Reflex control of diaphragm activation by thoracic afferents

Recent studies suggest that chest wall reflexes may have a role in modulating diaphragm activation. The purpose of this study was to more closely examine this issue by assessing the diaphragmatic motor response to airway occlusion. Studies were performed in vagotomized mongrel dogs anesthetized with pentobarbital sodium. Diaphragmatic electromyogram (EMG) and phrenic neurogram (ENG) responses to airway occlusion were evaluated at different precontractile respiratory muscle lengths, achieved by passive inflation and deflation with a volume syringe during the preceding expiration. Lung volume was expressed as the corresponding change in airway pressure. At functional residual capacity, deflation (-5 cmH₂O), and large inflation (+25 cmH₂O), phrenic ENG during occlusion was 90 ± 2 (SE), 84 ± 5, and 86 ± 3% of the preceding control breaths, respectively (n = 9). Qualitatively similar, but somewhat more pronounced, responses were observed on diaphragmatic EMG. With small lung inflations, the degree of reduction of phrenic ENG with airway occlusion was less. Consequently, the relationship between airway pressure and degree of inhibition was best described as a reverse parabola with the maximum at approximately +10-15 cmH₂O. Responses were not significantly affected by bilateral cervical phrenicotomy. Complete section of the spinal cord at the high thoracic level (T₁-T₂) abolished the observed reductions in phrenic ENG in response to airway occlusion. Our results demonstrate 1) the existence of nonvagal nonphrenic reflex control of diaphragm activation most likely secondary to activation of intercostal afferents and 2) that the magnitude of this reflex is highly dependent on factors related to lung volume.