ALTERATIONS IN HEART-RATE-VARIABILITY IN PATIENTS UNDERGOING DOBUTAMINE STRESS ECHOCARDIOGRAPHY, INCLUDING PATIENTS WITH NEUROCARDIOGENIC HYPOTENSION

Frequency-domain analysis of heart rate variability (HRV) appears to d istinguish sympathetic and parasympathetic modulation of heart rate. T he 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 echoc ardiography including during episodes of neurocardiogenically mediated hypotension. The low-frequency (LF; 0.04 to 0.15 Hz) and high-frequen cy (HF; 0.15 to 0.40 Hz) components of HRV decreased with increasing d ose of dobutamine (5 to 40 mu g/kg/min) in patients whose systolic blo od pressure, compared with baseline, remained within 20 mm Hg (normote nsive) or increased 20 mm Hg (hypertensive). The ratio of LF/HF became <1 in the hypertensive group at 30 and 40 mu g/kg/min of dobutamine. In eight patients in whom apparent neurocardiogenically mediated hypot ension developed (decrease in systolic blood pressure greater than or equal to 20 mm Hg from baseline), LF and HF measures became significan tly higher than those in the patients without hypotension, whereas the LF/HF ratio was unchanged. These findings suggest that the HF compone nt of HRV is a reliable measure of refer-augmented cardiac vagal activ ity and lend further support to the importance of parasympathetic modu lation of the LF component of HRV.