Sympathetic Responses to Dynamic Arm Ergometry in Humans

Wasmund, Stephen L, Sympathetic Responses to Dynamic Arm Ergometry. Doctor of Philosophy (Biomedical Sciences), May 2001; 96 pp; 1 table; 15 figures; bibliography. Cardiovascular control during exercise is of obvious importance due to the need for an increase in cardiac output and maintenance of blood pressure when metabolic demands increase. While investigations during exercise have been conducted for some time, and much is known about the responses to dynamic exercise, the understanding of the signals that elicit the cardiovascular changes, particularly as mediated by sympathetic nerve activity (SNA) is incomplete. Sympathetic nerve activity plays an important role during exercise by causing vasoconstriction in non-working vascular beds, probably causing vasoconstriction in the vascular beds of working muscles to partially counteract the profound vasodilation caused by locally produced metabolites and by stimulating the heart to increase contractility and heart rate. It is possible to directly measure electrical activity in sympathetic nerves supplying the vasculature of skeletal muscles, however few investigations have reported on this activity during strenuous dynamic exercise. The investigations described in this dissertation extend the understanding of muscle sympathetic nerve responses to dynamic exercise. The first investigation evaluated SNA during a graded arm ergometry test to near volitional fatigue and demonstrated that increases in SNA began to occur at approximately 40% of peak exercise and then increase in a linear fashion until exercise is stopped. This relation is more closely linked to relative workload rather than heart rate as previously suggested. We also sought to determine the relationship between the increase in SNA and the ventilator threshold, hypothesizing that the two would occur at similar times, and concluded that the exercise protocol utilized did not elicit a distinct breakpoint in ventilation. However, a ventilator threshold did occur in two subjects and there appeared to be an accelerated increase in SNA. The second investigation assessed the dynamics of SNA, blood pressure and heart rate responses during the onset and termination of dynamic arm ergometry at mild, moderate and intense workloads to determine the relationship between changes in sympathetic nerve activity and blood pressure. When analyzing data every 10 seconds we determined that modest increases in SNA tend to occur at the onset of exercise in most subjects, but this response did not reach significance. This finding suggests that a neural mechanism, likely central command, plays a minor role in the initial activation of SNA, although this is probably attenuated or overridden by cardiopulmonary reflex mediated sympathoinhibition as has been previously proposed. The delay (30 s) in frank sympathetic nerve activation during strenuous exercise strongly suggests that a delayed signal, probably muscle metaboreceptor stimulation, is the primary stimulus for activation of SNA. At the termination of 5 minutes of exercise SNA, blood pressure and heart rate all decreased significantly below peak values within 10 seconds. We propose that metabolites rapidly drop below a threshold level that allows SNA to decrease significantly towards baseline values. A rapid control mechanism, such as central command or mechanoreceptor stimulation, might also play an important role in returning SNA towards resting values following exercise. We conclude that SNA remains active throughout relatively strenuous dynamic exercise, and that multiple control mechanisms are likely responsible for its control during the onset and termination of exercise.