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Division of Cardiology, Toronto General and Mount Sinai Hospitals, and University of Toronto, Toronto, Ontario, Canada M5G 1X5
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 sympathetic
nerve activity (MSNA), but not the ratio of low- to high-frequency
harmonic component of spectral power (PL/PH), a
coarse-graining power spectral estimate of sympathetic heart rate (HR)
modulation. LBNP at 5 mmHg lowered central venous pressure and had no
effect on stroke volume (Doppler) or systolic blood pressure but
reduced vagal HR modulation. This latter finding, a manifestation of
arterial baroreceptor unloading, refutes the concept that low levels of
LBNP interrogate, selectively, cardiopulmonary reflexes. MSNA
increased, whereas PL/PH and HR were unchanged. This discordance is consistent with selectivity of efferent sympathetic responses to nonhypotensive LBNP and with unloading of tonically active
sympathoexcitatory atrial reflexes in some subjects. Hypotensive LBNP
(15 mmHg) increased MSNA and PL/PH, but there
was no correlation between these changes within subjects. Therefore, HR
variability has limited utility as an estimate of the magnitude of
orthostatic changes in sympathetic discharge to muscle.
arterial baroreceptor reflexes; Bainbridge reflex; cardiopulmonary reflexes; microneurography; muscle sympathetic nerve activity; parasympathetic nervous system; power spectral analysis
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