EFFECT OF HELIUM-INDUCED VENTILATORY UNLOADING ON BREATHING AND DIAPHRAGM EMG IN AWAKE PONIES

Two questions were addressed in this study: 1) Does respiratory resist ive unloading (inspired O-2 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 stu died at rest and during mild and moderate treadmill exercise. During s teady-state unloading at rest, duration of the diaphragm electromyogra m (EMG(di)) was less (P < 0.05) than control in I ponies, but there we re no additional significant changes in breathing or blood gases. Unlo ading 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 Pco(2) wa s 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 EMG(di) were reduced (P < 0.05) below control, beginnin g 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 differ ences in these EMG(di) responses. We conclude that resistive unloading during mild and moderate exercise in ponies results in a transient re duction in neural drive to the diaphragm that is not critically depend ent on diaphragm and pulmonary afferents.