REASSESSMENT OF BODY PLETHYSMOGRAPHIC TECHNIQUE FOR THE MEASUREMENT OF THORACIC GAS VOLUME IN ASTHMATICS

Abstract

Thoracic gas volume (Vtg) was measured by the body plethysmographic technique in 4 normal [human] subjects and in 9 asthmatics during remission and after bronchoconstriction induced with inhaled aerosolized histamine. Changes in both mouth (Pm) and esophageal pressure (Pes) were used to approximate changes in alveolar pressure (Palv), and both Vtg(m) and Vtg(es) were computed. Because small changes in lung recoil occur during panting, .DELTA.Pes exceeds .DELTA.Pm, leading to a value of Vtg(m) that is slightly greater than Vtg(es). In the normal subjects, Vtg(m) was significantly greater than Vtg(es), and the observed percentage error (E) between them (2.8%) corresponded with the expected value. In the asthmatics, during marked bronchoconstriction, there was an increased tendency for Vtg(m) to exceed Vtg(es), E averaging 12.2% and being greater than the predicted error. In some asthmatics, changes in volume led changes in Pm; the phase angle between volume and Pm averaged 6.3.degree. and was significantly greater than that between volume and Pes (1.7.degree.). In 3 asthmatics, increases in TLC(m) [total lung capacity] were not accompanied by similar changes in TLC(es). When a compliant upper airway is coupled with a high airway resistance during panting against an occluded airway, transmission of changes in alveolar pressure to the mouth is incomplete. The resulting underestimation of .DELTA.Palv by .DELTA.Pm leads to artifactual increases in TLC.