Sustained Hypoxia: Respiratory Muscles and Ventilation
Date
2015-02-02
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Abstract
The focus of this thesis was to systematically examine the effects of sustained hypoxia on ventilation and the function of the primary breathing muscles in an intact awake mammal. Successful execution of this thesis work relied upon the chronically instrumented awake canine model to test the physiologic questions and hypotheses, collaborative team work to execute the experiments, as well as development of appropriate software tools to analyze the immense physiologic dataset amassed over the years. This thesis is organized as a series of related investigative projects overarching the central theme of sustained hypoxia.
Ventilation in awake canines exhibited the characteristic biphasic pattern during sustained hypoxia, like that of humans and other mammals, with an initial peak followed by a subsequent decline - “roll-off” or hypoxic ventilatory decline (HVD) - to a lesser intermediate plateau. Our findings directly contest the longstanding canine controversy that ventilation does not roll-off in this species. Examination of the principle inspiratory muscle, the diaphragm, during sustained hypoxia revealed a biphasic contraction and neural activation of the costal and crural, with differential segmental function during initial and sustained hypoxia. Persistent effects of hypoxia caused a dramatic loss of the contractile output of the primary inspiratory chest wall muscle, the parasternal intercostal, accompanied by a decline in EMG activity. Immediate response to hypoxia elicited a marked recruitment of the primary expiratory abdominal muscle, the transversus abdominis, however, sustained hypoxia caused the initial acute expiratory abdominal contribution to be nearly abolished. Excitatory and inhibitory influence of hypoxia may largely account for the past discordance of expiratory activity with hypoxia in mammals. Attenuation of central drive appeared to persist following sustained hypoxia affecting the primary respiratory muscles.
We conclude that sustained hypoxic ventilatory roll-off is a universal mammalian characteristic without exception for canines. Attenuation of neural drive with sustained hypoxia is a widespread central phenomenon significantly impacting the primary respiratory muscles of the diaphragm, the chest wall and the abdominal wall, in a distinct and differential manner.
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Physiology, Health Sciences, Medicine and Surgery
Citation
Ji, S. J. (2015). Sustained Hypoxia: Respiratory Muscles and Ventilation (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/27946