Electromyogram of the Transversus Abdominis Expiratory Muscle during Chronic Hypoxia in Awake Ponies

Abstract

During acute hypoxia (10 min) in awake ponies and dogs, there is a reduction from normoxia in the duration (TTotal) and in the mean activity of expiratory muscles. The purpose of the present study was to determine the effect of chronic (1–120 h) hypoxia on expiratory muscle activity of awake ponies. From chronic wire implants into the transversus abdominis (TA) muscle and the diaphragm, we were able to monitor the electromyogram (EMG) of these muscles at sea level and during 120 h at a barometric pressure of 455 mm Hg. All ponies exhibited a normal acute ventilatory response to hypoxia and normal high altitude ventilatory acclimatization. At 10–60 min of hypoxia, bothTTotal and mean activity of the TA were markedly below normoxia; often the TA was quiescent during this period. In 4 of 5 ponies, there was some recovery of TA activity between 1 and 7 h of hypoxia, but in 4 of 5 ponies, TA activity remained significantly below control throughout the hypoxic period. In contrast, diaphragmatic activity was generally above control during hypoxia. Since stimulus level at the intracranial chemoreceptor is supposedly below normal during acute hypoxia and above normal during chronic hypoxia, it appears that the sustained TA diminished activity is not mediated by this chemoreceptor. Alternatively, it seems likely that hypoxic depression of brain stem expiratory neurons contributes to the reduced TA activity.