Frequency-domain analysis of heart rate variability (HRV) appears to distinguish sympathetic and parasympathetic modulation of heart rate. The effects of acute reflex, as opposed to tonically augmented, cardiac vagal tone on HRV are not clearly defined. Power spectral components of HRV were measured in 36 patients undergoing dobutamine stress echocardiography including during episodes of neurocardiogenically mediated hypotension. The low-frequency (LF; 0.04 to 0.15 Hz) and high-frequency (HF; 0.15 to 0.40 Hz) components of HRV decreased with increasing dose of dobutamine (5 to 40 micrograms/kg/min) in patients whose systolic blood pressure, compared with baseline, remained within 20 mm Hg (normotensive) or increased > or = 20 mm Hg (hypertensive). The ratio of LF/HF became < 1 in the hypertensive group at 30 and 40 micrograms/kg/min of dobutamine. In eight patients in whom apparent neurocardiogenically mediated hypotension developed (decrease in systolic blood pressure > or = 20 mm Hg from baseline), LF and HF measures became significantly higher than those in the patients without hypotension, whereas the LF/HF ratio was unchanged. These findings suggest that the HF component of HRV is a reliable measure of reflex-augmented cardiac vagal activity and lend further support to the importance of parasympathetic modulation of the LF component of HRV.