THE SIMPLIFIED VERSION OF BOYLES LAW LEADS TO ERRORS IN THE MEASUREMENT OF THORACIC GAS VOLUME

When using Boyle's Law for thoracic gas volume (V-tg) measurement, it is generally assumed that the alveolar pressure (P-alv) does not differ from barometric pressure (P-bar) at the start of rarefaction and compression and that the product of the change in volume and pressure (Delta P x Delta V) is negligibly small. In a gentle panting maneuver in which the difference between P-alv and P-bar is small, errors introduced by these assumptions are likely to be small; however, this is not the case when V-tg is measured using a single vigorous inspiratory effort. Discrepancies in the V-tg between the ''complete'' version of Boyle's Law, which does not ignore Delta P x Delta V and accounts for large swings in P-alv, and the ''simplified'' version, during both a panting maneuver and a single inspiratory effort were calculated for normal control subjects and patients with cystic fibrosis or asthma. Defining the V-tg from the complete version as ''correct,'' the errors introduced by the simplified version ranged from -3 to +3% for the panting maneuver whereas they ranged from 2 to 9% for the inspiratory maneuver. Using the simplified equation, the V-tg for the inspiratory maneuver was 0.135 +/- 0.237 L greater (p < 0.02) than for the panting maneuver. This discrepancy disappeared when the complete equation was used. While the errors introduced by the use of the simplified version of Boyle's Law are small, they are systematic and unnecessary.