An alternate method for the determination of functional residual capacity in a plethysmograph.

The validity of measuring thoracic gas volume using a single inspiratory effort against an occlusion (Vtginsp) was determined in children and young adults (8 normal control subjects and 17 patients with cystic fibrosis [CF] or asthma). In addition, the validity of the measurement of Vtg at FRC in children at a low panting frequency (Vtgpant) was also determined. During both the panting (encouraged to be about 1 Hz) and inspiratory maneuvers, mouth pressure (Pm) and esophageal pressure (Pes) were measured simultaneously. Hence, Vtgpant and Vtginsp were determined using both delta Pm and delta Pes. Vtginsp using delta Pm was found to be similar to Vtginsp using delta Pes. Values for Vtgpant using either delta Pm or delta Pes were also found to be similar. The percent difference between Vtginsp (using delta Pm) and the average of Vtgpant and Vtginsp (using delta Pes for both) was calculated as an indication of the error of the inspiratory method. The error ranged from -13 to +13% and did not correlate with indices of air-flow limitation or hyperinflation. We have found that Vtginsp can be used to determine FRC in normal children and young adults as well as in those with CF or asthma. In addition, we have also validated the use of Vtgpant at a low panting frequency in these subjects.