Abstract

We measured the changes in heart rate (HR) varability estimated from the standard deviation of the R-R intervals to evaluate parasympathetic tone noninvasively before and during activation of muscle metaboreflex induced by postexercise muscle ischemia. Eight healthy male subjects performed sustained handgrip at 50% maximal voluntary contraction followed by forearm occulsion. Mean arterial preassure, cardiac stroke volume, and ratio of cardiac preejection to left ventricular ejection time (PEP/LVET) were also measured. During the 2-min occulsion after 60 s of handgrip with voluntary respiration, HR variability and mean aeterial preassure were significantly increased from baseline abd PEP/LVET was descreased from resting level of 0.404 +- 0.022 to 0.363 +- 0.036. During occulsion and recovery, HR did not change from baselline level in any experiment. There was no influence of occulsion itself or of cessation of exercise per se on any parameters. Although overall enhanced HR variability was seen, probably due to lower breathing frequency and larger tidal volume, similar results were also obtained from an experiment with controlled respiration, showing that the increase in HR variability was not due to the changes in tidal volume or breathing frequency during occulation. In conclusion, the HR variability is increased during activation of the muscle metaboreflex induced by postexercise muscle ischemia in humans. This finding shows that the parasympathetic cardiac tone is enhanced during activation of the muscle metaboreflex and balances enhanced cardiac sympathetic activity to result in an unchanged HR. Thus, using gross HR to determine cardiac autonomic effects may be deceptive. Verf.-Referat