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(Circulation. 2001;104:2932.)
© 2001 American Heart Association, Inc.

Clinical Investigation and Reports

Sequential Modulation of Cardiac Autonomic Control Induced by Cardiopulmonary and Arterial Baroreflex Mechanisms

Raffaello Furlan, MD; Giris Jacob, MD, PhD; Laura Palazzolo, MD; Alexandra Rimoldi, MD; Andre Diedrich, MD; Paul A. Harris, PhD; Alberto Porta, MS, PhD; Alberto Malliani, MD; Rogelio Mosqueda-Garcia, MD, PhD; David Robertson, MD

From Medicina Interna II, Ospedale L. Sacco, Dipartimento Scienze Precliniche LITA di Vialba, Università degli Studi di Milano, Italy (R.F., L.P., A.R., A.P., A.M.); Recanati Autonomic Dysfunction Center, Rambam Medical Center, Haifa, Israel (G.J.); Dupont Pharmaceuticals, Wilmington, Del (R.M.-G.); and General Clinical Research Center, Department of Biomedical Engineering (P.A.H.), and Autonomic Dysfunction Center (A.D., D.R.), Vanderbilt University, Nashville, Tenn.

Correspondence to Dr Raffaello Furlan, Unità Sincopi e Disturbi della Postura, Medicina Interna II, Ospedale L. Sacco, Università di Milano, Via G.B. Grassi 74, 20157 Milano, Italy. E-mail Raffaellof{at}fisiopat.sacco.unimi.it

Background— Nonhypotensive lower body negative pressure (LBNP) induces a reflex increase in forearm vascular resistance and muscle sympathetic neural discharge without affecting mean heart rate. We tested the hypothesis that a reflex change of the autonomic modulation of heartbeat might arise during low intensity LBNP without changes of mean heart rate.

Methods and Results— Ten healthy volunteers underwent plasma catecholamine evaluation and a continuous recording of ECG, finger blood pressure, respiratory activity, and central venous pressure (CVP) during increasing levels of LBNP up to -40 mm Hg. Spectrum and cross-spectrum analyses assessed the changes in the spontaneous variability of R-R interval, respiration, systolic arterial pressure (SAP), and CVP and in the gain (_LF) of arterial baroreflex control of heart rate. Baroreceptor sensitivity was also evaluated by the SAP/R-R spontaneous sequences technique. LBNP began decreasing significantly: CVP at -10, R-R interval at -20, SAP at -40, and the indexes _LF and baroreceptor sensitivity at -30 and -20 mm Hg, compared with baseline conditions. Plasma norepinephrine increased significantly at -20 mm Hg. The normalized low-frequency component of R-R variability (LF_R-R) progressively increased and was significantly higher than in the control condition at -15 mm Hg.

Conclusions— Nonhypotensive LBNP elicits a reflex increase of cardiac sympathetic modulation, as evaluated by LF_R-R, which precedes the changes in the hemodynamics and in the indexes of arterial baroreflex control.

Key Words: nervous system, sympathetic • baroreceptors • hemodynamics • pressure

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