuracy of the measurements. Do not use an analyzer that is inaccurate.

b. Helium dilution equipment for measuring functional residual capacity

(See Figure 4-13 for the basic setup of the helium dilution equipment and breathing circuit.) Again, review the troubleshooting of volume-displacement and pneumotachometer spirometers; they are used with the helium dilution procedure to find the FRC. Both the helium dilution FRC test and the lung diffusion tests require the analysis of helium in the gas mixture. A thermal conductivity analyzer typically is used. It operates under the principle of a Wheatstone bridge, in which differences in gas density lead to different cooling rates of heated thermistor beads. The different rates of cooling change electrical resistances and cause different electrical currents to flow. In a helium analyzer, the greater the helium concentration, the faster the thermistor bead cools, and the more electricity flows through the circuit. This is then read off a meter as the helium percentage.

The thermal conductivity helium analyzer should be linear over the clinically used range of helium to an accuracy of ± 0.2% He. Minimally, a two-point calibration should be performed. A room-air sample can be drawn into the analyzer and should read zero helium. A known helium concentration may then be added and analyzed (for example, heliox, which contains 80% helium and 20% oxygen). A third point can be checked if needed by analyzing another known helium percentage.