Dm. Ainsworth et al., REGULATION OF RESPIRATORY MUSCLE ACTIVITIES DURING CHEMORECEPTOR STIMULATION IN ADULT HORSES, American journal of veterinary research, 56(3), 1995, pp. 366-373
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.