Relationship of thoracic volume and airway occlusion pressure: muscular effects


by contraction of inspiratory muscles was measured in sponta-neously breathing, anesthetized, vagotomized cats at thoracic volumes below, at, and above true FRC (range,-8 to +28 ml/ kg). A relatively constant neural drive at all volumes was shown by recordings of integrated phrenic nerve and external intercostal muscle activities. Peak occlusion pressure declined progressively with increasing thoracic volume and increased with occlusion volumes below FRC. Occlusion pressures mea-sured early in inspiration (0.3 and 0.5 s) decreased similarly. Since occluded airway pressure was a satisfactory index of muscle force output of all inspiratory muscles, the study dem-onstrates that inspiratory muscle performance decreases with increasing thoracic volume throughout the range studied. Analysis of all occluded breaths shows that, quantitatively, the decline in muscle performance from that occurring at tru

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