The spirometer measures changes in lung volume
The maximal volume of all the airways in an adult—the nasopharynx, the trachea, and all airways down to the alveolar sacs—is typically 5 to 6 L. Respiratory physiologists have defined a series of lung “volumes” and “capacities” that, although not corresponding to a particular anatomical locus, are easy to measure with simple laboratory instruments and that convey useful information for clinical assessment.
A spirometer measures the volume of air inspired and expired and therefore the change in lung volume. Spirometers today are complex computers, many so small that they can easily be held in the palm of one hand. The subject blows against a predetermined resistance, and the device performs all the calculations and interpretations. Nevertheless, the principles of spirometric analysis are very much the same as those for the “old-fashioned” spirometer shown in Figure 26-8A, which is far easier to conceptualize. This simple spirometer has a movable inverted bell that is partially submerged in water. An air tube extends from the subject's mouth, through the water, and emerges in the bell, just above water level. Thus, when the subject exhales, air enters the bell and lifts it. The change in bell elevation, which we can record on moving paper, is proportional to the volume of air that the subject exhales. Because air in the lungs is saturated with water vapor at 37°C (body temperature and pressure, saturated with water vapor, or BTPS), and the air in the spirometer is ambient temperature and pressure, saturated with water vapor, or ATPS, we must apply a temperature correction to the spirometer reading (see Box 26-3).
FIGURE 26-8 Workings of a simple spirometer.
The amount of air entering and leaving the lungs with each breath is the tidal volume (VT or TV). During quiet respirations, the VT is ~500 mL. The initial portion of the spirograph of Figure 26-8B illustrates changes in lung volume during quiet breathing. The product of VT and the frequency of breaths is total ventilation, given in liters (BTPS) per minute.
Because, with a typical Western diet, metabolism consumes more O2 (~250 mL/min) than it produces CO2 (~200 mL/min; see pp. 1187–1188), the volume of air entering the body with each breath is slightly greater (~1%) than the volume leaving. In reporting changes in lung volume, respiratory physiologists have chosen to measure the volume leaving—the expired lung volume (VE).
At the end of a quiet inspiration, the additional volume of air that the subject could inhale with a maximal effort is known as the inspiratory reserve volume (IRV). The magnitude of IRV depends on several factors, including the following:
1. Current lung volume. The greater the lung volume after inspiration, the smaller the IRV.
2. Lung compliance. A decrease in compliance, a measure of how easy it is to inflate the lungs, will cause IRV to fall as well.
3. Muscle strength. If the respiratory muscles are weak, or if their innervation is compromised, IRV will decrease.
4. Comfort. Pain associated with injury or disease limits the desire or ability to make a maximal inspiratory effort.
5. Flexibility of the skeleton. Joint stiffness, caused by diseases such as arthritis and kyphoscoliosis (i.e., curvature of the spine), reduces the maximal volume to which one can inflate the lungs.
6. Posture. IRV falls when a subject is in a recumbent position, because it is more difficult for the diaphragm to move the abdominal contents.
After a quiet expiration, the additional volume of air that one can expire with a maximal effort is the expiratory reserve volume (ERV). The magnitude of the ERV depends on the same factors listed before and on the strength of abdominal and other muscles needed to produce a maximal expiratory effort.
Even after a maximal expiratory effort, a considerable amount of air remains inside the lungs—the residual volume (RV). Because a spirometer can measure only the air entering or leaving the lungs, it obviously is of no use in ascertaining the RV. However, we will see that other methods are available to measure RV. Is it a design flaw for the lungs to contain air that they cannot exhale? Would it not be better for the lungs to exhale all their air and to collapse completely during a maximal expiration? Total collapse would be detrimental for at least two reasons: (1) After an airway collapses, an unusually high pressure is required to reinflate it. By minimizing airway collapse, the presence of an RV optimizes energy expenditure. (2) Blood flow to the lungs and other parts of the body is continuous, even though ventilation is episodic. Thus, even after a maximal expiratory effort, the RV allows a continuous exchange of gases between mixed-venous blood and alveolar air. If the RV were extremely low, the composition of blood leaving the lungs would oscillate widely between a high at the peak of inspiration and a low at the nadir of expiration.
The four primary volumes that we have defined—TV, IRV, ERV, and RV—do not overlap (see Fig. 26-8B). The lung capacities are various combinations of these four primary volumes:
1. Total lung capacity (TLC) is the sum of all four volumes.
2. Functional residual capacity (FRC) is the sum of ERV and RV and is the amount of air remaining inside the respiratory system after a quiet expiration. Because FRC includes RV, we cannot measure it using only a spirometer.
3. Inspiratory capacity (IC) is the sum of IRV and TV. After a quiet expiration, the IC is the maximal amount of air that one could still inspire.
4. Vital capacity (VC) is the sum of IRV, TV, and ERV. In other words, VC is the maximal achievable TV and depends on the same factors discussed above for IRV and ERV. In patients with pulmonary disease, the physician may periodically monitor VC to follow the progress of the disease.
At the end of the spirographic record in Figure 26-8B, the subject made a maximal inspiratory effort and then exhaled as rapidly and completely as possible. The volume of air exhaled in 1 second under these conditions is the forced expiratory volume in 1 second (FEV1). In healthy young adults, FEV1 is ~80% of VC. FEV1 depends on all the factors that affect VC as well as on airway resistance. Thus, FEV1 is a valuable measurement for monitoring a variety of pulmonary disorders and the effectiveness of treatment.
The volume of distribution of helium or nitrogen in the lung is an estimate of the RV
Although we cannot use a spirometer to measure RV or any capacity containing RV (i.e., FRC or TLC), we can use two general approaches to measure RV, both based on the law of conservation of mass. In the first approach, we compute RV from the volume of distribution of either helium (He) or nitrogen (N2). However, any nontoxic gas would do, as long as it does not rapidly cross the blood-gas barrier. In the second approach, discussed in the next section, we compute RV by the use of Boyle's law.
The principle underlying the volume of distribution approach is that the concentration of a substance is the ratio of mass (in moles) to volume (in liters). If the mass is constant, and if we can measure the mass and concentration, then we can calculate the volume of the physiological compartment in which the mass is distributed. In our case, we ask the subject to breathe a gas that cannot escape from the airways. From the experimentally determined mass and concentration of that gas, we calculate lung volume.
We begin with a spirometer containing air with 10% He—this is the initial He concentration, [He]initial = 10% (Fig. 26-9A). We use He because it is poorly soluble in water and therefore diffuses slowly across the alveolar wall (see pp. 661–663). In this example, the initial spirometer volume, VS,initial, including all air up to the valve at the subject's mouth, is 2 L. The amount of He in the spirometer system at the outset of our experiment is thus [He]initial × VS,initial, or (10%) × (2 L) = 0.2 L.
FIGURE 26-9 Volume-of-distribution and plethysmographic methods for measurement of lung volume. In C, the spirometer is usually replaced in modern plethysmographs by an electronic pressure gauge. In such instruments, the change in lung volume is computed from the change in pressure inside the plethysmograph (see Fig. 27-11).
We now open the valve at the mouth and allow the subject to breathe spirometer air until the He distributes evenly throughout the spirometer and airways. After equilibration, the final He concentration ([He]final) is the same in the airways as it is in the spirometer. The volume of the “system”—the spirometer volume (VS) plus lung volume (VL)—is fixed from the instant that we open the valve between the spirometer and the mouth. When the subject inhales, VL increases and VS decreases by an equal amount. When the subject exhales, VL decreases and VS increases, but (VL + VS) remains unchanged. Because the system does not lose He, the total He content after equilibration must be the same as it was at the outset. In our example, we assume that [He]final is 5%.
If the spirometer and lung volumes at the end of the experiment are the same as those at the beginning, N26-11
Contributed by Emile Boulpaep, Walter Boron
In the text, we assumed that the spirometer volume (VS) at the time we opened the stopcock in Figure 26-9A was the same as the spirometer volume at the end of the experiment. We also implicitly assumed that the same was true of the lung volume (VL). However, this need not be true. At the instant we open the stopcock, the total volume of the system—that is, the sum of VS and VL—is in principle a constant. As the subject inhales, the VL increases but VS decreases, and vice versa. If the spirometer volume at the end of the experiment (VS,final) were different from the spirometer volume at the beginning (VS,initial), then Equation 26-3 in the text would be replaced by the following:
Here, [He]initial is the initial concentration of helium in the spirometer and [He]final is the final concentration of helium in the spirometer and in the lung's airways. Thus, solving for final lung volume (VL,final), we get
If we use the above equation, it does not matter whether the initial and final spirometer volumes are the same. The computed VL,final is the lung volume at the instant we take a reading of VS,final. For example, even if the lungs were at FRC at the instant that the stopcock was opened (i.e., at the beginning of the experiment), if the lungs were at RV at the time we measure VL,final, (i.e., at the end of the experiment) then Equation NE 26-9 will yield RV. In fact, VL,final can be anywhere between the RV and TLC.
If, at the end of the experiment, the subject adjusts his or her lung volume to be the same as at the beginning, then the initial and final spirometer volumes will also be the same (i.e., VS,initial = VS,final). In this case, Equation NE 26-9 simplifies to Equation 26-4 (shown here as Equation NE 26-10):
Solving for lung volume, we get
If we now insert the values from our experiment,
VL corresponds to the lung volume at the instant we open the valve and allow He to begin equilibrating. If we wish to measure FRC, we open the valve just after the completion of a quiet expiration. If we open the valve after a maximal expiration, then the computed VL is RV. Because the subject rebreathes the air mixture in the spirometer until [He] stabilizes, the He-dilution approach is a closed-circuit method.
Imagine that you have a paper cup that contains a red soft drink. You plan to “empty” the cup but wish to know the “residual volume” of soft drink that will remain stuck to the inside of the cup (Vcup) after it is emptied. First, before emptying the cup, you determine the concentration of red dye in the soft drink; this is [red dye]cup. Now empty the cup. Although you do not yet know Vcup, the product [red dye]cup × Vcup is the mass of residual red dye that remains in the cup. Next, add hot water to the cup, swish it around inside the glass, and dump the now reddish water into a graduated cylinder. After repeating this exercise several times, you see that virtually all the red dye is now in the graduated cylinder. Finally, determine the volume of fluid in the cylinder (Vcylinder) and measure its red dye concentration. Because [red dye]cup × Vcup is the same as [red dye]cylinder × Vcylinder, you can easily calculate the residual volume of the soft drink that had been in the glass. This is the principle behind the N2-washout method.
Assume that the initial lung volume is VL and that the initial concentration of nitrogen gas in the lungs is [N2]initial. Thus, the mass of N2 in the lungs at the outset is [N2]initial × VL. We now ask the subject to breathe through a mouthpiece equipped with a special valve (see Fig. 26-9B). During inspiration, the air comes from a reservoir of 100% O2; the key point is that this inspired air contains no N2. During expiration, the exhaled air goes to a sack with an initial volume of zero. Each inspiration of 100% O2 dilutes the N2 in the lungs. Each expiration sends a fraction of the remaining N2 into the sack. Thus, the [N2] in the lungs falls stepwise with each breath until eventually the subject has washed out into the sack virtually all the N2 that had initially been in the lungs. Also entering the sack are some of the inspired O2 and all expired CO2. The standard period for washing out of the N2 with normal breathing is 7 minutes, after which the sack has a volume of Vsack and an N2 concentration of [N2]sack. Because the mass of N2 now in the sack is the same as that previously in the lungs,
Thus, the lung volume is
To illustrate, consider an instance in which [N2]initial is 75%. If the volume of gas washed into the sack is 40 L, there is roughly a total ventilation of 6 L/min for 7 minutes, and [N2]sack is 3.75%, then
What particular lung volume or capacity does VL represent? The computed VL is the lung volume at the instant the subject begins to inhale the 100% O2. Therefore, if the subject had just finished a quiet expiration before beginning to inhale the O2, VL would be FRC; if the subject had just finished a maximal expiratory effort, VL would represent RV.
The key element in the N2-washout method is the requirement that during the period of O2 breathing, all N2 previously in the lungs—and no more—end up in the sack. In other words, we assume that N2 does not significantly diffuse between blood and alveolar air during our 7-minute experiment. Because N2 has a low water solubility, and therefore the amount dissolved in body fluids is very low at normal barometric pressure, the rate of N2 diffusion across the alveolar wall is very low (see pp. 661–663). Thus, our assumption is very nearly correct. In principle, we could preload the lung with any nontoxic, water-insoluble gas (e.g., He), and then wash it out with any different gas (e.g., room air). Because the subject inhales from one reservoir and exhales into another in the N2-washout method, it is an open-system technique.
The plethysmograph, together with Boyle's law, is a tool for estimation of RV
We also can compute VL from small changes in lung pressure and volume that occur as a subject attempts to inspire through an obstructed airway. This approach is based on Boyle's law, which states that if the temperature and number of gas molecules are constant, the product of pressure and volume is a constant:
To take advantage of this relationship, we have the subject step inside an airtight box called a plethysmograph (from the Greek plethein [to be full]), which is similar to a telephone booth. The subject breathes through a tube that is connected to the outside world (see Fig. 26-9C). Attached to this tube is a gauge that registers pressure at the mouth and an electronically controlled shutter that can, on command, completely obstruct the tube. The Mead-type plethysmograph shown in Figure 26-9C has an attached spirometer. As the subject inhales, lung volume and the volume of the subject's body increase by the same amount, displacing an equal volume of air from the plethysmograph into the spirometer, which registers the increase in lung volume (ΔVL).
As the experiment starts, the shutter is open, and the subject quietly exhales, so that VL is FRC. Because no air is flowing at the end of the expiration, the mouth and alveoli are both at barometric pressure, which is registered by the pressure gauge (P). We now close the shutter, and the subject makes a small inspiratory effort, typically only ~50 mL, against the closed inlet tube. The subject's inspiratory effort will cause lung volume to increase to a new value, VL + ΔVL (see graph in Fig. 26-9C). However, the number of gas molecules in the airways is unchanged, so this increase in volume must be accompanied by a decrease in airway pressure (ΔP) to a new value, P − ΔP. Because no air is flowing at the peak of this inspiratory effort, the pressure measured by the gauge at the mouth is once again the same as the alveolar pressure. According to Boyle's law,
Rearranging Equation 26-10 yields the initial lung volume:
As an example, assume that ΔVL at the peak of the inspiratory effort is 50 mL and that the corresponding pressure decrease in the airways is 12 mm Hg. If the initial pressure (P) was 760 mm Hg,
What lung volume or capacity does 3.1 L represent? Because, in our example, the inspiratory effort against the closed shutter began after a quiet expiratory effort, the computed VL is FRC. If it had begun after a maximal expiration, the measured VL would be RV.