Effectiveness and Limitations of the Fallback I Algorithm for Transient Supraventricular Tachyarrhythmias in DDD Pacing

: To examine the function of the Fallback I algorithm (Chorus I, Ela Medical Inc., Montrouge, France), which automatically changes the DDD mode to VDI during transient supraventricular tachyarrhythmias to prevent high-rate tracking of the ventricle, a total of 45 patients who were preoperatively diagnosed with sick-sinus syndrome (SSS) (Group 1; n = 19) or with advanced or complete atrioventricular block (AVB) (Group 2; n = 26) were followed up and analyzed. Mean follow-up times (mean ± SD) were 22.4 ± 9.7 and 12.4 ± 10.9 months, respectively. Each of the groups was further divided into subgroups according to the preoperative existence of paroxysmal atrial fibrillation (or flutter) (PAF) or paroxysmal supraventricular tachyarrhythmia (PSVT). During follow up, the fallback started through a given cycle of ventricular pacing at an upper-rate limit (URL) to avoid a continuous high-rate tracking during the arrhythmias specifically in the patients with PAF (17/18), but it did not start in the patients with PSVT (0/4) (p = 0.0004). The fallback, however, sometimes started in the patients who had AVB not associated with either PAF or PSVT (4/21) during normal exercise because the fallback algorithm did not distinguish a normal P wave from the abnormal atrial waves. To further clarify the behavior of the fallback, a treadmill test was conducted in 25 of the 45 patients. The fallback start was observed in 12 of the 17 patients with AVB (Group 2). In such patients, the use of a long fallback delay and/or a high URL setting prevented the fallback starts during normal exercise. These results suggest that, in DDD pacing, the fallback algorithm in Chorus I is useful in patients with SSS or AVB to avoid high-rate tracking of the ventricle during transient supraventricular tachyarrhythmias, but special care must be taken to avoid the fallback starts during the normal exercise in AVB patients.