AJP - Heart and Circulatory Physiology, Vol 243, Issue 6 998-1009, Copyright © 1982 by American Physiological Society
Cardiovascular regulation in canines during low-frequency acceleration
C. F. Knapp, J. M. Evans, D. C. Randall and J. A. Marquis
Integrated, reflex, cardiovascular regulation in unanesthetized
(tranquilized), chronically-instrumented dogs was studied using sinusoidal
whole-body spinal-axis acceleration (+/- 2 Gz) at frequencies below 0.25
Hz. The participation of neurally mediated cardiac and vascular control
mechanisms was examined by comparing the responses of the same animal in a
normal (reflexive) state and ina pharmacologically blockaded (nonreflexive)
state. Integrated neural mechanisms were found to be the most effective in
minimizing acceleration-induced, arterial pressure disturbances for
frequencies below 0.012 Hz; became progressively out of phase with the
disturbances between 0.012 and 0.052 Hz, resulting in decreased
effectiveness; and failed to significantly participate in the regulatory
process for frequencies between 0.052 and 0.25 Hz, where "protection" was
provided by the hydraulic and intrinsic biomechanical characteristics of
the circulatory system. An analysis of the relative contribution of
peripheral vascular and cardiac mechanisms indicated that 1) neurally
mediated, systemic vascular responses were largest for frequency; 2) heart
rate oscillations were large for frequencies below 0.052 Hz (corner
frequency) and then decreased rapidly with increasing frequency; and 3)
neurally mediated stroke volume oscillations were the largest for the
lowest frequencies and decreased with increasing frequency.
