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

Withdrawal of muscle sympathetic nerve activity (MSNA) may not be necessary for the precipitous fall of peripheral arterial resistance and blood pressure (BP) during vasovagal syncope (VVS). We tested the hypothesis that the MSNA‐BP baroreflex entrainment is disrupted prior to VVS regardless if MSNA is withdrawn. We measured phase synchronization during 70° head‐up tilt (HUT) in 14 individuals including 8 VVS and 6 healthy control (C). Heart rate (HR), BP and MSNA were measured during supine baseline and at early, mid, and late stages of HUT. The phase synchronization index (PhSI), the rate of change of MSNA phase with BP phase (neural arc) and that of BP with MSNA (peripheral arc) were computed. During HUT, HR was greater in VVS compared to C (time × group; p=.04). BP did not change during the early stage, but significantly decreased during mid and late stages in VVS only (time × group; p=.001). MSNA was not different between groups during HUT (time × group; p=.700). However, PhSI significantly decreased during mid and late stages in VVS but not C (time × group; p<.001) indicating breakdown of MSNA‐BP phase locking, and therefore interaction, in VVS. In addition, the neural arc was significantly affected more than the peripheral arc prior to syncope. This provides a mechanism for the loss of vasoconstriction and drop in BP independent of late sympathetic nerve activity at the time of vasovagal faint. Supported by NIH (HL‐074873).