AUTONOMIC CARDIAC CONTROL .2. NONINVASIVE INDEXES AND BASAL RESPONSE AS REVEALED BY AUTONOMIC BLOCKADES

Heart period, systolic time intervals, low and high frequency heart pe riod variability, blood pressure, and respiration were measured in fem ale subjects under three drug conditions (saline, atropine sulfate, me toprolol) while sitting and standing on three consecutive days. Follow ing preinfusion baseline recordings, saline, metoprolol (14 mg), or at ropine sulfate (2 mg) was infused for 15 min (by using a double-blind procedure). Recordings were taken during a postinfusion baseline and i n response to an orthostatic stressor (standing versus sitting posture s). At the end of the metoprolol session, atropine sulfate was infused and responses were monitored during the postinfusion (i.e., double bl ockade) baseline and during orthostatic stressor. Analyses of the bloc kade data revealed that the preejection period (PEP) reflected sympath etic but not vagal influences on the heart, and high frequency (HF, 0. 12-0.40 Hz) heart rate variability (respiratory sinus arrhythmia) refl ected vagal but not sympathetic influences on the heart. No other meas ure provided a specific index of the tonic sympathetic or vagal activa tion of the heart. Postinfusion PEP under saline predicted individual differences in postinfusion cardiac sympathetic activation, whereas po stinfusion heart period (but not HF variability) under saline predicte d individual differences in postinfusion cardiac vagal activation.