Resetting of the Carotid Arterial Baroreflex during Dynamic Exercise




Bryant, Kristin Hannah Norton


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Byrant, Kristin, Resetting of the Carotid Arterial Baroreflex during Dynamic Exercise Doctor of Philosophy (Biomedical Sciences), August, 1997; 121 pp; 8 tables; 20 figures, bibliography, 82 titles. Following the initial response to the onset of dynamic exercise, prolonged exercise at a constant workload is characterized by a progressive decrease in stroke volume (SV) and mean arterial pressure (MAP) and concomitant rise in heart rate (HR). These data raise the question as to whether there is a loss of baroreflex regulation of arterial blood pressure during prolonged dynamic exercise. However, we propose that the carotid barareflex (CBR) is continually reset during prolonged exercise, with the operating point being shifted toward the reflex threshold, in relation to a progressive increase in central command activity as motor fibers are recruited in response to muscle fatigue. Therefore, the baroreflex is unresponsive to the fall in MAP. In order to investigate the hypothesis, volunteer subjects performed one hour of dynamic leg cycling exercise at 65% of maximal oxygen uptake (VO2max) with: I) no intervention; and ii) maintenance of cardiac filling volume via continuous infusion of a 6% dextran in saline solution to counteract the fall in SV. At 10 and 50 minutes of exercise, CBR stimulous-response curves were generated using the neck pressure/neck suction technique. The maintenance of cardiac filling volume and thus SV resulted in a diminished drift in MAP. However, indices of central command such as HR, VO2 and ratings of perceived exertion (RPE) increased to the same extent regardless of exercise condition. Furthermore, there was augmented resetting of the CBR at 50 minutes of exercise as compared to 10 minutes under both exercise conditions. In order to further investigate the effects of central command on baroreflex control of blood pressure, a second investigation was designed to demonstrate the effects of exercise type and intensity on CBR function. Stimulus-response relationships were compared during dynamic exercise at a wide range of exercise intensities performed with either leg exercise alone or leg exercise combined with arm exercise. Increases in exercise intensity to maximal exercise resulted in increases in indices of central command such as HR and VO2 as well as an augmentation of the magnitude of the lateral shift in the CBR stimulus-response curve (with the operating point being shifted further toward the threshold of the reflex) relative to the activation of central command. In addition, the performance of combined arm and leg exercise elicited an augmented shift in the carotid-vasomotor stimulus-response relationship as compared to leg exercise alone at the same exercise intensity. As arm exercise compared to leg exercise performed at the same absolute VO2 results in an increased lactate accumulation in the venous system, the augmented resetting of the CBR is likely due to a disproportionate activation of the muscle metaboreflex component of the muscle pressor reflex. Therefore we propose that the central command is the primary mechanism by which the CBR is reset at the onset of dynamic exercise through feed-forward control. However, additional, feed-back modulation can be exerted by the muscle pressor reflex upon the development of mechanical or chemical error signals in the exercising muscle.