Angiotensin II modulates cardiovascular autonomic control in the absence of baroreflex loading.

OBJECTIVE To investigate the effects of angiotensin II in the absence of baroreflex activation. DESIGN Ten healthy male volunteers were studied in a single blind, randomised, crossover study of heart rate variability during intravenous angiotensin II infusion (5-20 ng/kg/min) compared with a control pressor infusion of phenylephrine (0.7-2.8 micrograms/kg/min). Each infusion was titrated to increase mean blood pressure by 20 mm Hg; sodium nitroprusside was then infused simultaneously to restore blood pressure to baseline values. RESULTS During concomitant angiotensin II (AII) and sodium nitroprusside (SNP) infusion, the mean (SD) RR interval (864 (117) ms) was significantly shorter than during phenylephrine (PE) and sodium nitroprusside infusion (1057 (163) ms), and was significantly shorter than at baseline (999 (164) ms), despite comparable levels of blood pressure. Values of high frequency heart rate variability measured in the time and frequency domains were significantly lower during AII/SNP infusion than during PE/SNP: percentage of successive RR interval differences exceeding 50 ms, 30(16)% v 57(21)%; root mean square of successive RR interval differences, 63 (39) v 90 (40) ms; high frequency power 0.48 (0.19) v 0.66 (0.26) nu. CONCLUSIONS When the pressor response is controlled by sodium nitroprusside, angiotensin II infusion is associated with tachycardia. Analysis of heart rate variability suggests that this reflects inhibition of cardiac vagal activity.