Disruption of phase synchronization between blood pressure and muscle sympathetic nerve activity in postural vasovagal syncope

Abstract

Schwartz CE, Lambert E, Medow MS, Stewart JM. Disruption of phase synchronization between blood pressure and muscle sympathetic nerve activity in postural vasovagal syncope. Am J Physiol Heart Circ Physiol 305: H1238 -H1245, 2013. First published August 9, 2013 doi:10.1152/ajpheart.00415.2013.-Withdrawal of muscle sympathetic nerve activity (MSNA) may not be necessary for the precipitous fall of peripheral arterial resistance and arterial pressure (AP) during vasovagal syncope (VVS). We tested the hypothesis that the MSNA-AP baroreflex entrainment is disrupted before VVS regardless of MSNA withdrawal using the phase synchronization between blood pressure and MSNA during head-up tilt (HUT) to measure reflex coupling. We studied eight VVS subjects and eight healthy control subjects. Heart rate, AP, and MSNA were measured during supine baseline and at early, mid, late, and syncope stages of HUT. Phase synchronization indexes, measuring time-dependent differences between MSNA and AP phases, were computed. Directionality indexes, indicating the influence of AP on MSNA (neural arc) and MSNA on AP (peripheral arc), were computed. Heart rate was greater in VVS compared with control subjects during early, mid, and late stages of HUT and significantly declined at syncope (P ϭ 0.04). AP significantly decreased during mid, late, and syncope stages of tilt in VVS subjects only (P ϭ 0.001). MSNA was not significantly different between groups during HUT (P ϭ 0.700). However, the phase synchronization index significantly decreased during mid and late stages in VVS subjects but not in control subjects (P Ͻ .001). In addition, the neural arc was significantly affected more than the peripheral arc before syncope. In conclusion, VVS is accompanied by a loss of the synchronous AP-MSNA relationship with or without a loss in MSNA at faint. This provides insight into the mechanisms behind the loss of vasoconstriction and drop in AP independent of MSNA at the time of vasovagal faint. vasovagal syncope; phase synchronization; blood pressure; muscle sympathetic nerve activity MANY INDIVIDUALS will have at least one vasovagal syncope (VVS; fainting) episode in their lifetime, which often appears without warning. Others experience multiple episodes, sometimes several within a span of months. This can compromise quality of life and, when occurring in certain situations, can be life threatening. Although this is a common event, and we do not fully understand all the mechanisms involved in VVS, a disruption in the arterial baroreflex has been suggested to contribute to fainting (17,28).The arterial baroreflex is one of the main cardiovascular autoregulatory mechanisms in the body. Orthostatic stress results in a redistribution of blood to the lower body due to the pull of gravity. After a short delay, blood pressure is restored by sympathetic noradrenergic vasoconstriction and venoconstriction and by an increase in heart rate (HR) (39). During a VVS episode, there is evidence that ineffective reflex mechanisms may lead to excessive venous poo...

Reduced Cerebral Blood Flow With Orthostasis Precedes Hypocapnic Hyperpnea, Sympathetic Activation, and Postural Tachycardia Syndrome

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Effect of coronary artery bypass grafting on blood pressure response to head-up tilting

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Phase Coupling Between Baroreflex Oscillations of Blood Pressure and Heart Rate Changes in 21-Day Dry Immersion

“…Orthostatic intolerance after dry immersion is associated with a decrease in MSNA, HR, and blood pressure (Iwase et al, 2000), as in vasovagal syncope which is clearly linked to the impaired baroreflex regulation of hemodynamics (Ogoh et al, 2004;Guasti et al, 2010;Schwartz et al, 2013b). Phase synchronization of baroreflex blood pressure and MSNA oscillations disappears in patients with vasovagal syncope during head-up tilt test a few minutes before the drop in blood pressure (Schwartz et al, 2013a). Regarding the phase synchronization of blood pressure and HR during syncope, we showed, for the first time, that the absence of PSI increase at an early stage of the head-up tilt test is associated with subsequent decompensation of hemodynamics and orthostatic intolerance (Borovik et al, 2019).…”