Published Online
on April 14, 2003

A more recent version of this article appeared on April 29, 2003

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Submitted on October 23, 2002
Revised on February 7, 2003
Accepted on February 7, 2003

Cardiovascular Control During Exercise. Insights From Spinal Cord-Injured Humans

Flemming Dela MD^*, Thomas Mohr MD, Christina M.R. Jensen , Hanne L. Haahr PhD, Niels H. Secher MD, Fin Biering-Sørensen MD, and Michael Kjær MD

From the Department of Medical Physiology (F.D., C.M.R.J.), The Panum Institute; Copenhagen Muscle Research Center (F.D., N.H.S., M.K.); Department of Clinical Physiology/Nuclear Medicine (F.D.), Herlev Hospital; Department of Anesthesia (T.M., H.L.H., N.H.S.) and Center for Spinal Injury (F.B.-S.), Rigshospitalet; and Sports Medicine Research Unit (M.K.), Bispebjerg Hospital, University of Copenhagen, Denmark.

^* To whom correspondence should be addressed. E-mail: F.Dela{at}mfi.ku.dk.

Background--We studied the role of the central nervous system, neural feedback from contracting skeletal muscles, and sympathetic activity to the heart in the control of heart rate and blood pressure during 2 levels of dynamic exercise.

Methods and Results--Spinal cord-injured individuals (SCI) with (paraplegia, n=4) or without (tetraplegia, n=6) sympathetic innervation to the heart performed electrically induced exercise. Responses were compared with those established by able-bodied individuals (control, n=6) performing voluntary exercise at a similar pulmonary oxygen uptake. In all subjects, cardiac output and leg blood flow increased, but in SCI they reached a maximal value. The increase in cardiac output was mainly elicited by an increase in stroke volume in individuals with tetraplegia, whereas in individuals with paraplegia it was by heart rate. The increase in SCI was slow compared with that in controls. During exercise, blood pressure was stable in controls, whereas it decreased over time in SCI and especially in individuals with tetraplegia.

Conclusions--The autonomic nervous system provides for acceleration of the heart at the onset of exercise, but a slow increase in heart rate is established even without central command, neural feedback from working muscles, or autonomic influence on the heart. Yet an intact autonomic nervous system is a prerequisite for a large rise in cardiac output and in turn leg blood flow during exercise. Thus, when the sympathetic nervous system is injured at a level where it influences the heart, vasodilatation in working muscles challenges blood pressure.

Key words: electrical stimulation • nervous system, autonomic • heart rate • blood pressure

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