1094 July 2002 PACE, Vol. 25, No. 7 Introduction The hemodynamic and clinical usefulness of automatic mode switching (AMS) to control the ventricular pacing rate of dual chamber pacemak- ers was reviewed previously. 1 The clinical behav- ior and programmability of the various types of AMS algorithms are reviewed in this part of the ar- ticle. Current AMS algorithms can be classified ac- cording to the way atrial tachyarrhythmias are de- tected: (1) “Rate cutoff” criterion: the sensed atrial (As) rate exceeds a programmable value; (2) “Run- ning average rate” criterion: the atrial rate exceeds a mean atrial rate calculated by the pacemaker from the duration of the preceding atrial rate; (3) “Sensor-determined” physiological rate to distin- guish sinus rhythm from atrial tachyarrhythmia; and (4) Complex algorithms that combine one or more of the above criteria, with or without addi- tional methods such as examining the atrioven- tricular (AV) relationship. Illustrative Types of AMS (Table I) This discussion focuses mainly on atrial tach- yarrhythmia detection by algorithms developed by several major pacemaker companies to illus- trate their complexity and evaluation (Table I). Clinical results on these algorithms are presented when available. Telectronics AMS Function The Telectronics (St. Jude Medical, St. Paul, MN, USA) algorithms give an historical perspec- tive of AMS development. The first algorithm (Model 1250, Meta DDDR) used the postventricu- lar atrial refractory period (PVARP) to monitor atrial tachyarrhythmia, and an atrial event sensed in the PVARP resulted in a mode switch from DDDR to VVIR. The PVARP duration was further controlled by the minute ventilation sensor, giv- ing a sensor determined rate band for defining a pathological rate. 2 While this algorithm gave an immediate AMS response at the onset of atrial tachyarrhythmia, it suffered from frequent unde- sirable mode changes in the presence of sinus tachycardia and frequent atrial premature beats. In one report on 24 patients, 3 Holter monitoring showed that patients spent up to 50% of the time in effective VVIR pacing due to low specificity of Automatic Mode Switching of Implantable Pacemakers: II. Clinical Performance of Current Algorithms and Their Programming CHU-PAK LAU,* SUM-KIN LEUNG,† HUNG-FAT TSE,* and S. SERGE BAROLD‡ From the *Cardiology Division, Department of Medicine, Queen Mary Hospital, and the Institute of Cardiovascular Science and Medicine, Faculty of Medicine, University of Hong Kong, †Kwong Wah Hospital, Hong Kong, and the ‡Broward General Hospital, Ft. Lauderdale, Florida LAU, C.-P., ET AL.: Automatic Mode Switching of Implantable Pacemakers: II. Clinical Performance of Current Algorithms and Their Programming. While the hemodynamic and clinical significance of auto- matic mode switching (AMS) in patients with pacemakers has been demonstrated, the clinical behavior of AMS algorithms differ widely according to the manufacturers and pacemaker models. In general, a “rate-cutoff” detection method of atrial tachyarrhythmias provides a rapid AMS onset and resynchro- nization to sinus rhythm at the termination of atrial tachyarrhythmias, but may cause intermittent oscil- lations between the atrial tracking and AMS mode. This can be minimized with a “counter” of total num- ber of high rate events before the AMS occurs. The use of a “running average” algorithm results in more stable rate control during AMS by reducing the incidence of oscillations, but at the expense of delayed AMS onset and resynchronization to sinus rhythm. Algorithms may be combined to fine tune the AMS re- sponse and to avoid rapid fluctuation in pacing rate. Appropriate programming of atrial sensitivity, and the avoidance of ventriculoatrial cross-talk are essential for optimal AMS performance. (PACE 2002; 25:1094–1113) automatic mode switching, dual chamber pacemakers, atrial flutter, atrial fibrillation, pacemaker programming Address for reprints: C.P. Lau, M.D., Cardiology Division, De- partment of Medicine, University of Hong Kong, Queen Mary Hospital, Hong Kong. Fax: (852) 2855-1143 or (852) 2818-6304; e-mail: cplau6hkucc.hku.hk Received May 29, 2001; revised August 28, 2001; accepted September 4, 2001. Reprinted with permission from JOURNAL OF PACING AND CLINICAL ELECTROPHYSIOLOGY , Volume 25, No. 7, July 2002 Copyright © 2002 by Futura Publishing Company, Inc., Armonk, NY 10504-0418.