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1Department of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, Osaka; 2Core Laboratory, Nagoya City University Graduate School of Medical Sciences, and 3Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Nagoya; and 4Department of Cardiovascular Medicine, Kyusyu University Graduate School of Medical Sciences, Fukuoka, Japan
Submitted 6 October 2004 ; accepted in final form 31 May 2005
Sympathetic activation during orthostatic stress is accompanied by a marked increase in low-frequency (LF, 
0.1-Hz) oscillation of sympathetic nerve activity (SNA) when arterial pressure (AP) is well maintained. However, LF oscillation of SNA during development of orthostatic neurally mediated syncope remains unknown. Ten healthy subjects who developed head-up tilt (HUT)-induced syncope and 10 age-matched nonsyncopal controls were studied. Nonstationary time-dependent changes in calf muscle SNA (MSNA, microneurography), R-R interval, and AP (finger photoplethysmography) variability during a 15-min 60° HUT test were assessed using complex demodulation. In both groups, HUT during the first 5 min increased heart rate, magnitude of MSNA, LF and respiratory high-frequency (HF) amplitudes of MSNA variability, and LF and HF amplitudes of AP variability but decreased HF amplitude of R-R interval variability (index of cardiac vagal nerve activity). In the nonsyncopal group, these changes were sustained throughout HUT. In the syncopal group, systolic AP decreased from 100 to 60 s before onset of syncope; LF amplitude of MSNA variability decreased, whereas magnitude of MSNA and LF amplitude of AP variability remained elevated. From 60 s before onset of syncope, MSNA and heart rate decreased, index of cardiac vagal nerve activity increased, and AP further decreased to the level at syncope. LF oscillation of MSNA variability decreased during development of orthostatic neurally mediated syncope, preceding sympathetic withdrawal, bradycardia, and severe hypotension, to the level at syncope.
autonomic nervous system; baroreflex; blood pressure; heart rate variability; hemodynamics
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