Cardiovascular regulation in canines during low-frequency acceleration

Abstract

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.