Adjustment of reflex bradycardia to elevated arterial pressure in conscious rabbits.

• An acute pressure rise in the vascularly isolated carotid sinus causes a decrease in total peripheral resistance and a bradycardia which lead to a fall in arterial pressure. A shift in posture that raises blood pressure at the carotid bifurcation elicits a similar reflex response. Stimulation of most of the other baroreceptor areas of the systemic arterial and cardiopulmonary systems produces a depressor response and bradycardia. However, none of these baroreceptor areas prevents the development of experimental or clinical hypertension, and there is evidence that in chronic experimental hypertension, the carotid sinus nerves carry a normal nerve-impulse pattern to the central nervous system in response to high carotid sinus pressure. It is also a fact that hypertensive animals and humans have normal heart rates under basal conditions. The present work was designed to find out exactly how long it takes the baroreceptor reflex mechanism to adjust to a continuous pressure elevation. One report sheds some light on this subject: Ivubicek et al. have shown that the pulse rate had returned to normal at the end of a 20-hour period of elevated pressure due to splanchnic nerve stimulation. Thus, adjustment of the cardiac part of the reflex mechanism occurs in less than 20 hours. This paper is a report of a group of experiments done in the intact conscious rabbit to observe the effect of a continuously elevated arterial pressure on reflex bradycardia. Spe-