Use of a new fallback function to prevent endless-loop tachycardias: first clinical results. The Investigators of the Multicenter Study.

The methods used for preventing endless-loop tachycardias (ELTs) most often consist of initiating a long postventricular atrial refractory period (PVARP) with the sensing of every event likely to induce ELTs, such as sensed premature ventricular contractions (PVCs). A new fallback function may be useful to prevent the initiation of ELTs. A window of atrial rate acceleration detection (WARAD) is initiated with the sensing of every sinus event and equals 75% of the preceding PP interval. If an atrial event is sensed during this period, as are premature atrial contractions (PACs), no atrioventricular (AV) delay is initiated, but an atrial pulse output is delivered and a subsequent 31-msec AV delay is started. Theoretically retrograde P waves are premature compared to sinus rhythm. They are therefore detected as PACs, and do not initiate AV delay, thus prohibiting the induction of ELTs. This function was tested in six patients, using external or implanted Chorus 2 pacemakers. Short PVARP (203 msec) and high atrial sensibility were programmed. Retrograde conduction was induced either by inefficient atrial pacing or a long programmed AV delay. Two different dual chamber settings were tested: dual chamber pacing with the fallback function On or Off. In every situation, the function proved effective in preventing ELTs: the number of tachycardia episodes went from 124 with the function programmed Off to 5 with the function programmed On for comparable durations. More than 75 ELTs effectively prevented by fallback have been recorded.