Autonomic mechanisms in the initial heart rate response to standing

Abstract

Autonomic mechanisms underlying the initial heart rate response to standing were analyzed in nine normal subjects. The normal pattern of response was altered by atropine to a small and gradual R-R interval shortening over 30 beats, with no rebound R-R interval lengthening. With additional propranolol, R-R interval shortening was even less and confined to the first 15-20 beats, whereas propranolol alone did not affect the normal response pattern, showing that this is under vagal control with increased cardiac sympathetic activity occurring only if the vagus is blocked. The response was reproducible in 23 normal subjects. Heart rate variation during quiet standing was almost completely abolished by atropine, but unaffected by propranolol, confirming that it is also under vagal control. In four normal subjects no rebound R-R interval lengthening occurred during either "fast" or "slow" tilt, whereas it was present during both "slow" and "fast" standing. The rebound R-R interval lengthening is determined by the muscular activity involved in standing up, rather than by the speed of the maneuver.

Relationship between impaired parasympathetic and sympathetic cardiovascular control in diabetes mellitus

“…A spectrum of abnormalities in adrenergic physiology and circulatory control after standing has been described in diabetic patients [5][6][7][8][9][10][11]. Measurement of parasympathetic HR control and testing of the postganglionic neuron in the legs made it possible to classify the different abnormal responses in this study.…”

“…HR initially increases predominantly by means of parasympathetic withdrawal [3,5,20], whereas sustained tachycardia of later phases depends almost exclusively on sympathetic stimulation [20]. Severe impairment of vagal HR control in group 1 was established by the virtual absence (< 5 beats/min) of HR variation with forced breathing [2,14].…”

“…A delay of more than I s in cardioacceleration after standing in 9/12 of group 1 A patients implies that vagal HR control was completely lost [21,22]. A sluggish sympathetic HR response remained [5,21,22]. In group 1 A the HR increase after 5 min standing was abnormally small demonstrating impaired sympathetic HR control as well [20].…”

“…The initial HR response after standing is also used to assess vagal HR control [1][2][3][4][5]. Orthostatic hypotension and an abnormal plasma catecholamine response after standing in diabetic patients has been reported as a sign of impaired sympathetic control [6][7][8][9][10][11].…”

Cardiovascular Adjustments to Gravitational Stress

“…An increase in plasma catecholamines reflects sympathetic activation. Ewing et al (94) thought that primarily vagal withdrawal mediated the early orthostatic heart-rate response, but data on autonomic blockade during head-up tilt or LBNP indicate that the true reflex response consists of a combined a-and ,B-adrenergic activation and vagal withdrawal (34-36, 211, 242, 293, 349). ,B-Adrenergic blockade attenuates the heart-rate response, and vagal withdrawal only partially compensates.…”