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(Hypertension. 1995;25:1270-1275.)
© 1995 American Heart Association, Inc.

Articles

Modulation of Cardiac Autonomic Control in Humans by Angiotensin II

Jonathan N. Townend; Muzahim Al-Ani; John N. West; William A. Littler; John H. Coote

From the University of Birmingham Departments of Cardiovascular Medicine (J.N.T., J.N.W., W.A.L.) and Physiology (M.A.-A., J.H.C.), Queen Elizabeth Hospital, Edgbaston, Birmingham, UK.

Correspondence to J.N. Townend, Department of Cardiovascular Medicine, University of Birmingham, Queen Elizabeth Hospital, Edgbaston, Birmingham B15 2TH, UK.

Abstract Angiotensin II (Ang II) exerts an inhibitory action on vagal activity in animals and may also facilitate sympathetic activity. The object of this study was to compare autonomic activity resulting from equivalent steady-state baroreflex activation during intravenous Ang II infusion with that resulting from a control infusion of phenylephrine. Eight healthy subjects aged 22 to 34 years were studied in a single-blind, randomized, prospective crossover study. Autonomic activity was determined by computer analysis of RR interval variability in the time and frequency domains. Despite equal experimental hypertension with Ang II and phenylephrine infusion, at peak infusion rates the mean RR interval was significantly shorter with Ang II (983±179 milliseconds; mean±SD) than with phenylephrine (1265±187 milliseconds, P<.01). The variability of RR intervals was not significantly different, but the variability (median interquartile difference) of RR interval successive differences was significantly lower with Ang II (66 milliseconds) than with phenylephrine (104 milliseconds, P<.02). Power spectral analysis revealed the power of the 0.25-Hz component in normalized units to be significantly smaller during Ang II infusion (20.5±12.7 U) than during phenylephrine (38.2±14.7 U, P<.05), whereas the power of the 0.1-Hz component was significantly greater during Ang II infusion (67.8±17.1 U) than phenylephrine (38.8±20.3 U, P<.05). Measures of vagal modulation of heart rate were significantly attenuated, and sympathetic modulation appeared to be increased during Ang II infusion compared with control phenylephrine infusions. These observations may underlie reports of increased vagal activity during angiotensin-converting enzyme inhibitor therapy.

Key Words: angiotensin II • autonomic nervous system • spectrum analysis • heart rate

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