The occluded airway pressure generated by contraction of inspiratory muscles was measured in spontaneously 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 measured 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 demonstrates 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 true FRC was -1.9% per ml/kg volume change from FRC. Zero effective muscle pressure generation occurred at 52 ml/kg above FRC, or 2.8 times FRC. These findings have important implications for the use of occlusion pressures and ventilatory responses in respiratory control system studies, since changes in FRC may affect muscle performance.
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