Sensor detection and pacing for impending reflex syncope

Reflex syncope occurs infrequently but may benefit from pacing intervention. However, reflex syncope poses special problems for successful cardiac pacing. Between episodes, monitoring by the implanted device is required to provide warning of an impending syncope and initiate pacing therapy if appropriate. This article aims to review available devices and their function in different forms of reflex syncope.

The first device dedicated to address detection of VVS was the Rate Drop Response (RDR) (Medtronic Inc., MN, USA). It used a modified rate hysteresis approach to detect falls in heart rate that are faster than physiological but slower than those in cardiac conduction tissue disease. Since the introduction of RDR more has been learnt about the development of vasodepression and cardiac inhibition in vasovagal syncope (VVS). Detection of vasodepression, which occurs substantially earlier than cardioinhibition, assumes great importance. Currently, such detection is only offered by the Closed Loop System (CLS) (Biotronik GmbH, Berlin, Germany). Carotid sinus syndrome (CSS) shows different haemodynamic features from VVS with early vasodepression absent which demands rethinking of device selection and programming. RDR yielded improved management of both VVS and CSS. CLS has demonstrated clear benefits in two important recent randomized controlled VVS pacing trials. So far, no comparative trial between RDR and CLS has been undertaken.

Pacing the bradycardia of reflex syncope requires a full appreciation of its haemodynamic events, including timing of vasodepression and cardioinhibition requiring sophisticated detection and therapy delivery.