Failure of Reflex Venoconstriction During Exercise in Patients With Vasovagal Syncope

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Abstract
Background In this study, we tested two hypotheses. First, we tested the hypothesis that reflex constriction of the venous capacitance beds in patients with vasovagal syncope is impaired during both subhypotensive lower-body negative pressure. Second, we proposed that splenic venoconstriction may be impaired during exercise in patients with vasovagal syncope. Methods and Results We evaluated 25 patients with vasovagal syncope (age, 45.0±15.9 years; 12 men, 13 women) and 24 control subjects (age, 41.3±13.7 years; 16 men, 8 women). A nuclear technique was used to assess changes in forearm venous tone during lower-body negative pressure and in splenic venous volume during cycle exercise. Changes in forearm vascular resistance (FVR) during cycle exercise were assessed with a strain-gauge plethysmography technique. The percentage reduction in unstressed forearm vascular volume during lower-body negative pressure was similar in patients and control subjects (9.0±8.0% versus 9.7±5.9%, P =NS). During exercise, splenic venous volume decreased less in patients than in control subjects (15.8±21.7% versus 42.6±12.6%, P <.0001). FVR decreased by 2±32% in patients but increased 108±90% in control subjects ( P <.0001). There was no relation between percentage change in splenic volume and percentage change in FVR during exercise in either patients or control subjects ( r =−.06, P =NS and r =−.18, P =NS, respectively). Conclusions Patients with vasovagal syncope exhibit a failure of the normal increase in tone in the splenic capacitance bed and in forearm resistance vessels during dynamic exercise. Forearm venous tone increases normally during lower-body negative pressure.