Closed-loop analysis of the reflex function in orthostatic hypotension.

The arterial baroreceptors and cardiopulmonary receptors are important for the control of blood pressure during orthostatic stress. We assessed the baroreflex loop gain of arterial pressure compensation to orthostatic hypotension in pentobarbital-anesthetized and gallamine-paralyzed dogs subjected to 45 degrees and 90 degrees head-up tilt. The orthostatic hypotension before and after sinus denervation and cervical vagotomy was designated as D' and D, respectively. The loop gain (G) was calculated using closed-loop analysis: G = (D/D')-1. In 10 dogs, the D', D and G were 17.2 +/- 2.2 (mean +/- SE) mmHg, 64.2 +/- 4.4 mmHg and 2.73 +/- 0.26, respectively in 45 degrees tilt; and 30.6 +/- 3.2 mmHg, 80.7 +/- 5.2 mmHg and 1.64 +/- 0.24, respectively in 90 degrees tilt. Atropine sulfate (0.5 mg/kg, i.v.) eliminated the reflex tachycardia, but did not affect the SAP change during the head-up tilt. The effects of cervical vagotomy on the orthostatic hypotension were thus attributed to interruption of the aortic nerves and vagal afferents. The data provide a quantitative measure of the baroreflex function during the orthostatic stress. The loop gain is not a constant function because the value is significantly less in 90 degrees than in 45 degrees tilt (P less than 0.05). The nonlinear control system implies that the baroreflex compensation became less efficient in the face of a greater blood pressure perturbation.