Nonlinear Dynamics Of Cardiopulmonary Responses during Exercise

Abstract

Interpretation of cardiopulmonary transients due to a step change in exercise in terms of fast (neural or central command) and slow (humoral or reflex) components was initiated by Krogh and Lindhard¹ and further developed by Dejours². The respective roles of humoral and reflex components have been difficult to define due to the apparent lack of an error signal in the blood gases or arterial blood pressure. A recent review by Rowell and O’Leary³ has proposed a specific hypothesis dealing with sympathetic reflexes during exercise involving muscle chemoreceptors and baroreceptors. According to their reasoning, in human exercise below a heart rate of 100 beats/min. vagal withdrawal dominates heart rate control largely by central command. Above this range, sympathetic reflexes involving muscle chemoreceptors and baroreceptors become important. Since vagal and sympathetic efferent cardiac nerve stimulation lead to different dynamics⁴, a shift from vagal to sympathetic control mechanisms should lead to identifiable differences in transient response dynamics. Muscle chemoreceptor⁵ and baroreceptor^{6, 7} stimulation can both lead to ventilatory response so ventilatory transient dynamics could be coupled to cardiovascular transients. In this way, some of the reflex or humoral component of ventilatory adjustments during exercise could be accounted for. While exercise transients have been extensively studied^{8, 9, 10, 11}, the currently available experimental evidence does not provide a clear indication of whether a shift from vagal to sympathetic cardiovascular mechanisms is associated with a change in heart rate dynamics. Further, it is uncertain whether cardiovascular transients are coupled with corresponding ventilatory transients. A change in dynamics as a function of exercise level constitutes nonlinear system behavior. This study is directed at exploring whether such a nonlinearity is present.