The baroreflex is counteracted by autoregulation, thereby preventing circulatory instability

The aims of this study were (a) to apply in the animal with intact baroreflex a two‐point method for estimation of overall, effective open‐loop gain, G 0e , which results from the combined action of baroregulation and total systemic autoregulation on peripheral resistance; (b) to predict specific baroreflex gain by correcting the effective gain for the autoregulation gain; and (c) to discuss why the effective gain is usually as low as 1–2 units. G 0e was estimated from two measurements of both cardiac output, Q , and mean systemic arterial pressure, P : one in the reference state (set‐point) and the other in a steady‐state reached 1–3 min after a small cardiac output perturbation. In anaesthetized cats and dogs a cardiac output perturbation was accomplished by partial occlusion of the inferior vena cava and by cardiac pacing, respectively. Average (± s.e.m. ) estimates of G 0e were 1.4 ± 0.2 ( n = 8) in the cat and 1.5 ± 0.4 ( n = 5) in the dog. The specific baroreflex open‐loop gain, G 0b , found after correction for total systemic autoregulation, was 3.3 ± 0.4 in the cat and 2.8 ± 0.8 in the dog. A model‐based analysis showed that, with G 0e as low as 1.4, the closed‐loop response of P to a stepwise perturbation in Q results in damped oscillations that disappear in about 1 min. The amplitude and duration of these oscillations, which have a frequency of about 0.1 Hz, increase with increasing G 0e and cause instability when G 0e is about 3. We conclude that autoregulation reduces the effectiveness of baroreflex gain by about 55%, thereby preventing instability of blood pressure response.