Differential sympathetic nerve and heart rate spectral effects of nonhypotensive lower body negative pressure

Lower body negative pressure (LBNP; -5 and -15 mmHg) was applied to 14 men (mean age 44 yr) to test the hypothesis that reductions in preload without effect on stroke volume or blood pressure increase selectively muscle sympa thetic nerve activity (MSNA), but not the ratio of low- to high-frequency h armonic component of spectral power (P-L/P-H), a coarse-graining power spec tral estimate of sympathetic heart rate (HR) modulation. LBNP at -5 mmHg lo wered central venous pressure and had no effect on stroke volume (Doppler) or systolic blood pressure but reduced vagal HR modulation. This latter fin ding, a manifestation of arterial baroreceptor unloading, refutes the conce pt that low levels of LBNP interrogate, selectively, cardiopulmonary reflex es. MSNA increased, whereas P-L/P-H and HR were unchanged. This discordance is consistent with selectivity of efferent sympathetic responses to nonhyp otensive LBNP and with unloading of tonically active sympathoexcitatory atr ial reflexes in some subjects. Hypotensive LBNP (-15 mmHg) increased MSNA a nd P-L/P-H, but there was no correlation between these changes within subje cts. Therefore, HR variability has limited utility as an estimate of the ma gnitude of orthostatic changes in sympathetic discharge to muscle.