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.

3. Minute volume

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.

4. Alveolar 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.

EXAMPLE 1

A 154-lb (70-kg) person has a measured tidal volume of 500 mL and an estimated anatomic dead space of about 154 mL.

The minute alveolar ventilation () is the volume of gas that reaches the alveoli in 1 minute. It is found by multiplying the alveolar volume by the respiratory rate in 1 minute.

EXAMPLE 2

A 154-lb (70-kg) person has a measured tidal volume of 500 mL and an estimated anatomic dead space of about 154 mL. The respiratory rate is 14/min.

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.

EXAMPLE 1

Normal patient: f = 12, tidal volume = 500 mL, anatomic dead space = 154 mL

This patient should have a normal carbon dioxide level.

EXAMPLE 2

Tachypneic patient: f = 24, tidal volume = 250 mL, anatomic dead space = 154 mL

This patient should have a high carbon dioxide level.

EXAMPLE 3

Bradypneic patient: f = 6, tidal volume = 1,000 mL, anatomic dead space = 154 mL

This patient should have a low carbon dioxide level.

5. Maximum inspiratory pressure

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:

1. Obtain a pressure gauge capable of measuring at least −60 cm H₂O pressure.

2. Have the patient sit upright. Place a note in the chart if the patient is lying down.

3. Describe the procedure to the patient.

4. Simulate a demonstration of the procedure.

5. Place nose clips over the patient’s nose. Have the patient seal the lips and teeth around the mouthpiece and breathe through the open port.

6. Tell the patient to exhale completely. Seal the port when residual volume has been reached.

7. Tell the patient to breathe in as hard as possible and hold it for 1 to 3 seconds.

8. Reteach if necessary.

9. Repeat until at least three good efforts have been performed. Record the greatest stable value seen after the first second of effort. This eliminates any artifact created by the cheeks or by chest wall movement.

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

Age and Gender	Lower Limits of Normal
Males: 143 – (0.55 × age)	− 75 cm H₂O
Females: 104 – (0.51 × age)	− 50 cm H₂O

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.

Exam Hint 4-2 (ELE)

Expect to see at least one question in which the MIP value is used to help determine whether a patient is getting stronger or weaker. If the patient’s MIP is at least −20 cm H₂O (e.g., −35 cm H₂O), and other conditions are acceptable, the patient can be weaned from the mechanical ventilator. Conversely, if the patient’s MIP is not at least −20 cm H₂O (e.g., −15 cm H₂O), and other conditions are unacceptable, the patient should not be weaned from the mechanical ventilator.

6. Maximum expiratory pressure

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:

1. Obtain a pressure gauge capable of measuring at least +60 cm H₂O pressure.

2. Have the patient sit upright. Place a note in the chart if the patient is lying down.

3. Describe the procedure to the patient.

4. Simulate a demonstration of the procedure.

5. Place nose clips over the patient’s nose. Have the patient seal the lips and teeth around the mouthpiece and breathe through the open port.

6. Tell the patient to inhale completely. Seal the port when TLC has been reached.

7. Tell the patient to breathe out as hard as possible. Hold it for 1 to 3 seconds.

8. Reteach if necessary.

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

Age and Gender	Lower Limits of Normal
Males: 268 – (1.03 × age)	+140 cm H₂O
Females: 170 – (0.53 × age)	+95 cm H₂O

7. Vital capacity

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.

Exam Hint 4-3 (ELE)

A bedside vital capacity test is widely used to help in the assessment of a mechanically ventilated patient’s ability to breathe effectively. This issue and weaning guidelines are discussed in Chapter 15. In general, a decreasing VC is a sign that the patient is tired and failing. (Racial adjustment and BTPS correction are not typically done when the VC is used for bedside spirometry.)

8. Forced expiratory volume in one second

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.

9. Peak flow

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:

• Green zone: Peak flow is 80% to 100% of predicted or personal best.

• Yellow zone: Peak flow is 50% to 79% of predicted or personal best.

• Red zone: Peak flow is less than 50% of predicted or personal best.

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.

MODULE C

Pulmonary function testing laboratory studies

1. Recommend pulmonary function testing (Code: IC6) [Difficulty: ELE: R, Ap; WRE: An]

Because the pulmonary function testing laboratory has more advanced equipment and specially trained personnel, the more challenging cases requiring a pulmonary diagnosis are sent there for testing. The following tests are either specifically listed as testable by the NBRC or have been tested in recent examinations.

2. Exhaled nitric oxide

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:

• Adult: 15 to 25 ppb, with an upper limit of normal of 35 ppb.

• Child: 5 to 22 ppb, with an upper limit of normal of 25 ppb.

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.

3. Forced vital capacity

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.)

Exam Hint 4-4 (ELE, WRE)

The NBRC can show an FVC tracing from any system and expect it to be interpreted. The start of the FVC effort can be determined by the near-vertical portion of the tracing and the relatively small expiratory reserve volume (ERV) compared with the inspiratory reserve volume (IRV). You must be able to determine the various volumes and capacities from a spirometry tracing.

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:^*

EXAMPLE

Calculate the predicted FVC for a 50-year-old Caucasian man who is 6 feet (72 inches) tall.

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.

Exam Hint 4-5 (WRE)

You may need to calculate how close a patient’s FVC or other breathing effort has come to his or her predicted value. This is known as the patient’s percentage of predicted. It is found by this equation:

For example, in the previous discussion on FVC, it was determined that a patient had a predicted FVC of 5.166 L. Calculate the percentage of predicted if the actual FVC result is 4.65 L.

Therefore, the patient exhaled 90% of predicted FVC and is within the normal range for that test result.

4. Peak flow

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.

Timed forced expiratory volume tests

All of the timed forced expiratory volume tests are derived from a properly performed FVC test (see Figure 4-3). When the FVC is done correctly, the following values can be properly calculated and evaluated to determine the patient’s condition. As discussed earlier, an FVC within 80% of predicted is interpreted as within normal limits. Therefore, if the results of the following tests show patient values within 80% of predicted, the results are interpreted as being within normal limits.

5. Forced expiratory flow_25%-75% (FEF_25%-75%)

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.