Baroreflex responses to acute changes in blood volume in humans

Abstract

To test the hypothesis that acute changes in plasma volume affect the stimulus-response relations of high- and low-pressure baroreflexes, eight men (27-44 yr old) underwent measurements for carotid-cardiac and cardiopulmonary baroreflex responses under the following three volemic conditions: hypovolemic, normovolemic, and hypervolemic. The stimulus-response relation of the carotid-cardiac response curve was generated using a neck cuff device, which delivered pressure changes between +40 and -65 mmHg in continuous steps of 15 mmHg. The stimulus-response relationships of the cardiopulmonary baroreflex were studied by measurements of forearm vascular resistance (FVR) and peripheral venous pressure (PVP) during low levels of lower body negative pressure (0 to -20 mmHg). Altered vascular volume had no effect on response relations of the carotid-cardiac baroreflex but did alter the gain of the cardiopulmonary baroreflex (-7.93 +/- 1.73, -4.36 +/- 1.38, and -2.56 +/- 1.59 peripheral resistance units/mmHg for hypovolemic, normovolemic, and hypervolemic, respectively) independent of shifts in baseline FVR and PVP. These results indicate greater demand for vasoconstriction for equal reductions in venous pressure during progressive hypovolemia; this condition may compromise the capacity to provide adequate peripheral resistance during severe orthostatic stress. Fluid loading before reentry after spaceflight may act to restore vasoconstrictive capacity of the cardiopulmonary baroreflex but may not be an effective countermeasure against potential post-flight impairment of the carotid-cardiac baroreflex.

Hypovolemia and neurovascular control during orthostatic stress

“…That is, the MSNA response for a given change in CVP was as expected. The repeatability of MSNA burst frequency on different days (9,47,50) provides confidence in this conclusion.…”

Short-term variability of blood pressure: effects of lower-body negative pressure and long-duration bed rest

“…Data during bed rest suggested that cardiopulmonary baroreflex-related effects are blunted and that blood pressure maintenance in the presence of an orthostatic stimulus relies mostly on arterial control. arterial blood pressure; variability; baroreflex; lower body negative pressure; bed rest MILD OR NONHYPOTENSIVE LOWER body negative pressure (LBNP; Ϫ20 mmHg Ͻ LBNP Ͻ 0 mmHg) has been used to selectively unload cardiopulmonary baroreceptors (25,44,46,50). However, straightforward observations of the time course of mean arterial pressure (MAP) following rapid onset of mild LBNP (17, 18) evidenced a transient decrease followed by a recovery of pre-LBNP onset values, suggesting that arterial baroreceptors are affected by mild LBNP as well.…”

“…MILD OR NONHYPOTENSIVE LOWER body negative pressure (LBNP; Ϫ20 mmHg Ͻ LBNP Ͻ 0 mmHg) has been used to selectively unload cardiopulmonary baroreceptors (25,44,46,50). However, straightforward observations of the time course of mean arterial pressure (MAP) following rapid onset of mild LBNP (17,18) evidenced a transient decrease followed by a recovery of pre-LBNP onset values, suggesting that arterial baroreceptors are affected by mild LBNP as well.…”

Neural Control of Vascular Function in Skeletal Muscle

“…Orthostatic stress can be induced nonpharmacologically by upright posture, or simulated using lower body negative pressure (LBNP). Many studies illustrate an increase in the groupaveraged forearm and leg vascular resistance during these tests (29,164,253,283) that are sensitive to norepinephrine (NE) release (253). In humans, head-up tilt elicits predictable increases in sympathetic outflow and total vascular resistance, the magnitude of which expresses interindividual variability (105).…”