Heart Rate Variability and Muscle Sympathetic Nerve Activity Response to Acute Stress: the Effect of Breathing.

Previous research has suggested a relationship between low-frequency power of heart rate variability (HRV, in normalized units LFnu) and muscle sympathetic nerve activity (MSNA). However, investigations have not systematically controlled for breathing, which can modulate both HRV and MSNA. Accordingly, the aims of this experiment were to investigate: the possibility of parallel responses in MSNA and HRV (LFnu) to selected acute stressors; and, the effect of controlled breathing. After obtaining data at rest, twelve healthy males (28 +/- 5 years) performed isometric handgrip exercise (30% MVC) and the cold pressor test in random order, and then hypoxia (F(I)O(2)=0.105) for 7 minutes, during randomly assigned spontaneous and controlled breathing conditions (20 breathsmin(-1), constant tidal volume, isocapneic). MSNA was recorded from the peroneal nerve, while HRV was calculated from ECG. At rest, controlled breathing did not alter MSNA but decreased LFnu (p<0.05, for all) relative to spontaneous breathing. MSNA increased in response to all stressors regardless of breathing. LFnu increased with exercise during both breathing conditions. During cold pressor, LFnu decreased when breathing was spontaneous, while in the controlled breathing condition LFnu was unchanged from baseline. Hypoxia elicited increases in LFnu when breathing was controlled, but not during spontaneous breathing. The parallel changes observed during exercise, and controlled breathing hypoxia, suggested that LFnu may be an indication of sympathetic outflow in select conditions. However, since MSNA and LFnu did not change in parallel with all stressors, a cautious approach to the use of LFnu as a marker of sympathetic activity is warranted.