Statics: Snapshots of the Ventilatory Pump

Measurement of Lung Volume

With the spirometer shown in Animated Figure 3-7, we can calculate changes in volume from FRC to TLC (i.e., the inspiratory capacity) and from TLC to RV (i.e., the vital capacity). This allows us to quantify the vital capacity, inspiratory capacity, and expiratory reserve volume (Table 3-1). To know the true values for FRC, TLC, and RV, however, we need to know the absolute volume for at least one of them. With that information in hand, we can then derive the other two by looking at changes in volume with a spirometer. For example, if you know the value for TLC, you could ask the person to exhale from TLC until she could not exhale any more (perform a vital capacity maneuver). The TLC minus the VC gives you the RV.

Given the choice between measuring TLC, FRC, or RV directly, which would you prefer to measure? Which would likely give the most reproducible results?

Functional residual capacity is typically measured directly. Its measurement is more consistent than the measurement of TLC and RV because it is not dependent on the use of ventilatory muscles. FRC is the volume that is achieved at the end of a relaxed exhalation when the force exerted by the elastic recoil of the lung is equal to and opposite the force exerted by the chest wall to spring outward (recall Animated Figs. 3-2 and 3-7). To reach TLC requires the patient to make a maximal inspiration; to reach RV requires the patient to make a maximal exhalation. In either case, you must depend on the cooperation of the person doing the test to achieve accurate measurements. If the individual were to give less than a maximal effort, all other volumes derived with the spirometer would be in error.

To measure the volume of gas in the lung at FRC, one needs a marker gas that is inert and relatively insoluble in blood. This permits the gas to stay in the alveoli because it will not be absorbed into the blood or metabolized as it is inhaled by the patient. Thus, the gas is diluted as it moves into the lung and mixes with the air already in the lung. The technique commonly used to measure FRC is termed helium dilution (Fig. 3-8).

The person breathes in a relaxed manner on a mouthpiece connected to a valve that leads to a container with a known volume and a known concentration of helium. After the individual becomes comfortable on the apparatus and the technician notes the person is at FRC, the valve is turned so that the individual is now breathing from and into the container. The concentration of helium in the container is monitored as it gradually decreases from its initial concentration. After 2 to 3 minutes in an individual with normal lungs, the concentration of helium levels off as it comes into equilibrium with the gas originally in the lungs. You know the original volume (V₁) and concentration of helium (C₁), and you now know the final concentration of helium (C₂). With this information, you can calculate the new volume (container + lungs, or V₂) into which the helium is distributed:

Subtraction of the volume of the box (V₁) from the new volume of distribution (V₂) gives you FRC:

There are two alternative methods for measuring FRC. The first is termed the nitrogen washout test. This test makes use of the fact that approximately 79% of the gas we breathe from the atmosphere is nitrogen (the remaining 21% is primarily oxygen). Because there is no net movement of nitrogen from the lungs into the blood, the amount of nitrogen exhaled is the same as the amount inhaled. To do this test, ask a person to breathe 100% Oxygen, starting at FRC (V₁ in this example). The nitrogen in the lungs will be gradually replaced by oxygen as the person exhales nitrogen and replaces it with oxygen. Measure the volume of exhaled gas for a fixed period of time (V₂) (e.g., 5 minutes) after oxygen breathing is started, along with the concentration of exhaled nitrogen in that gas (C₂). At the end of the 5-minute collection of gas, measure the nitrogen concentration of the very last gas exhaled (C₃). This represents the concentration of the nitrogen still remaining in the alveoli. The original volume of the lung at the time that oxygen breathing was started can be calculated as follows:

The nitrogen washout method has the same limitation as does the helium dilution technique. You are measuring only the lung units that have good communication with the central airways.

To deal with this issue of lung units that do not communicate well with the central airways, a third method called body plethysmography can be used (Fig. 3-9). The body plethysmograph is a device that makes use of Boyle's law to measure the volume of gas within the thorax. In this test, a person is seated within an airtight box and breathes on a mouthpiece connected to a tube that links the individual to equipment outside the box. The volume of the box is known, and the pressure within the box is measured. When the person is at FRC, a valve is closed in the tube on which the person is breathing and he is instructed to make quick inspiratory and expiratory efforts against the closed airway. During this time, the pressure changes in the person's airways (and hence, the lungs because there is no flow) are measured as he compresses and decompresses the gas in the thorax. Simultaneously, the changes in the pressure of the box, which occur as the person's chest expands and contracts during the breathing efforts, is also measured. Knowing the volume of the box, the changes in pressure in the box and in the individual's airway, the volume of gas in the thorax can be calculated.