FAILURE OF REFLEX VENOCONSTRICTION DURING EXERCISE IN PATIENTS WITH VASOVAGAL SYNCOPE

Background In this study, we tested two hypotheses. First, we tested t he hypothesis that reflex constriction of the venous capacitance beds in patients with vasovagal syncope is impaired during both subhypotens ive lower-body negative pressure. Second, we proposed that splenic ven oconstriction may be impaired during exercise in patients with vasovag al syncope. Methods and Results We evaluated 25 patients with vasovaga l syncope (age, 45.0+/-15.9 years; 12 men, 13 women) and 24 control su bjects (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 ne gative pressure and in splenic venous volume during cycle exercise. Ch anges in forearm vascular resistance (FVR) during cycle exercise were assessed with a strain-gauge plethysmography technique. The precentage reduction in unstressed forearm vascular volume during lower-body neg ative pressure was similar in patients and con trol subjects (9.0+/-8. 0% versus 9.7+/-5.9% P=NS). During exercise, splenic venous volume dec reased 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 incre ased 108+/-90% in control subjects (P<.0001). There was no relation be tween percentage change in splenic volume and percentage change in FVR during exercise in either patients or control subjects (r =-.06, P= N S and r=-.18, P=NS, respectively). Conclusions Patients with vasovagal syncope exhibit a failure of the normal increase in tone in the splen ic capacitance bed and in forearm resistance vessels during dynamic ex ercise. Forearm venous tone increases normally during lower-body negat ive pressure.