ABNORMAL REFLEX VENOUS FUNCTION IN PATIENTS WITH NEUROMEDIATED SYNCOPE

Objectives. We sought to compare the forearm reflex venous response to mental arithmetic stress in patients with neuromediated syncope and i n normal subjects. Background. Patients with neuromediated syncope hav e a paradoxic arterial vasodilation in response to stressors that usua lly provoke vasoconstriction. Given the postulated role of diminished preload in provoking the reflex responses resulting in syncope, we hyp othesized that mental stress might provoke paradoxic reflex venodilati on in patients with neuromediated syncope. Methods. Twelve normal subj ects (mean age [+/-SD] 47 +/- 9 years) and 27 patients with neuromedia ted syncope (mean age 42 +/- 13 gears) were studied before and during a mental arithmetic stress test. Forearm venous pressure-volume relati ons were determined by using radionuclide plethysmography. Results. Du ring mental arithmetic stress, heart rate and systolic and diastolic b lood pressure increased significantly and similarly both in normal sub jects and in patients with neuromediated syncope. The heart rate and b lood pressure changes were qualitatively similar in both groups. Howev er, with mental arithmetic stress, forearm venoconstriction of 13 +/- 2% (mean +/- SEM) was noted in normal subjects (p < 0.001) but not in patients with neuromediated syncope (mean 2%, p = NS). This group resp onse of patients with neuromediated syncope did not result from a lack of individual responses but occurred because these patients had a wid e range of responses. The normal physiologic and methodologic variabil ity of the method was +/-4%. Thirteen of the 27 patients with neuromed iated syncope had forearm venoconstriction of 14.5 +/- 6.8% during men tal arithmetic stress,whereas 7 had paradoxic forearm venodilation of 14.6 +/- 8.8%, and 7 were considered nonresponders (-1.3 +/- 3.4%). Th us, 14 (52%) of the 27 patients with syncope did not have normal vasoc onstriction in response to mental stress. Conclusions. Patients with n euromediated syncope have an abnormal range of forearm venomotor respo nses to mental arithmetic stress. Reflex control of the veins may play an important role in the pathogenesis of neuromediated syncope.