To examine the effect of FRC changes, we simultaneously measured tidal volume by the inductive plethys-mograph (as the sum of the abdominal and chest wall signals) and also by a mouthpiece, nose clip, and a water-sealed spirometer in nine normal subjects, breathing both at FRC and at FRC plus 1 L. Analysis of at least 30 breaths at each of these lung volumes showed that increasing the end-tidal lung volume (FRC plus 1 L) did not make a significant difference to the ratio between the summed Respitrace signal and the tidal volume as measured by the water-sealed spirometer. Thus, at FRC plus 1 L this ratio (Respitrace/ spirometer volume) was 0.89 ±0.09 compared with a Respitrace/ spirometer volume ratio of 1.00 at FRC (p>0.1). Link
To investigate the accuracy of the Respitrace after the induction of bronchoconstriction, we compared the sum of the Respitrace signals to the tidal volume as measured by the water-sealed spirometer before and after histamine challenge in four of our patients on a separate occasion. The ratio of the sum of the Respitrace signals (in arbitrary units) to the tidal volume measured by the spirometer fell as bronchoconstriction developed, from 1.0 arbitrary unit/L before histamine challenge to a mean of 0.88 arbitrary unit/ L (range, 0.81 to 0.95) after the maximal bronchoconstriction. We made the same comparisons when the Respitrace was calibrated by a multiple linear regression technique. Again, bronchoconstriction reduced the ratio of the sum of the Respitrace signal/spirometer volume from 1.0 arbitrary unit/L to a mean of 0.92 (range, 0.87 to 1.03 arbitrary unit/L). We conclude that with both calibration techniques, the Respitrace may therefore underestimate tidal volume by some 10 percent following histamine-induced bronchoconstriction.