M. J. Brunner

Carotid sinus baroreceptor reflex control of respiration.

We have studied the effect of the carotid sinus baroreceptor reflex on respiration in 10 vagotomized, spontaneously breathing, pentobarbital anesthetized dogs. The carotid body chemoreceptor reflex response was eliminated by surgically excluding the carotid bodies from the carotid sinus baroreceptor area. Steady state frequency, tidal volume, and minute ventilation were measured after 25 mm Hg step changes in intrasinus pressure between 50 and 200 mm Hg. Over this range, the step decreases in intrasinus pressure caused concomitant increases in mean arterial pressure from 86 to 182 mm Hg. All of the respiratory response curves were sigmoidal in shape. Decreasing intrasinus pressure from 200 to 50 mm Hg caused respiratory frequency to increase from 4.8 to 9.7/min, and tidal volume to decrease from 704 to 515 ml. The calculated total ventilation, however, increased from 3180 to 4530 ml/min. The time of inspiration decreased from 3.7 to 2.4 seconds, and the time of expiration decreased from 9.8 to 4.1 seconds. These ventilatory responses are shown to be baroreceptor reflex mediated, and not secondary to changes in arterial pressure. These findings indicate that not only does the carotid sinus baroreceptor reflex control arterial pressure, but it also simultaneously influences ventilation, through changes in both respiratory frequency and tidal volume.

Carotid sinus baroreceptor reflex control and the role of autoregulation in the systemic and pulmonary arterial pressure-flow relationships of the dog.

To understand more fully the role of the arterial baroreceptor reflex on systemic and pulmonary vascular resistance, we studied the influence of the carotid sinus baroreceptor reflex control system on the entire systemic and pulmonary arterial pressure-flow relationships. Ten pentobarbital-anesthetized dogs, whose carotid sinuses were isolated, were used in a preparation in which the right and left hearts were bypassed to control systemic and pulmonary blood flows. At intrasinus pressures of 50, 125, and 200 mm Hg, systemic and pulmonary arterial pressures were measured in response to step changes in systemic and pulmonary blood flows from 0 up to 200 ml/min per kg. The systemic arterial pressure-flow relationship exhibited a marked nonlinearity, especially at either high or low flows. A third-order polynomial fit was found to represent the steady state systemic arterial pressure-flow relationship best. Blood flow autoregulation was reflected as a secondary change in systemic arterial pressure at constant flow approximately 15 seconds after the initial response. Blood flow autoregulation was seen in the entire systemic vascular bed over the entire range of flows studied. The degree of autoregulatory significantly contributed to the shape of the systemic arterial pressure-flow relationship. The steady state arterial pressure-flow relationship shifted upward and toward the pressure axis, increasing the calculated incremental resistance and total peripheral resistance as intrasinus pressure was decreased. The systemic zero-flow arterial pressure was found to increase with decreases in intrasinus pressure. The pulmonary arterial pressure-flow relationship was found to be linear in the range of flows studied from 25 up to 200 ml/min per kg.(ABSTRACT TRUNCATED AT 250 WORDS)

The importance of the spleen in blood volume shifts of the systemic vascular bed caused by the carotid sinus baroreceptor reflex in the dog.

To quantify the relative importance of the spleen in the carotid sinus baroreceptor reflex control of total blood volume, we studied the reflex control of systemic vascular capacity before and after removal of the spleen in 18 sodium pentobarbital-anesthetized dogs. Venous return was diverted into a reservoir while cardiac output and venous pressure were maintained constant. Intrasinus pressure was either raised or lowered between 50 and 200 mm Hg, and this mobilized blood into or out of the reservoir. Two sets of experiments were performed: one in which the spleen was acutely removed and the other in which a snare ocoluder was placed around the spleen. In the first series with the spleen intact, the volume shift amounted to 8.42 ml/kg and after splenectomy the volume shift was attenuated to 5.17 ml/kg. The average difference in these responses amounted to 2.69 ml/kg. In the second series of experiments in which we repeatedly measured volume shifts with spleen intact and spleen removed, the volume shift amounted to 10.57 ml/kg with spleen intact and 8.20 ml/kg with spleen removed. The average difference amounted to 2.37 ml/kg. There was a significant increase in total systemic vascular compliance for all dogs tested in both sets of experiments from 2.15 ± 0.08 to 2.41 ± 0.12 ml/nun Hg per kg when intrasinus pressure was increased from SO to 200 mm Hg before spleen removal. After spleen removal, the compliance still increased significantly from 2.17 ± 0.12 to 2.47 ml/mm Hg per kg when intrasinus pressure was increased from 50 to 200 mm Hg. However, total systemic vascular compliance before and after spleen removal at the same intrasinus pressure showed no significant differences. We conclude that, although the spleen contributes singificantly to the blood volume mobilization, it is not the major contributor to total systemic blood volume shifts caused by carotid sinus baroreceptor reflex in dogs.

Interaction of canine carotid sinus and aortic arch baroreflexes in the control of total peripheral resistance.

Interaction of carotid sinus and aortic arch reflex control of total peripheral resistance was studied in eight dogs anesthetized with sodium pentobarbital and placed on constant flow cardiac bypass. Carotid sinus and aortic arch baroreceptor areas were isolated and separately perfused at controlled pressures. Combinations of carotid sinus and aortic arch pressures were delivered at random in steps of 25 mm Hg over the 50–225 mm Hg pressure range, and systemic arterial pressure was measured. Changes in arterial pressure reflected changes in total peripheral resistance. A multiple linear regression showed that both carotid sinus and aortic arch pressures exhibited a sigmoidal relationship with arterial pressure. Independent of carotid and aortic baroreceptor pressures, arterial pressure was found to be a periodic function of time (period = 2 hours) in all dogs. The average carotid sinus reflex open loop gain was found to be 0.231 ± 0.092, while average aortic arch open loop gain was 0.141 ± 0.088. The gain of either the carotid sinus or aortic arch reflex was not influenced by the absolute pressure level of the other receptor area. In a separate series of experiments performed in the same dogs, we tested the hypothesis that a nonlinear temporal summation of the reflex control of total peripheral resistance might exist when the inputs to carotid and aortic baroreceptors are changed simultaneously. With both inputs held at the region of maximum gain, 25 mm Hg step changes were imposed first on carotid sinus pressure, then on aortic arch pressure, and then on both simultaneously. A temporal inhibition of the two reflexes showed that simultaneous excitation of both receptors resulted in a smaller reflex response than the sum of individual responses.