EFFECTS OF INCREASED END-EXPIRATORY LUNG-VOLUME ON BREATHING IN AWAKE PONIES

We studied the changes in breathing and respiratory muscle electromyograms (EMG) during passively induced increases in end-expiratory lung volume (EELV) in awake normal (N), hilar nerve-denervated (HND), carotid body-denervated (CBD), and HND + CBD ponies. EELV was increased by applying continuous negative pressure (-10 and -20 cmH2O) around the torso of the standing pony. In all groups, negative pressure produced sustained increases in EELV that were linearly related to the degree of negative pressure. Elevated EELV decreased breathing frequency (f) in N and CBD ponies but increased f in HND and HND + CBD ponies. When EELV was increased, tidal volume was unchanged or above control in N ponies but was below or near control in the other groups. In all groups during elevated EELV, arterial PCO2 initially decreased but then increased relative to control with isocapnia achieved after approximately 1.5 min. In all groups, the elevated EELV was accompanied by increased stimulation of the diaphragm as indicated by increased rate of rise of the integrated EMG (P < 0.05). During elevated EELV, the duration of diaphragm EMG was reduced, but only in HND ponies was this reduction significant (P < 0.05). In N ponies, the major effect of elevated EELV on the expiratory transversus abdominis (TA) muscle was an increase (P < 0.05) in duration of activity and therefore total activity. The work of breathing was thus presumably shifted more to this muscle during elevated EELV. These changes in TA timing were not observed in HND and HND + CBD ponies during elevated EELV. We conclude that elevation of EELV, which presumably places the diaphragm on a less favorable portion of its length-tension relationship, results in compensatory increased stimulation of the diaphragm that is not critically dependent on hilar and carotid chemoreceptor afferents. However, hilar afferents do contribute to the changes in diaphragm and TA duration of activity during elevated EELV.