Voluntary factors influencing the distribution of inspired gas.

Abstract

This study was designed to assess the influence of voluntary patterns of respiratory muscle contractions on the distribution of inspired gas at low flows (0.4 liter per sec). Three types of inspiration were studied in 9 normal subjects in upright, supine, and lateral decubitus posture: (1) natural, (2) intercostal, during which rib cage expansion was accentuated by preferential use of intercostal and accessory muscles, and (3)abdominal, during which abdominal motion was enhanced. The types of inspiration were assessed by monitoring transdiaphragmatic pressure and the anteroposterior diameter of the rib cage and abdomen. Distribution of inhaled gas was measured directly in 4 seated subjects using xenon-133 as tracer gas, and indirectly, by inference from helium bolus washouts in 5 subjects in upright, supine, and lateral postures. In all postures the tracer gas inhaled at functional residual capacity could be distributed preferentially to dependent lung regions by an abdominal inspiration, whereas an intercostal inspiration resulted in a more even distribution, or one preferentially to the nondependent zones. When the tracer gas was inhaled at residual volume, differences in distribution between abdominal and intercostal inspirations were detected only in the lateral posture. Thus, in normal subjects distribution of inspired gas may be altered by voluntary use of different muscle groups. This implies unequal regional pleural pressure swings that differ in the various breathing maneuvers.