Hypovolemia and neurovascular control during orthostatic stress

Abstract

Humans exposed to real or simulated microgravity experience decrements in blood pressure regulation during orthostatic stress that may be related to autonomic dysregulation and/or hypovolemia. We examined the hypothesis that hypovolemia, without the deconditioning effects of bed rest or spaceflight, would augment the sympathoneural and vasomotor response to graded orthostatic stress. Radial artery blood pressure (tonometry), stroke volume (SV), brachial blood flow (Doppler ultrasound), heart rate (electrocardiogram), peroneal muscle sympathetic nerve activity (MSNA; microneurography), and estimated central venous pressure (CVP) were recorded during five levels (Ϫ5, Ϫ10, Ϫ15, Ϫ20 and Ϫ40 mmHg) of randomly assigned lower body negative pressure (LBNP) (n ϭ 8). Forearm (FVR) and total peripheral vascular resistance (TPR) were calculated. The test was repeated under randomly assigned placebo (normovolemia) or diuretic (spironolactone: 100 mg/day, 3 days) (hypovolemia) conditions. The diuretic produced an ϳ16% reduction in plasma volume. Compared with normovolemia, SV and cardiac output were reduced by ϳ12% and ϳ10% at baseline and during LBNP after the diuretic. During hypovolemia, there was an upward shift in the %⌬MSNA/⌬CVP, ⌬FVR/⌬CVP, and ⌬TPR/⌬CVP relationships during 0 to Ϫ20 mmHg LBNP. In contrast to normovolemia, blood pressure increased at Ϫ40 mmHg LBNP during hypovolemia due to larger gains in the %⌬MSNA/⌬CVP and ⌬TPR/⌬CVP relationships. It was concluded that acute hypovolemia augmented the neurovascular component of blood pressure control during moderate orthostasis, effectively compensating for decrements in SV and cardiac output.baroreflex; muscle sympathetic nerve activity; Doppler ultrasound; lower body negative pressure; vascular resistance; spironolactone EXPOSURE TO REAL OR SIMULATED MICROGRAVITY leads to cardiovascular deconditioning with the associated reductions in blood pressure regulation during orthostatic stress. The effect of this deconditioning on baroreflex neurovascular control in humans is not known, and the pathophysiology of postflight difficulties in blood pressure control remains a focus of debate.To maintain adequate blood pressure and cerebral perfusion during orthostatic stress, reflex adjustments occur to increase heart rate (HR) and peripheral vasoconstriction to compensate for a decreased venous return and stroke volume (SV). Primary contributors to the diminished ability to maintain blood pressure in many individuals after spaceflight or bed rest are believed to include reductions in plasma volume (PV) (3, 7, 10, 11), diminished baroreflex control of HR (10, 21), and/or vascular resistance (36,48). From these findings, two separate hypotheses have been proposed to explain difficulties in postural blood pressure control after spaceflight or bed rest.The first hypothesis recognizes the positive correlation between the duration of microgravity exposure and the degree of PV reduction reaching 12-15% or 350-500 ml (3). Hypovolemia leads to larger decreases in both venous return and ...

Effect of acute sympathetic nervous system activation on flow-mediated dilation of brachial artery

“…Significant elevations were observed in plasma norepinephrine in the muscle chemoreflex, cold pressor, and lower body suction tests. These data are consistent with observations from direct measurement of muscle SNA (MSNA) during muscle ischemia (39), cold pressor test (33), and lower body suction (23,31). The effects of mental stress are equivocal, with some studies showing a small increase in MSNA (5), whereas with others there was a decrease (16).…”

“…In the study of Hijmering et al, subjects were supine for at least 90 min before the application of the final level of lower body suction, and this might have accounted for the Ͼ15% elevation in heart rate (18), whereas heart rate in the current study and other research (3,31) showed little or no increase until lower body suction exceeded Ϫ20 mmHg. Even though heart rate increased very little with Ϫ20 to Ϫ30 mmHg lower body negative pressure, MSNA is increased by lower body suction (23,31,38). It is possible that prolonged supine posture might have interacted with the lower body suction to modify the cardiovascular response, resulting in elevated heart rate, as has been observed after very short durations of slight head-down bed rest (2,25).…”

Lower vascular tone and larger plasma volume in Parkinson's disease with orthostatic hypotension

“…This assumption is based on the important role of the sympathetic nervous system in volume homeostasis via renal innervation and the renin-angiotensin system (6). In addition, a relation is present between baroreflex-mediated vascular control (26), time to presyncope (10), and blood pressure during orthostatic challenges in autonomic failure (46). In addition, plasma volume expansion is considered an effective therapy in PD patients with OH (38).…”

Tilt-table testing as a predictor of + GZ tolerance

“…It has been demonstrated in numerous studies that, during tilting to erect position or HUT, noradrenaline, adrenaline and vasopressin concentrations increased significantly after several minutes of the test, reaching a constant value, which remained unchanged until test completion. [18][19][20] This was true, even if a significant BP decrease and/or HR reduction (fainting) developed in the subject.In addition, the Renin-AngiotensinAldosterone (R-A-A) system activation occurred only after about 15-20 min of HUT. [20] In our HUT study, syncope developed between minutes 15th and 17th.…”