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 diffe r from barometric pressure (P-bar) at the start of rarefaction and com pression and that the product of the change in volume and pressure (De lta 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 introduc ed by these assumptions are likely to be small; however, this is not t he case when V-tg is measured using a single vigorous inspiratory effo rt. Discrepancies in the V-tg between the ''complete'' version of Boyl e's Law, which does not ignore Delta P x Delta V and accounts for larg e swings in P-alv, and the ''simplified'' version, during both a panti ng maneuver and a single inspiratory effort were calculated for normal control subjects and patients with cystic fibrosis or asthma. Definin g the V-tg from the complete version as ''correct,'' the errors introd uced by the simplified version ranged from -3 to +3% for the panting m aneuver 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. Wh ile the errors introduced by the use of the simplified version of Boyl e's Law are small, they are systematic and unnecessary.