The loop gain of autonomic reflex function in orthostatic hypotension

The loop gain (G) of the autonomic reflex function in orthostatic stress was assessed in anesthetized dogs subjected to 45 and 90 degrees head-up tilt. We observed the magnitude of orthostatic hypotension before and after 1) sinus denervation and vagotomy (SDVT), or 2) ganglionic blockade (GB) with hexamethonium. The decreases in arterial pressure during the orthostatic stress before and after interruption of the autonomic reflex from either the afferent or efferent limb were defined as E and D, respectively. The loop G of the compensatory system was calculated using closed-loop analysis: G = (D/E) - 1. In the SDVT experiments, the average values of E, D, and G were 18.6 mmHg, 62.6 mmHg, and 2.36, respectively, for 45 degrees tilt; and 31.2 mmHg, 82.7 mmHg, and 1.63, respectively, for 90 degrees tilt. The data provide a quantitative measure of the autonomic reflex function in orthostatic hypotension. Furthermore, we found that the corresponding G values in the SDVT and GB experiments were not significantly different. In each experiment, the G value in 90 degrees tilt was lower than that in 45 degrees tilt. The findings suggest that reflexes from the arterial baroreceptors and cardiopulmonary receptors account for a large part of the autonomic compensation to the orthostatic stress. The whole control system operates in a nonlinear fashion, because the gain value tends to decrease as the degree of tilt is increased.