Venous mechanics of contracting gastrocnemius muscle and the muscle
pump theory.
Magder, S.
Royal Victoria Hospital, Critical Care Division, McGill University,
Montreal, Quebec (CANADA)
APStracts 2:0329A, 1995.
The time constant of venous drainage ([tau]v) is an important
determinant of the return of blood to the heart. The lower the
[tau]v, the higher the flow for a given volume. To determine if the
[tau]v of muscle decreases during muscle contraction we anesthetized
dogs with pentobarbital and mechanically ventilated them. We isolated
the vasculature of the gastrocnemius muscle and attached the muscle
to a force transducer. The muscle was pump perfused and flows () were
measured with electromagnetic flow probes on the artery and vein.
Pressure in the venous compliant region (Pel) was obtained by
simultaneously occluding the artery and vein. Changes in volume
([delta]v) were produced by changing inflow and obtaining the
integral of the difference between inflow and outflow. [tau]v was
obtained from ([delta]v) divided by change in flow. Compliance was
calculated from ([delta]v) divided by change in Pel from before to
after a change in flow. The venous resistance was calculated from
[tau]v divided by Cv. The muscle was set at the optimal length and
contractions were produced by stimulating the nerve to the muscle
with supramaximal voltage at either 1 or 5 Hz trains with
stimulations at 20 Hz, 0.2 msec duration and duty cycle of 0.25. The
[tau]v at rest was 4.06 +/- 2.16 sec and decreased to 2.44 +/- 1.07
sec (p < 0.05) at 1 Hz and 1.81 +/- 0.4 sec at 5 Hz. There were
no significant changes in Cv or venous resistance. In conclusion,
muscle contractions can reduce the time constant of venous drainage
of muscle and could thereby contribute to the increased venous return
and cardiac output during exercise.
Received 9 May 1994; accepted in final form 3 July 1995.
APS Manuscript Number A438-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.