Chapter 10
Specialized Test Regimens
1. Describe the indications for respiratory muscle strength testing.
2. Identify the normal range for exhaled nitric oxide values.
3. List two indications for preoperative pulmonary function testing.
4. Understand the difference between open- and closed-circuit calorimetry.
1. Judge the reliability of metabolic measurements.
2. Select appropriate tests to evaluate disability in either chronic obstructive pulmonary disease (COPD) or pulmonary fibrosis.
3. Understand the equation of motion and the impact of impedance and elastance on flow.
4. Evaluate the clinical implications of an elevated level of exhaled nitric oxide (eNO).
Respiratory muscle strength testing
Description
Technique
The patient is connected to a valve, shutter apparatus, or the PF testing system with a mouthpiece (a flanged mouthpiece may facilitate testing in subjects with excessive muscle weakness) and noseclip in place. The mouthpiece can be placed in the mouth or the lips pressed against the mouthpiece opening, as would be done with a bugle, thus the early term for the procedure “bugles” (Figure 10-1). Regardless of the mouth-device interfaced used, there should be a tight fit so that the patient can exert maximal pressure. The airway is occluded by blocking a port in the valve or by closing a shutter. In either system, a small, fixed leak is introduced between the occlusion and the patient’s mouth. The leak may be a small opening such as a large-bore needle or created by the PFT system in their hardware. The leak eliminates pressures generated by the cheek muscles during the MEP maneuver by allowing a small amount of gas to escape the oral cavity. Likewise, the leak prevents glottic closure during the MIP maneuver. The incorporation of this small leak does not significantly change lung volume or the pressure measurement.
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A simple and historical method of measuring maximal inspiratory and expiratory pressures involves the use of a small metal tube with a fixed leak and pressure gauges. During the expiratory maneuver, the subject would blow into the device with his or her lips within the mouthpiece, earning it the name, “bugle testing.”
Pressure may be measured using a manometer, an aneroid-type gauge, or a pressure transducer. The pressure-monitoring device should be linear over its range. It should be able to record pressures from −200 to approximately +200 cm H2O. Pressure transducers are typically used in PF testing systems with the output incorporated into a data acquisition screen and algorithms used to calculate the sustained pressure. If a manometer or aneroid gauge is used, the technologist directly observes the pressure and records it. This type of monitoring is common in the hospital where measuring vital capacity and respiratory pressures are used to assess ventilatory adequacy (Figure 10-2). The technologist should record the plateau pressure that the patient can maintain for 1–2 seconds.
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Measurement of vital capacity and respiratory muscle strength at the bedside.
For the MIP test, the patient is instructed to expire maximally. Monitoring expiratory flow or having the patient signal helps determine when maximal expiration has been achieved. Then the airway is occluded as described. The patient inspires maximally and maintains the inspiration for 1–2 seconds. The first portion of each maneuver is disregarded because it may include transient pressure changes that occur initially (Figure 10-3). The most negative value from at least three efforts that vary less than 20% is recorded.
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These tracings plot the respective pressures recorded in cm H2O against time on a single graph. Maximal inspiratory pressure shows a downward or negative deflection. Maximal expiratory pressure shows a positive or upward deflection. Each maneuver is conducted with the airway occluded (see text). The occlusion is maintained for a short interval (1 to 3 seconds), and any initial transient tracings are discarded.
MEP is recorded similarly. The patient inhales as much as possible, then exhales maximally against the occluded airway for 1–2 seconds. Longer efforts should be avoided as cardiac output can be reduced by the high thoracic pressures (e.g., Valsalva maneuvers) that are sometimes developed. MEP is usually larger than MIP in healthy patients. The pressure-monitoring device should be able to withstand the higher pressure without damage. The most positive value from at least three efforts that vary less than 20% is recorded. As for MIP, initial pressure transients during the MEP are disregarded. Both MIP and MEP require patient cooperation and effort. Low values may reflect a lack of understanding or insufficient effort. (Box 10-1)
Significance and Pathophysiology
Sniff nasal inspiratory pressure (SNIP) has been proposed as an alternative or complementary test to MIP. The measurement is performed by occluding a nostril during a maximal sniff maneuver performed through the contralateral nostril from functional residual capacity or residual volume. In the contralateral nostril, a nasal olive with a central catheter is connected to a pressure transducer. Instrumentation can be sophisticated with computer incentive screens to simple hand-held devices (Figure 10-4). Measurement limitations include nasal obstruction or collapse during the maneuver. Several papers have compared the test method to sniff pressures measured using an esophageal catheter with good correlation in subjects with neuromuscular disease. A sniff nasal-inspiratory force less than 40 cm H2O has been shown to predict mortality in patients with amyotrophic lateral sclerosis (sensitivity 97%, specificity 79%).
Exhaled nitric oxide
Description
Measurement of exhaled nitric oxide (eNO) provides a simple and noninvasive method for assessing airway inflammation. It is particularly useful in diagnosing and monitoring lung diseases characterized by eosinophilic inflammation, such as asthma. The fraction of eNO in an exhaled breath (FeNO) can be measured with a sensitive chemiluminescent or electrochemical analyzer (see Chapter 13). FeNO is usually reported in parts per billion (ppb). Because the measurement is dependent on flow during exhalation, the flow (in liters per second) may be subscripted to the term (e.g., FeNO0.05 represents the FeNO measured at a flow of 0.05 L/sec). NO is also sometimes measured from the nasal cavities, including the sinuses, as a marker of nasal inflammation.
Techniques
On-Line FeNO in Adults
Without breath holding, the patient then exhales slowly and evenly while exhaled gas is sampled continuously. To prevent contamination of the exhalate with nasal NO, the patient exhales against an expiratory resistance while receiving feedback to maintain a positive pressure at the mouth. This positive pressure (usually about +5 cm H2O) causes the velum in the posterior pharynx to close, preventing nasal NO from entering the air stream. The fractional concentration of NO in the exhaled gas varies inversely with the flow. To standardize on-line measurements, an exhaled flow of 0.05 L/sec (i.e., 50 mL/sec) ±10% is recommended. This flow allows dead space gas to be exhaled and a plateau in NO to be observed in about 10 seconds in adults (Figure 10-5). The correct flow can be maintained by using a pressure-sensitive flow controller and providing visual feedback to the subject. Current commercially available instrumentation (Chapter 11 uses visual incentives to assure standardized flow/pressure and will mark a maneuver as not meeting acceptability criteria if flow/pressure is not maintained).
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The upper graph shows three efforts in which flows are measured at the mouth. The subject exhales against a slight resistance to close the velum in the oropharynx. Visual feedback in the form of a computer display helps the patient maintain a constant pressure and thereby a constant flow at the recommended 0.05 L/sec. The lower graph displays the FeNO values for the three maneuvers over 10 seconds of exhalation. Exhaled NO rises to a plateau and the FeNO concentration is measured over a 3-second (i.e., 0.15 L) window from the plateau (see text).
FeNO is measured from the plateau of the single-breath exhalation profile (see Figure 10-5). The plateau phase may slope up or down slightly. The exhalation should last long enough to establish this plateau (>6 seconds for adults and children older than 12 years, >4 seconds for children ages 12 years and younger). Two points should be chosen on the plateau that represent a 3-second interval (about 0.15 L) in which the FeNO varies less than 10%. The FeNO is the mean concentration over this 3-second interval. For FeNO values of 10 ppb or less, variability of 1 ppb NO may be used in place of the 10% criteria. A minimum of 30 seconds should elapse between repeated measurements with the subject breathing air (off the NO sampling circuit).
Off-Line FeNO in Adults
The patient inhales through an NO scrubber or from a reservoir with NO-free gas. After inhaling to TLC, the patient exhales his or her VC into an appropriate sampling device without breath holding. Expiratory resistance (+5 cm H2O) is added just as is done for on-line measurements to minimize contamination by nasal NO. Flow is usually controlled by monitoring the back-pressure in the system; flows of 0.35 L/sec ±10% are recommended to allow the VC to be collected in a reasonable interval. The sample is collected in a balloon made of polyester (Mylar) or a similar material that is impermeable to NO and large enough to accommodate the adult’s VC. Off-line samples should be analyzed within 12 hours. Exhaled nitric oxide can be performed in children able to perform the single breath maneuver and is described in Chapter 8.
Nasal Nitric Oxide
Although various methods of sampling nasal NO have been described, the recommended method uses transnasal airflow in series. In this technique, air is aspirated into one naris and out the opposite side, where NO is sampled. The subject exhales against a resistance of approximately 10 cm H2O, while air is aspirated at a constant flow rate. As for measurement of FeNO from the lower airways, exhalation against resistance closes the velum in the posterior pharynx to prevent contamination of the sample. Airflow of 0.25–3.0 L/min is used for nasal NO measurements. Flows in this range allow a plateau in the NO signal within 20–30 seconds in most adults. The flow used for NO analysis, along with the transnasal flow, should be recorded (Figure 10-6).
Significance and Pathophysiology
Nasal NO levels are typically much higher than eNO. Values from 100 ppb up to more than 1000 ppb have been reported. Most of the nasal NO appears to be produced by the epithelial cells in the paranasal sinuses. Patients with allergic rhinitis show elevated levels of NO that appear to be responsive to nasal corticosteroids. One disease in which nasal NO measurements may be particularly useful is primary ciliary dyskinesia (PCD). Patients who have PCD have much lower levels of nasal NO (Interpretive Strategies Box 10-1).
Forced oscillation technique
Impedance of the respiratory system (Zrs) represents the net force that must be overcome to move gas in and out of the respiratory system (upper airway, lungs, and chest wall). Applying oscillatory flow is appropriate because that is how we breathe, in a regular in-and-out fashion. Imagine the lungs modeled as a stiff pipe (the airways) with a balloon on the end (the alveoli). The pressure required to push gas down the pipe and into the balloon must overcome three basic forces: the resistance (R) of the pipe, the elastance (E), or stiffness, of the balloon, and the inertia (I) of the gas itself (Figure 10-7). Stated mathematically, the pressure necessary to move gas down the pipe and into the balloon is as follows:
![image](/wp-content/uploads/2015/06/B9780323085052000101_f10-07-9780323085052.jpg)
Measuring the pressure, flow, and volume generated at the mouth during breathing, one can solve for the parameters resistance (R), elastance (E), and inertia (I) that define the mechanical characteristics of the model.
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To measure Z, one could apply flow at various single frequencies and measure the resulting pressures generated at each frequency. Alternatively, one could apply a flow signal consisting of many different frequencies at once and then use a mathematical function known as the fast Fourier transform (FFT) to break down the output into unique sine waves, each of its own frequency. Despite involving complex mathematics with real and imaginary numbers, computers have made this method quick and accurate, and it has become the technique most commonly used (Figure 10-8, A).
![image](/wp-content/uploads/2015/06/B9780323085052000101_f10-08-9780323085052.jpg)
A, A typical apparatus for measuring input Zrs involves a loudspeaker generating a flow signal consisting of many, simultaneous frequencies. These pulsations are delivered to the mouth while mouth pressure (P) and flow (VY) are measured. These measurements are then processed using the fast Fourier transform (FFT). B, The real (R) and imaginary (X) parts of Zrs are shown in relation to frequency. R is frequency dependent, falling to lower values with higher frequency.
Traditional devices use loudspeakers pulsating at different, predetermined frequencies to generate the broadband flow signals (Figure 10-9, A). One commercially available device (Figure 10-9, B) generates the broadband flow signal by an electronically controlled deflection of an internal speaker to create an impulse of flow containing many frequencies, although the frequencies analyzed range from 5–35 Hz. For all devices, the flow signal can be applied during quiet spontaneous breathing and thus requires little subject effort or cooperation. Once performed, the output is recorded in two parts: the part of Z that is in phase with the flow signal, which represents the real part of the Z and is due to flow resistive properties of the respiratory system, R, and the part that is out of phase with the flow signal, called the reactance (X), which represents the imaginary part of Z, and encompasses E and I. These real and imaginary parts are plotted against frequency, as shown in Figure 10-8, B.
![image](/wp-content/uploads/2015/06/B9780323085052000101_f10-09a-9780323085052.jpg)
![image](/wp-content/uploads/2015/06/B9780323085052000101_f10-09b-9780323085052.jpg)
Mayo Clinic pulmonary physiology lab in the 1960s testing FOT using a loudspeaker and recorder. B, A built-in loud speaker generates an impulse of flow containing a wide range of frequencies (5–35 Hz). (Courtesy Carefusion, Cardiopulmonary Diagnostics, Yorba Linda, Calif.)
In most clinical applications, the FOT is used to measure R. Because the technique is noninvasive, fast, and easy, it can be applied in many situations where other methods of measuring R are difficult or cumbersome. These situations include use in pediatric patients or others who cannot perform the panting maneuvers to measure Raw by the body box technique. However, as with Raw measured in the body box, the maneuver does require some degree of subject cooperation, including cheek holding to reduce the effect of facial noise in the signal (Figure 10-10). The R derived from the FOT has been shown to be sensitive to bronchoconstriction and bronchodilatation and thus can be used to assess airway hyperresponsiveness. An important disadvantage of the technique is that, unless an esophageal balloon is placed to measure transpulmonary pressure, the R measured is not of the lungs alone but of the entire respiratory system, thus including the upper airway and chest wall. The upper airway, in particular, can markedly influence the measurement because of its high compliance and the propensity for glottic interference.
Preoperative pulmonary function testing
1. To estimate postoperative lung function in candidates for pneumonectomy or lobectomy
2. To plan perioperative care (preoperative preparation, type and duration of anesthetic during surgery, postoperative care) to minimize complications
3. To enhance the estimate of risk involved in the surgical procedure (i.e., morbidity and mortality) derived from history and physical examination
Preoperative pulmonary function testing may be indicated in patients who have the following:
2. Symptoms of pulmonary disease (e.g., cough, sputum production, shortness of breath)
3. Abnormal physical examination findings, particularly of the chest (e.g., abnormal breath sounds, ventilatory pattern, respiratory rate)