REGULATION OF RESPIRATORY MUSCLE ACTIVITIES DURING CHEMORECEPTOR STIMULATION IN ADULT HORSES

We examined the electromyographic activity of the costal portion of th e diaphragm and the transverse abdominal and external oblique muscles in 6 chronically instrumented awake adult horses during eupneic breath ing, during 2 levels of hypercapnia (fractional concentration of inspi red CO2 FICO2 = 0.4 and 0.6), and during 2 levels of hypocapnic hypoxi a (FIO2 = 0.15 and 0.12). Using the inert gas technique, we also measu red the end-expiratory lung volumes of the 6 horses during eupnea, 6% CO2 challenge, and 12% O-2 breathing. During eupneic breathing, phasic electrical activity of these 3 muscles was always present and was pre ceded by the onset of mechanical flow. At progressive levels of hyperc apnia, the magnitude of inspiratory and expiratory electrical activity increased, and for the expiratory muscles, this recruitment coincided with significant (P < 0.05) in creases in peak expiratory gastric pre ssure. However, during hypocapnic hypoxia, differential recruitment pa tterns of the respiratory muscles were found. The electrical activity of the diaphragm increased in magnitude and occurred sooner relative t o the onset of mechanical now. The magnitude and onset of abdominal ex piratory activity failed to increase significantly during these episod es of hyperpnea and this pattern of activity coincided with decrements in peak expiratory gastric pressure. Despite alterations in muscle re cruitment patterns during these hyperpnea episodes, end-expiratory lun g volume remained unchanged. Thus, we conclude that adult horses respo nd similarly to awake dogs during peripheral and central chemoreceptor stimulation.