Pulmonary Function Measurements

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Pulmonary Function Measurements

Pulmonary function refers to the role of the lungs in gas exchange. Pulmonary function testing is a practical application of respiratory physiology and is necessary for understanding abnormal lung function and the effects of treatment. Tests of healthy humans have identified factors that are associated with normal variations in lung function, such as age, gender, height, and body size. In clinical medicine, pulmonary function testing has helped define the natural progression of pulmonary diseases and produced a useful classification of basic functional defects. Pulmonary function tests help determine the severity of functional impairments and the extent to which treatment restores normal function. Although pulmonary function tests are diagnostic in nature, they are rarely the key factor in a definitive diagnosis. These tests reflect the combined function of the airways, alveolar-capillary membrane, respiratory muscles, and neural control mechanisms. The same pattern of abnormal pulmonary function often overlaps several diseases. The main purpose of this chapter is to provide a physiological basis for understanding pulmonary function measurements and abnormal test results. Detailed instruction on testing technique and definitive interpretation of results are beyond the scope of this chapter.

Static Lung Volumes

Static lung volumes and their spirometric measurement are discussed in Chapter 3. Normal values and interrelationships among volumes and capacities are illustrated in Figure 5-1. Anthropometric differences (differences in body type and size) affect normal values, creating ranges in which normal function is presumed. Normal ranges are derived from a statistical analysis of large populations of “normal” people (i.e., people who are healthy, have no history of lung disease, and have minimal exposure to risk factors such as smoking and environmental pollution). Physical characteristics that influence pulmonary function most are age, gender, height, ethnic origin, and body size or surface area.1 Normal values are predicted for an individual based on these physical characteristics. Function is generally classified as normal if values are within 20% of the predicted value (i.e., 80% to 120% of the predicted value). Table 5-1 displays the relationship between the percentage of the predicted normal value and the degree of functional impairment.

TABLE 5-1

Severity of Pulmonary Impairments Based on the Percentage of the Predicted Normal Value or Standard Deviations of the Mean Predicted Normal Value

Degree of Impairment Percentage of Predicted SD Above and Below Mean
Normal 80-120 ±1
Mild 65-79 ±1-2
Moderate 50-64 ±2-3
Severe 35-49 > ±3
Extremely severe <35

SD, Standard deviation.

SD, Standard deviation.

Modified from Wilkins RL, Stoller JK, Scanlan CL: Egan’s fundamentals of respiratory care, ed 8, St Louis, 2003, Mosby.

Theoretical Basis for Measurement

Because residual volume (RV) cannot be exhaled, it cannot be measured via direct spirometry. Therefore, no capacity containing RV can be directly measured. RV and capacities containing it are measured indirectly via one of the following methods: helium dilution, nitrogen washout, or body plethysmography. The dilution and washout techniques measure gas in the lung only if the gas communicates with unobstructed airways. These tests are started at the end of a normal expiration (i.e., the functional residual capacity [FRC] level). Washout and dilution tests measure the FRC, which is the logical starting point because it is highly reproducible and its achievement requires no patient effort—only ventilatory muscle relaxation. As shown in Figure 5-1, after FRC is determined, spirometric measurement of the expiratory reserve volume (ERV) allows the RV to be calculated (RV = FRC − ERV). Likewise, spirometric measurement of inspiratory capacity (IC) allows total lung capacity (TLC) to be calculated (FRC + IC = TLC). Alternatively, if RV is measured, TLC can be calculated as follows: RV + vital capacity (VC) = TLC.

Helium Dilution Method

The helium dilution method requires the person to rebreathe a helium gas mixture through a closed circuit from a spirometer of known volume (Figure 5-2). Helium is a foreign gas to the lung and is inert and insoluble in the blood. Therefore, the blood does not absorb helium during the test. The lung contains no helium initially (Figure 5-2, A), but rebreathing causes lung and spirometric gases to mix (Figure 5-2, B). A helium analyzer in the circuit continuously monitors helium concentration. As rebreathing progresses, lung gases dilute the helium in the spirometer. Consequently, helium concentration decreases in the spirometer and increases in the lung until equilibrium is reached (Figure 5-2, C). During the rebreathing process, the spirometric volume is kept constant through the addition of oxygen to the circuit to replace oxygen taken up by the blood. Likewise, a chemical in the circuit absorbs carbon dioxide entering the lung from the blood.

The principle of the closed-circuit helium dilution method is that the amount of helium in the lung spirometric system is the same at the end of the test as at the beginning. For example, after it is charged with helium, the spirometer’s volume may be 3000 mL and the helium concentration 10% (see Figure 5-2, A). In this example, the amount of helium present at the beginning of the test is 10% of 3000, or 300 mL (0.10 × 3000 = 300 mL). Because the lung contains no helium initially, 300 mL of helium must be present in the lung spirometer system at the end of the test after helium equilibrium is achieved. Consider an example in which the helium concentration at equilibrium is 5% (see Figure 5-2, C). The lung volume diluting the spirometer’s helium mixture is the FRC if the first inspiration of the helium mixture started from the resting FRC level. In principle, the FRC is calculated as follows:2

Initial helium volume=final helium volume:(FinHe)(FRC)+(FinHe)(Vs)=(FfinHe)(FRC+Vs) 1.

image 1.

In this equation, FinHe and FfinHe represent the initial and final helium fractional concentrations, and Vs represents the spirometric volume.

With the values from the previous example, the following can be calculated:

0+(0.10)×(3000)=(0.05)×(FRC+3000) 2.

image 2.

300=0.05(FRC)+0.05(3000) 3.

image 3.

300=0.05(FRC)+150 4.

image 4.

150=0.05(FRC) 5.

image 5.

3000mL=FRC 6.

image 6.

This gas volume is measured under ambient temperature, pressure, and saturated (ATPS) conditions. Therefore, this gas volume must be converted to the volume it occupied in the lung under body temperature, pressure, and saturated (BTPS) conditions. (Factors for converting ATPS volumes to BTPS volumes are available in pulmonary function procedure manuals.) Modern pulmonary function machines automatically correct measured volumes to BTPS values. If the person exhales to RV at the end of the test, the RV can be calculated by subtracting ERV from FRC.

Nitrogen Washout Method

The nitrogen washout method requires the person to breathe a nitrogen-free gas (100% oxygen) until the nitrogen in the lung is “washed” out. The lung volume is unknown at the beginning of the test, but the nitrogen concentration in the lung is about 80% if room air is breathed. If the volume of nitrogen in the lung can be measured (in milliliters), the lung volume can be calculated as follows:

Nitrogen(mL)=0.80(lung volume)Nitrogen(mL)/0.80=lung volume

image

In the open-circuit nitrogen washout procedure (Figure 5-3), the person breathes through a one-way valve, inspiring 100% oxygen and expiring into a large gas-collecting spirometer previously flushed with 100% oxygen. Each inspiration is nitrogen-free, and each expiration eliminates some of the nitrogen in the lung. Normally, less than 7 minutes is required to eliminate most of the nitrogen in the lung, but patients with poorly ventilated areas may require longer washout times. The test usually ends when the expired nitrogen is less than 2%. The amount of nitrogen exhaled is calculated by analyzing the spirometer’s nitrogen concentration (FN2) and multiplying it by the Vs (Nitrogen [mL] = FN2 × Vs). If 35 L (35,000 mL) of expired gas is collected during the test and its nitrogen content is 6%, the amount of nitrogen exhaled is calculated as follows: 35,000 × 0.06 = 2100 mL. As the following indicates, 2100 mL is 80% of the lung volume:

2100mL=0.80×FRCFRC=2625mL

image

The measured FRC must be corrected to reflect the volume it occupies under BTPS conditions.

Neither the helium dilution nor the nitrogen washout techniques measure gas trapped behind occluded airways. Total thoracic gas volume (TGV), which includes air that does not communicate with patent airways, can be measured by plethysmography, described in the next section.

Body Plethysmographic Method

The plethysmographic method is based on Boyle’s law. This law states that when gas volume and pressure changes occur, the initial volume (V1) multiplied by the initial pressure (P1) equals the final volume (V2) multiplied by the final pressure (P2): P1V1 = P2V2.

The body plethysmograph is an airtight chamber, or “body box,” in which the patient sits while breathing through a mouthpiece (Figure 5-4). Sensitive pressure transducers measure mouthpiece and box pressures. Box calibration is carried out with an empty, sealed box; a known volume of gas is injected into the box with a large calibration syringe, and the change in box pressure is noted. This calibration procedure allows subsequent box pressure changes to be converted to volume changes.

At the end of expiration when gas flow ceases, mouthpiece pressure equals alveolar pressure. The lung volume at this point (the FRC) is unknown. An electrically controlled shutter occludes the mouthpiece precisely at the end expiratory point. The subsequent inspiratory effort cannot move gas. However, the respiratory muscles enlarge the thorax, decompressing the gas in the lung, decreasing its pressure (see Figure 5-4). In the absence of gas flow, intrapulmonary pressure equals mouthpiece pressure. At the same time, the enlarging thorax compresses air inside the box around the patient, increasing the box pressure. The change in box pressure is converted to the change in volume from the initial calibration procedure. The variables in Boyle’s law are assigned as follows: P1 equals initial mouth pressure (atmospheric pressure), V1 equals FRC (unknown), P2 equals final mouth pressure (measured during inspiratory effort through the occluded mouthpiece), and V2 equals FRC plus the change in lung volume (ΔV). The ΔV is known. In principle, the FRC is calculated as follows:

(P1)(FRC)=(P2)(FRC+ΔV)(P1)(FRC)=(P2)(FRC)+(P2)(ΔV)(P1)(FRC)(P2)(FRC)=(P2)(ΔV)FRC(P1P2)=(P2)(ΔV)FRC=(P2)(ΔV)(P1P2)

image

The plethysmographic method is quite rapid; successive FRC measurements can be made as the patient pants against the occluded mouthpiece. This technique measures both the ventilated and the nonventilated air space in the thorax.

Significance of Changes in Functional Residual Capacity and Residual Volume

Physical factors that change FRC and RV are summarized in Box 5-1. Changes in lung recoil affect FRC because FRC is determined by the balance point between lung and thoracic recoil forces. An abnormally increased FRC represents hyperinflation, which may be caused by a loss of elastic recoil or partial airway obstruction. Partial airway obstruction caused by bronchospasm is generally reversed by bronchodilator drugs, and the associated increase in FRC is reversible. Increased FRC caused by a permanent loss of elastic recoil is irreversible. In severe emphysema, this loss of lung elasticity is associated with passive airway compression and collapse during expiration, causing air trapping. Bronchodilator drugs are not useful in these circumstances. (See the discussion on air trapping and auto-PEEP in Chapter 3).