Cardiovascular control during exercise: insights from spinal cord-injured humans.
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.


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.
Dela
Flemming
F
Department of Medical Physiology, The Panum Institute, Building 12.4.7, Blegdamsvej 3, DK-2200 Copenhagen N, Denmark. F.Dela@mfi.ku.dk
Mohr
Thomas
T
Jensen
Christina M R
CM
Haahr
Hanne L
HL
Secher
Niels H
NH
Biering-Sørensen
Fin
F
Kjaer
Michael
M
D001794:Blood Pressure; D002490:Central Nervous System / Q000503:physiopathology; D015444:Exercise; D006321:Heart / Q000294:innervation / Q000503:physiopathology*; D006339:Heart Rate; D006801:Humans; D007866:Leg / Q000098:blood supply; D009068:Movement; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000378:metabolism / Q000503:physiopathology; D010264:Paraplegia / Q000503:physiopathology; D011782:Quadriplegia / Q000503:physiopathology; D012039:Regional Blood Flow; D012119:Respiration; D013119:Spinal Cord Injuries / Q000378:metabolism / Q000503:physiopathology*; D013318:Stroke Volume; D013564:Sympathetic Nervous System / Q000503:physiopathology; D014655:Vascular Resistance