Baroreflex Sensitivity in Patients With Vasovagal Syncope

Background In the present study, we tested the hypothesis that baroreflex sensitivity is reduced in patients with vasovagal syncope compared with normal control subjects. Methods and Results We investigated 30 patients with vasovagal syncope (mean age, 43.6±16.7 years; 14 men and 16 women) and 32 normal control subjects (mean age, 41.8±17.0 years; 24 men and 8 women). Cardiopulmonary baroreceptor sensitivity was assessed by measuring the change in forearm vascular resistance during subhypotensive lower body negative pressure (LBNP). Carotid baroreflex sensitivity was assessed by measuring the change in RR interval during the manipulation of carotid transmural pressure. Phenylephrine baroreceptor sensitivity was assessed on the basis of the linear regression slope of the RR interval versus systolic arterial blood pressure during the increment in blood pressure after intravenous administration of phenylephrine. In patients with vasovagal syncope, during the application of −10 mm Hg LBNP, forearm vascular resistance decreased by 0.7±11.6 U versus an increase of 8.3±6.2 U in control subjects ( P =.002). Phenylephrine baroreceptor sensitivity was 11±7 ms/mm Hg in patients versus 14±6 ms/mm Hg in control subjects ( P =NS). Carotid baroreflex sensitivity was 4±6 versus 4±2 ms/mm Hg in patients and control subjects, respectively ( P =NS). Conclusions In patients with vasovagal syncope, during the application of subhypotensive LBNP, there is impaired forearm vasoconstriction or paradoxical forearm vasodilation. This suggests impaired cardiopulmonary baroreceptor inactivation or paradoxical activation of these receptors and is consistent with reduced cardiopulmonary baroreceptor sensitivity.