Mesenteric blood flow during vasovagal syncope

      Rapid vasoconstriction of the splanchnic arterioles, controlled by the sympathetic nervous system is thought to be the most important mechanism for maintaining blood pressure during posture change, exercise and haemorrhage. It has been hypothesised that during vasovagal reactions, the baroreflexes transiently fail, allowing blood to pool in the splanchnic vessels. This results in decreased venous return and a steady fall in stroke volume output during the minutes preceding syncope. We aimed to measure mesenteric artery blood flow during tilt-induced syncope to see if “active” vasoconstriction was maintained as blood pressure fell. We selected 40 patients with a history of fainting and during 70-degree head-up tilt, we continuously monitored blood pressure [MAP] [Finapress], heart rate [HR] and muscle sympathetic nerve activity [MSNA]. We also did pulsed-doppler measurements from the superior mesenteric artery each minute. Using the pixel-averaging technique we quantified the area under the wave signal representative of diastolic flow and normalised this for systolic flow [NDF]. Patients were tilted for 30 minutes [GTN given at 20 minutes] or until presyncope. 18 patients [age 41 ± 15 yrs, 9 females] had tilt-induced presyncope after 21 ± 8 minutes. During the last 5 minutes of tilt: BP fell from 98 ± 12 mmHg to 70 ± 18 [p = 0.001]; HR from 82 ± 12 bpm to 74 ± 18 [p = 0.09]; MSNA from 47 ± 9 bursts/min to 37 ± 13 [p = 0.04] and NDF from 0.92 units to 0.75 [p = 0.02]. Despite a fall in MSNA during presyncope, active vasoconstriction is maintained in the splanchnic arterioles resulting in decreased diastolic blood flow. Stroke volume falls at this time, therefore a relative increase in systolic flow is not contributing to the fall in NDF. The dominant hypotensive mechanism in vasovagal syncope is likely to be splanchnic venous pooling which occurs despite decreased arterial inflow.
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