Regulation of respiratory muscle activities during chemoreceptor stimulation in adult horses.
We examined the electromyographic activity of the costal portion of the diaphragm and the transverse abdominal and external oblique muscles in 6 chronically instrumented awake adult horses during eupneic breathing, during 2 levels of hypercapnia (fractional concentration of inspired CO2; FICO2 = 0.4 and 0.6), and during 2 levels of hypocapnic hypoxia (FIO2 = 0.15 and 0.12). Using the inert gas technique, we also measured the end-expiratory lung volumes of the 6 horses during eupnea, 6% CO2 challenge, and 12% O2 breathing. During eupneic breathing, phasic electrical activity of these 3 muscles was always present and was preceded by the onset of mechanical flow. At progressive levels of hypercapnia, the magnitude of inspiratory and expiratory electrical activity increased, and for the expiratory muscles, this recruitment coincided with significant (P < 0.05) increases in peak expiratory gastric pressure. However, during hypocapnic hypoxia, differential recruitment patterns of the respiratory muscles were found. The electrical activity of the diaphragm increased in magnitude and occurred sooner relative to the onset of mechanical flow. The magnitude and onset of abdominal expiratory activity failed to increase significantly during these episodes of hyperpnea and this pattern of activity coincided with decrements in peak expiratory gastric pressure. Despite alterations in muscle recruitment patterns during these hyperpneic episodes, end-expiratory lung volume remained unchanged. Thus, we conclude that adult horses respond similarly to awake dogs during peripheral and central chemoreceptor stimulation.
Ainsworth
D M
DM
Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA.
Ducharme
N G
NG
Hackett
R P
RP
Eicker
S W
SW
Snedden
K
K
D000818:Animals; D002628:Chemoreceptor Cells / Q000502:physiology*; D005260:Female; D006734:Horse Diseases / Q000139:chemically induced / Q000503:physiopathology; D006736:Horses / Q000502:physiology*; D006935:Hypercapnia / Q000139:chemically induced / Q000503:physiopathology / Q000662:veterinary; D000860:Hypoxia / Q000139:chemically induced / Q000503:physiopathology / Q000662:veterinary; D008297:Male; D009119:Muscle Contraction; D012132:Respiratory Muscles / Q000502:physiology*; D013268:Stimulation, Chemical