Reduced baroreflex sensitivity with volume loading in conscious dogs.

Abstract

The Bainbridge reflex, i.e., the effect of rapid saline infusion (1.1 ± 0.1 liters) on heart rate and arterial and atrial blood pressures, was examined in 12 intact conscious dogs; mean arterial blood pressure rose by 33 ± 3 (SE) mm Hg, mean atrial pressure by 14 ± 1 mm Hg, and heart rate by 75 ± 9 beats/min. After beta-receptor blockade, heart rate rose slightly less (+49 =t 5 beats/min, P = 0.05). Cholinergic blockade, combined cholinergic and beta-receptor, or beta-receptor blockade after vagotomy blocked the heart rate response to the infusion. The rise in heart rate in the face of an increase in arterial blood pressure with volume loading suggested that the arterial baroreceptor reflex was not responding appropriately to the increase in arterial blood pressure. In conscious dogs after denervation of the arterial baroreceptors, the increase in heart rate with volume loading was no greater than that in those dogs with their arterial baroreceptors intact, suggesting that the baroreceptor reflex was not restraining heart rate in the normal response to volume loading. The relationship between the pulse interval (PI) and the systolic arterial blood pressure (SAP) following an intravenous injection of methoxamine was used to evaluate the sensitivity of the baroreceptor reflex in intact conscious dogs. After a mild amount of volume loading, when atrial pressure was 8 ± 2 mm Hg, the PI/SAP slope was significantly depressed from normal. When atrial pressure was elevated further to 28 ± 1 mm Hg by volume loading, the slope was further depressed. Thus, arterial baroreflex sensitivity is reduced progressively as atrial pressure is raised by volume loading, an observation that explains how heart rate can rise strikingly in the face of an elevated arterial blood pressure.• In 1915, Bainbridge (1) observed that a rapid infusion of saline or blood induces tachycardia, which is abolished by vagotomy; this response has been referred to as the Bainbridge reflex. The Bainbridge reflex is one of several physiological situations that does not follow Marey's law (2), which describes the inverse relationship between cardiac rate and arterial blood pressure, since volume loading results in an increase in both arterial blood pressure and cardiac rate. The infusion-induced elevation of arterial blood pressure would be expected to stimulate the arterial baroreceptors to reduce heart rate reflexly, unless a change in the gain of the reflex, the set point of the reflex, or both occurs (3). The goal of the present study was to evaluate the effects of acute volume 236loading on heart rate in the conscious dog and to examine the extent to which volume loading alters the properties of the arterial baroreceptor system.Since one of the controversies concerning the Bainbridge reflex involves the questions of whether heart rate consistently rises with volume loading and to what extent the response is related to the initial heart rate prior to volume loading (4-6), the present study was conducted in conscious dogs in which resting heart ...

Hemodynamic Characteristics of Regional Vascular Beds in Cerebral Ischemic Pressor Responses

“…This is in agreement with the results in this paper. Volume loading not only redistributed blood flow but decreased the gain of the carotid sinus reflex (CASTENFORS and SJOSTRAND, 1973;VATNER et al, 1975). As described in this paper, volume loading slightly reduced the pressor response produced by cerebral ischemia.…”

“…It has been indicated that the gain in the carotid sinus reflex decreases after hemorrhaging and volume loading (CASTENFORS and SJOSTRAND, 1973;VATNER et al, 1975 also appeared slightly before the top of the oscillation. Namely, the sequential order of the kidney, the intestine and the skeletal muscles was maintained at the start of both the descent and the ascent of the undulation, which always preceded the ascent and descent of the oscillation, respectively.…”

Mild hypovolemic stress alters autonomic modulation of heart rate.

“…Animal studies by Bainbridge 9 ' 10 and Vatner et al 11 have convincingly demonstrated reflex vagal changes in HR in response to both increases and decreases in central venous volume. Also, some reports have suggested that the sensitivity of the arterial HR baroreceptor reflex may be increased with decreases in central venous pressure.…”

Mechanism of Alteration in Baroreflex Cardiovascular Responses due to Volume Loading

“…Alterations in the baroreflex sensitivity have been observed in the conscious dog with heart failure (HIGGINS et al, 1972) and volume loading (VL) (VATNER et al, 1975). These studies have demonstrated that the reflex cardiovascular responses to a diminished and an augmented baroreceptor input are reduced under both experimental conditions.…”

“…It has been speculated that heart failure may cause dysfunction of the baroreceptor reflex arc per se at either the receptor, central or effector site (HIGGINS et al, 1972). VATNER et al (1975) have suggested that VL may be analogous to heart failure and that the vagal afferent inputs from the cardiopulmonary receptors are responsible for the baroreflex alteration in both conditions. STINNETT et al .…”