Pacing in vasovagal syncope: A physiological paradox?

The physiological principles underlying pacemaker treatment in patients with vasovagal syncope have never been reviewed. Current knowledge suggests that pacing the right heart is unlikely to correct blood pressure during a vasovagal reaction. In adults, the reason for this is that stroke volume is dictated by central blood volume contained in the cardiopulmonary vessels within the chest (ie, left ventricular preload). Preceding posture-triggered vasovagal syncope, there is a significant fall in central blood volume and therefore in stroke volume and cardiac output long before the onset of bradycardia. This explains why high rate cardiac pacing does not improve cardiac output or blood pressure during presyncope. Contradictory results between physiological theory and trial evidence underlying pacemaker treatment at present cannot be explained. Placebo effects during pacing for vasovagal syncope should be considered. More work is needed to solve the dilemma.