Effect of helium-induced ventilatory unloading on breathing and diaphragm EMG in awake ponies

Two questions were addressed in this study: 1) Does respiratory resistive unloading (inspired O2 fraction = 0.21, inspired He fraction = 0.79) elicit a compensatory reduction in stimulation of the diaphragm? 2) Do diaphragm and lung afferents contribute to compensatory responses to unloading? Ten intact (I), five diaphragm-deafferented (DD), four hilar nerve-denervated (HND), and seven DD+HND adult ponies were studied at rest and during mild and moderate treadmill exercise. During steady-state unloading at rest, duration of the diaphragm electromyogram (EMGdi) was less (P < 0.05) than control in I ponies, but there were no additional significant changes in breathing or blood gases. Unloading during mild and moderate exercise increased (P < 0.05) pulmonary ventilation in all groups, and this response did not differ (P > 0.05) among the groups. With unloading during exercise, arterial PCO2 was within 1 Torr of control except in the DD+HND ponies, which were 1-2 Torr hypocapnic (P < 0.05). During exercise, the duration and rate of rise of the EMGdi were reduced (P < 0.05) below control, beginning at about the third unloaded breath. The decrease in rate of rise was usually not sustained, inasmuch as there was a gradual return toward control over 2 min of unloading. There were no consistent group differences in these EMGdi responses. We conclude that resistive unloading during mild and moderate exercise in ponies results in a transient reduction in neural drive to the diaphragm that is not critically dependent on diaphragm and pulmonary afferents.