Breathing In: The Determinants of Lung Volume

Abstract

The measurement of lung volume is as old as the spirometer. Measurements of lung volume and its subdivisions yield critical diagnostic information about the principle cause of disease, either restrictive with decreased volume or obstructive with increased volume. There are numerous simple and complex techniques for measuring lung volume including simple spirometry, body plethysmography, and the CT scan.

The interpretation of lung volume measurements requires knowledge of the interaction of the respiratory muscles on the chest wall and the static elastic forces at work. A step-by-step series of assessments and considerations are presented aimed at extracting the maximal amount of useful information from the measurement of the lung volume subdivisions (i.e., volumes and capacities). The volumes and capacities that are most useful for detailed assessment of the effect of disease on the lung are; residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC). These three measure the boundaries of the operational change in volume from the normal functional volume to the extremes of vital capacity. They also yield the maximum amount of information. Four cases illustrate the utility of measuring lung volumes and the approach to interpretation.

Appendix

Boyle’s law states that the volume of gas in a container varies inversely with the pressure within a container, assuming constant temperature. Thus, under initial conditions of pressure (P1) and volume (V1), the product equals a constant such that under new conditions P2 and V2, the following equation applies:

$$ \mathrm{P}1\mathrm{V}1=\mathrm{P}2\mathrm{V}2 $$

So if P2 is a situation where a step change in P occurs (∆P), then V2 is the new volume which is smaller, i.e., compression. During a panting maneuver against an obstruction airway (closed container), no air moves in or out, and therefore mouth pressure (Pmouth) approximates alveolar pressure (Palv); the pressure P2 and volume V2 in the lung will vary slightly by ∆P and ∆V with respect to the initial pressure P1 and volume V1 in the lung; hence,

$$ \mathrm{P}1\mathrm{V}1=\left(\mathrm{P}1+\varDelta P\right)\times \left(\mathrm{V}1+\varDelta V\right) $$

If the obstruction is generated by a shutter closure right at end expiration or FRC (Vtg), then V1 becomes FRC and P1 is atmospheric pressure just before the shutter is closed. We therefore can solve for V1. First, the terms of the equation are rearranged:

$$ \mathrm{P}1\varDelta V+\mathrm{V}1\varDelta P+\varDelta V\varDelta P=0 $$

Then, V1 is defined:

$$ \mathrm{V}1\left(\mathrm{FRC}\right)=-\frac{\varDelta V}{\varDelta P}\times \left(\mathrm{P}1+\varDelta P\right) $$

Now we make the assumption that ∆P is very small compared to P1 + ∆P, so that P1 + ∆P is approximated by P1 alone:

$$ \mathrm{V}1=-\mathrm{P}1\times \frac{\varDelta V}{\varDelta P} $$

To solve for V1, we need to know P1 (which is atmospheric pressure-Patm) and ∆V/∆P. The latter is simply the inverse slope of the pressure tracing made during the closed-shutter panting maneuver that plots Pmouth vs. Pbox, because changes in Pmouth approximate changes in Palv (∆P) and changes in Pbox are calibrated to measure the small volume changes in the lung (∆V). Plugging in the inverse slope and the atmospheric pressure (P1) into the equation (and ignoring the sign) yields V1, which is FRC:

$$ \mathrm{Vtg}\ \left(\mathrm{FRC}\right)=\mathrm{Patm}\times \frac{\varDelta V}{\varDelta P} $$

Strictly speaking, V1 is actually thoracic gas volume (V_tg), since it includes all compressible gas at that moment, and the shutter may or may not have been closed precisely at FRC. Therefore, in practice one adds or subtracts the volume distance away from true FRC as determined by the position of the stable, end-expiratory lung volume recorded during the previous tidal breathing preceding the panting maneuver. This correction is sometimes called the “switch-in volume,” because it was the volume error created by switching to closed-shutter panting should that occur not precisely at FRC. When measured in a body plethysmograph, this corrected FRC is commonly reported as V_tg or TGV.