Pacing in vasovagal syncope: a physiological paradox?

Abstract CThe physiological principles underlying pacemaker treatment in patients with vasovagal syncope (VVS) have never been reviewed. Current knowledge suggests that pacing the right heart is unlikely to correct blood pressure (BP) during a vasovagal reaction. In adults, the reason for this is that stroke volume (SV) is dictated by central blood volume (CBV) contained in the cardiopulmonary vessels within the chest (i.e. left ventricular preload). Preceding posture-triggered VVS, there is a significant fall in CBV and therefore in SV and cardiac output (CO) long before the onset of bradycardia. This explains why high rate cardiac pacing does not improve CO or BP during presyncope. Contradictory results between physiologic 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.