Muscle sympathetic nerve activation during the Valsalva maneuver: interpretive and analytical caveats

We investigated the relationship between arterial pressure and muscle sympathetic nerve activity (MSNA) to test the hypothesis that the Valsalva maneuver may be used to estimate magnitudes of sympathetic baroreflex activation.

We recorded the ECG, beat-by-beat arterial pressure, and MSNA in 33 subjects (25 men and 8 women, aged 18-25 yr) who performed three Valsalva maneuvers at 40 mmHg expiratory pressure for 15 s. Valsalva phases were identified and the magnitude of pressure changes were correlated with MSNA. Arterial pressure-MSNA relations were probed further with beat-by-beat linear regression analysis after subjects had been separated into responders (n = 20) and non-responders (n = 13) (> or < 10 mmHg decrease in diastolic pressure, respectively).

We detected no significant correlations among the magnitudes of either systolic or diastolic pressure reductions and total MSNA. Slopes relating MSNA to beat-by-beat diastolic pressure decreases were greater (p = 0.01) for responders (-3.4 bursts x min(-1) x mmHg(-1)) than non-responders (0.8 bursts x min(-1) x mmHg(-1)), but total MSNA during straining was not different between the two groups. With both groups combined, total MSNA during phase II and III was positively correlated to both systolic (r = 0.41) and diastolic (r = 0.57) pressure during phase IV.

Sympathetic activation during the Valsalva maneuver does not necessarily reflect arterial baroreflex mechanisms alone. Phase IV increases of arterial pressure correlate positively to MSNA during phase II and III, and therefore gross estimations of sympathetic neural activation are possible through examination of terminal arterial pressure elevations after release from strain.