- Implantation of dual chamber devices in patients with paroxysmal atrial tachyarrhythmias who require permanent pacemakers may lead to significant complications due to an inappropriately triggered ventricular response. VVI/VVIR units cause loss of AV synchrony in the presence of sinus activity. A new DDDR device (THERA DR, model 7940), with an automatic mode switching (AMS) algorithm, was evaluated. When the mean atrial rate is > 182 beats/min, atrial tachyarrhythmia is detected, and AMS is activated. Twenty-three patients (12 males, mean age 71 ± 7 years) underwent implantation of a THERA DDDR device with the AMS algorithm. Seventeen patients had AV block and/ or sick sinus syndrome (SSS) and atrial arrhythmias, and 6 patients (2 with hypertrophic obstructive cardiomyopathy) had SSS and paroxysmal atrial fibrillation (PAF). The follow-up period was from 1–9 months. During follow-up, Holter monitoring and treadmill tests were performed. Results: Eighty-seven episodes of AMS were recorded. Telemetered AMS recordings demonstrated episodes in which the DDDR mode switched to the DDIR mode in the presence of PAF, and reverted to DDDR when sinus rhythm returned. Paroxysmal supraventricuiar arrhythmias with a heart rate < 182 beats/min did not activate tbe mode switch. Conclusions: This early, short-term clinical experience with a DDDR device capable of AMS from DDDR to DDIR demonstrated appropriate clinical function and response to PAF. These preliminary results suggest that DDDR pacemakers with AMS to DDIR may significantly extend the current indications for dual chamber pacing.