RESPIRATORY MUSCLE ELECTROMYOGRAM RESPONSES TO ACUTE-HYPOXIA IN AWAKE PONIES

We determined the effect of acute hypoxia on the ventilatory (V̇E) and electromyogram (EMG) responses of inspiratory (diaphragm) and expiratory (transversus abdominis) muscles in awake spontaneously breathing ponies. Eleven carotid body-intact (CBI) and six chronic carotid-denervated (CBD) ponies were studied during normoxia (fractional inspired O2 concn [FI(O2)] = 0.21) and two levels of hypoxia (FI(O2) ~0.15 and 0.12; 6-10 min/period). Four CBI and five CBD ponies were also hilar nerve (pulmonary vagal) denervated. Mean V̇E responses to hypoxia were greater in CBI ponies (Δarterial PCO2 = -4 and -7 Torr in CBI during hypoxic periods; -1 and -2 Torr in CBD). Hypoxia increased the rate of rise and mean activity of integrated diaphragm EMG in CBI (P < 0.05) and CBD (P > 0.05) ponies relative to normoxia. Duration of diaphragm activity was reduced in CBI (P < 0.05) but unchanged in CBD ponies. During hypoxia in both groups of ponies, total and mean activities per breath of transversus abdominis were reduced (P < 0.05) without a decrease in rate of rise in activity. Time to peak and total duration of transversus abdominis activity were markedly reduced by hypoxia in CBI and CBD ponies (P < 0.05). Hilar nerve denervation did not alter the EMG responses to hypoxia. We conclude the following: 1) hypoxia increases the intensity of diaphragm stimulation as indicated by increased rate of rise and mean activity of the integrated EMG; 2) transversus abdominis total and mean activities per breath are reduced by hypoxia consequent to a decrease in the time this muscle is active; and 3) the qualitative transversus abdominis response to hypoxia is not dependent on carotid chemoreceptor or lung vagal afferents.