Dual Demand Pacing Using Retriggerable Refractory Periods for ...

REVIEW

Dual Demand Pacing Using Retriggerable Refractory Periods for Ventricular Rate Control During Paroxysmal Supraventricular Tachyarrhythmias in Patients with Dual Chamber Pacemakers SHENTHAR JAYAPRAKASH, PAUL B. SPARKS, JONATHAN M. KALMAN, and HARRY G. MOND From the Department of Cardiology, The Royal Melbourne Hospital, Victoria, Australia

JAYAPRAKASH, S., ET AL.: Dual Demand Pacing, Using Retriggerable Refractory Periods for Ventricular Rate Control During Paroxysmal Supraventricular Tachyarrhythmias in Patients with Dual Chamher Pacemakers: A Review. The use of dual chamber pacing in patients with atrioventricular block and paroxysmal supraventricular tachyarrhythmias may present a clinical dilemma because of the rapid and erratic triggering of ventricular pacing. To avoid this, a variety of pacing methods have now been described, including the use of retriggerable atrial refractory periods or dual demand pacing. This review details the use, advantages, and limitations of this poorly understood algorithm referred to as "pseudo-mode switching." (PACE 2000; 23:1156-1163) dual demand pacing, dual chamber pacemakers, supraventricular Introduction Dual chamber pacing (DDDR, VDDR) is being increasingly used in preference to single chamber pacing (VVIR) as the restoration of atrioventricular (AV) synchrony not only improves the hemodynamic and functional status of pacemaker patients, but is also associated with a reduced incidence of pacemaker syndrome, atrial fibrillation, stroke, and mortality.^"^ A substantial number of patients requiring dual chamber pacemakers also have associated paroxysmal supraventricular tachyarrhythmias (PSVTs). These patients include those with atrioventricular (AV) block, the tachycardia-bradycardia syndrome, or atrial fibrillation who have undergone AV junction ablation. Previously, the presence of PSVT, particularly in the presence of AV block, was considered a contraindication for DDDR or VDDR pacing, as tracking the PSVT resulted in rapid, erratic ventricular pacing with potentially deleterious hemodynamic consequences.^•^° A number of pacing methods to deal with this situation are now available includAddress for reprints: Harry G. Mond, M.D., Suite 22, Private Medical Centre, The Royal Melbourne Hospital, Victoria, 3050, Australia. Fax: 61-3-9347-6760; e-mail: hmond@ bigpond.net.au Received October 23, 1998; revised December 14, 1998; accepted October 14, 1999.

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ing DDIR pacing, limiting the upper tracking rate and/or total atrial refractory period (TARP), SmarTracking® (Intermedics, Inc., Freeport, TX, USA), and automatic mode conversion or switching. ^^ The use of retriggerable or resettable atrial refractory periods (RARPs), otherwise known as dual demand pacing, is another method designed to avoid a rapid ventricular response in the presence of PSVT. Dual demand pacing is available in certain Biotronik (Biotronik Gmbh & Co, Woermannkehre 1, Berlin, Germany) and Telectronics (Telectronics Pacing Systems, now a St. Jude Medical Company, Sylmar, CA, USA) pacemaker models. The Use of Retriggerahle Refractory Periods for Antitachycardia Pacing One of the earliest and simplest methods of pace termination of PSVT was asynchronous atrial or ventricular pacing during the tachycardia. Custom-built, physician or patient-activated pacemakers with underdrive or overdrive capability were used in the early 1970s to treat certain patients with PSVT.^^" During the late 1970s, automatic, single chamber, dual demand pacemakers were successfully used in the treatment of drug refractory PSVT."-^^ These predominantly atrial pacemakers, paced at 70 ppm, not only when the sensed heart rate was < 70 beats/min, but also when it exceeded an upper sensed limit such as

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RARPS FOR VENTRICULAR RATE CONTROL DURING PSVT WITH DUAL CHAMBER PACEMAKERS 150 beats/min. Hence the term "dual demand." In pulse generators demonstrating dual demand function, the interference reversion timing interval within the refractory period was set at 400 ms instead of the standard 200 ms. Electrical potentials sensed within this interference window were regarded as a tachyarrhythmia, and provided these potentials, continued to be sensed. The pulse generator responded as asynchronous underdrive pacing. During the 1980s, dual demand AV sequential pacing was introduced for drug resistant pgy-p 18,19 -pj^jg £QJ,J^ Q£ automatic underdrive DOO pacing, however, failed to become popular because of concomitant advances in curative surgical and catheter ablation techniques. The Use of Retriggerable Atrial Refractory Periods for Ventricular Rate Control During PSVT

The use of dual chamber pacing in patients with PSVT rekindled interest in dual demand pacing as an algorithm for ventricular rate control during PSVT.^° For dual demand pacing to allow ventricular rate control during PSVT, the following characteristics should be satisfied:

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Figure 1. Schematic diagram to demonstrate dual demand pacing with Biotronik pacemaker models during paroxysmal supraventricular tachyarrhythmia (PSVT). In these pulse generators the retriggerable atrial refractory period (RARP) is the total atrial refractory period (TARP) and defines the detection and intervention interval. Once an atrial electrical potential is sensed in this period, a new TARP is triggered and provided these atrial signals continue to be sensed, the atrial channel remains refractory. At the end of the low rate or sensor-indicated interval (LRI), the atrium is paced (Ap) and an atrioventricular (AV) delay starts, which is terminated by ventricular pacing (Vp), provided there is no sensed spontaneous ventricular activity. DVIR pacing is present during the period of sensed PSVT. After Ap and Vp there are 56-ms atrial blanking periods.

1, There must be a RARP, which defines the detection and intervention rate. 2, During PSVT, the intervention rate is asynchronous atrial pacing (i.e., DVIR). 3, Following PSVT reversion, the pacing system should revert to, and perform as, a standard dual chamber pacemaker.

grammable and designed specifically for PSVT. The nonprogrammable Telectronics algorithm has been present for many years in all dual chamber models and was designed primarily for interference reversion (Table I).

Knowledge of the timing cycles of these dual chamber pacemakers is essential not only to program dual demand pacing where possible, but also to understand the complexities of this seemingly simple algorithm. During DDDR pacing, initiation of the dual demand algorithm is achieved by sensing electrical potentials within the RARP. This period is then retriggered without starting an AV delay (Fig. 1). For example, a RARP of 400 ms will trigger dual demand pacing for atrial rates > 150/min. At the end of the programmed low rate interval (DVI) or the sensor-indicated interval (DVIR), an atrial pulse is delivered and an AV delay initiated. Consequently, for the duration of the PSVT, DVIR pacing is established and rapid atrial triggered ventricular pacing prevented. For pacemakers programmed VDDR, atrial pacing is not possible and thus the response is VVIR. Although two pacing companies have dual chamber pacing systems demonstrating dual demand function, the designs and clinical objectives are quite different. The Biotronik algorithm is pro-

These dual chamber pulse generator models have dual demand pacing as a programmable option. In pacing modes where sensed atrial signals can trigger ventricular pacing, dual demand DVIR pacing is initiated by the sensing of an atrial signal in the RARP. For Biotronik pacemakers, the TARP, which is the sum of the AV delay and the postventricular atrial refractory period (PVARP), forms the RARP, This period is programmable from 775 to 200 ms in 25-ms steps, which sets the intervention rates from 77 to 300 ppm. Unlike other dual chamber pacemakers, where the PVARP is programmable, the PVARP in these models can only be altered by separately programming the TARP and the AV delay. The AV delay can also be programmed to respond physiologically to changes in the spontaneous atrial rate (dynamic AV delay]. When programmed ON, shortening of the AV delay will occur with increasing atrial rates such as with exertion or a PSVT and because the TARP is fixed, widening of the PVARP will occur. Because dynamic AV delay is applica-

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Biotronik (Dromos®, Physios®, Actros®, and Eikos® Families)

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JAYAPRAKASH, ET AL. Table I.

Automatic mode switching Dual demand Programmable ON/OFF Response Intervention interval (TARP) Separate noise window Programmable atrial blanking period After A pace After A sense After V pace After V sense Programmable ventricular blanking period Dynamic hysteresis

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Telectronics

No Yes Yes DDDR to DVIR VDDR to VVIR Programmable; 250-750 ms Yes; 125 ms No 56 ms 0 ms 56 ms 0 ms Yes; 12-72 ms

Yes; VVIR Yes No DDDR to DVIR VDDR to VVIR Fixed; 240 ms No No 120 ms 120 ms 150 ms 150 ms No; 100 ms

Yes; - 6 , - 5 0

No

TARP = total atrial refractory period.

ble to atrial sensing and pacing, AV delay shortening will also occur during dual demand DVIR pacing and exertion. Chest wall stimulation can be used to test the dual demand algorithm by mimicking the PSVT (Fig. 2).^^ By using the ECG interpretation channel, the timing intervals can be confirmed. Following atrial and ventricular pacing stimuli, there are 56-ms blanking periods that lie within, but are independent of, the TARP (Table I, Fig, 1), These blanking periods are not reset during dual demand pacing to allow the maximum sensing window during the PSVT. The timing of the atrial blanking periods is particularly important in patients with paroxysmal atrial flutter. Flutter waves may on occasion fall within these blanking periods and are consequently not sensed. The next flutter wave may then fall outside the RARP and be sensed in a normal fashion. This starts an AV delay and a premature ventricular paced beat results (Fig. 3).^^ Because of the influence of the dynamic AV delay, the measured AV delays following sensing of an atrial flutter wave and dual demand atrial pacing can be shown to markedly differ. The sensed rapid atrial flutter waves have a short AV delay, whereas dual demand atrial pacing demonstrates the maximum AV delay while the patient is at rest (Fig. 3). When such timing issues are encountered with atrial flutter, the problems can occasionally be overcome by adjusting the TARP or the AV delay to allow all appropriate atrial flutter waves to be sensed. This highlights the high sensitivity but low specificity of dual demand pacing, which requires only one impulse within or outside the 1158

RARP to activate or terminate the algorithm. This continual oscillation between normal dual chamber and dual demand function is difficult to manage with an algorithm that requires only a single atrial impulse for activation or termination. One of the advantages of using DVIR pacing during dual demand pacing is the ability to pace the ventricle in a rate adaptive manner during PSVT. However, to achieve this effectively, it may become necessary to program a high sensor gain or rate response so that during activity with sinus rhythm there may be competitive sensor-driven atrial pacing. With the Biotronik dual chamber models, there is a programmable function called dynamic hysteresis that encourages P wave tracking rather than sensor-driven pacing when free of the PSVT. This feature, which is linked to the sensor-driven rate, creates a hysteresis between it and the sinus rate of a programmable 0 ppm (programmed OFF) to - 5 0 ppm depending on the pulse generator model. When programmed ON, the patient remains in P wave tracking, rather than sensor-driven pacing unless the sinus rate falls below the hysteresis limit. If this occurs, then the sensordriven rate functions as programmed. For example, when dynamic hysteresis -12 ppm is programmed and the calculated sensor-driven rate is 112 ppm, then the patient will retain P wave tracking, unless the sinus rate falls below 100 ppm. In this situation, sensor-driven pacing at 112 ppm commences. If the patient should then go into PSVT, the DVIR rate will be 112 ppm. Dynamic hysteresis is particularly useful in patients with paroxysmal chronotropic incompetence or therapeutic beta blockade and catecholamine induced PSVT.

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Figure 2. Dual demand pacing response of the Physios CTM 01 DDD pacemaker to chest wall stimulation (CWS). The pulse generator has been programmed DDD with the atrial channel unipolar sensing at maximum sensitivity to facilitate detection of CWS. The low rate has been programmed 80 ppm, the upper rate 150 ppm, and the AV delay 160 ms. There is bipolar ventricular sensing and both channels have unipolar pacing, so that the stimulus artefacts can be seen on the surface ECG. The Riotronik EPR 1000 programmer printout demonstrates simultaneously from above: ECG interpretation channel, surface EGG lead II, calibrated atrial intracardiac electrogram (A lEGM) and the ventricular intracardiac electrogram (V IEGM). Below, there is a sim ultaneous high quality surface EGG, lead II. The GWS is delivered usingan external programmed stimulator at a rate of 300 ppm (200 ms) and is recognized during refractory sensing as A,js. The MC also differentiates between atrial pacing (Ap), atrial sensing (As), ventricular pacing (Vp), and ventricular sensing (Vs). (Above) Dual demand pacing programmed OFF. The GWS is detected by the atrial channel and tracked by the ventricle with 2:1 block, close to the upper rate of 150 ppm. (Relow) Dual demand pacing programmed ON, with a RARP of 400 ms (intervention rate 150 ppm). The GWS continually retriggers the RARP and the pacing is DVI, thus avoiding rapid ventricular rates. RARP = retriggerable atrial refractory period. PACE, Vol, 23

Figure 3. EGG interpretation channel, EGG lead II, atrial electrogram, and ventricular electrogram demonstrating the dual demand pacing response of the Physios GTM 01 DDD pacemaker to paroxysmal atrial flutter at a rate of approximately 240 ppm. The pulse generatorhas been programmed DDD, lowrate 70 ppm, upper rate 120 ppm, and the pace AV delay 180 ms. Dynamic AV delay (physiological AV shortening) has been programmed ON at medium. The legend to the EGG interpretation channel is described in Figure 2. (Top) Dual demand programmed OFF. Alternate flutter waves are sensed resulting in ventricular pacing at the upper rate limit. (Bottom) Dual demand programmed ON. The fourth and eighth ventricular paced beats are due to tracking of the atrial flutter waves, which fall outside the RARP resulting in an irregular ventricular paced rhythm. The atrial flutter wave immediately prior to each of these tracked waves falls within the atrial blanking period, thus terminating dual demand pacing. The next flutter wave is sensed and because the Dynamic AV delay has been programmed ON, the actual AV delay is shortened to 100 ms, which is the calculated value for atrial rates between 111 and 130 ppm. Note that for dual demand DVIR pacing, the AV delay remains fixed at 180 ms because the patient is at rest. AV = atrioventricular.

With all Biotronik models, there are also, within the atrial and ventricular refractory periods, interference or noise detection windows of 125 ms, referred to as Logical A blanking, which are separate from the RARP, The sensing of signals in these windows results in asynchronous pacing in the corresponding chamber.

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JAYAPRAKASH, ET AL. Telectronics (Quadra®, Aurora®, Reflex®, Meta®, and Tempo® Families) All Telectronics dual chamber pacemakers exhibit dual demand pacing under certain conditions. The function is nonprogrammable and is present by default. Dual demand pacing can be demonstrated in Telectronics DDD models (Quadra®, Aurora®, and Reflex®) and the minute ventilation sensor DDDR models (Meta® and Tempo®). The dual demand function depends on a design peculiarity of the noise detection algorithm. These pacemakers have a fixed RARP of 240 ms, which was primarily designed as an interference or noise detection interval (Fig. 4). The sensing window is further narrowed by the 120-ms atrial blanking periods making the algorithm suitable mainly for atrial fibrillation and maybe atrial flutter (Table I). Electrical potentials sensed within this narrow window reverts only the atrial channel to asynchronous pacing, while the ventricular channel functions in the inhibited mode resulting in DVIR pacing. Because of the fixed short RARP, Telectronics dual chamber pacemakers can only sense PSVT at rates of > 250 ppm. The Meta® and Tempo® famiAtrial Blanking 120 150 ms

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Figure 4. Sciiematic diagram to demonstrate dual demand pacing with the Teiectronics Quadra^', Aurora®, Reflex®, Meta, and Tempo® families of dual chamber pacemakers during a paroxysmal supraventricular tachyarrhythmia (PSVT) such as atrial fibrillation. The 240-ms retriggerable atrial refractory period (RARP) defines the detection and intervention interval. Once an atrial electrical potential is sensed in this period, a new RARP is triggered and provided these atrial signals continue to be sensed, the atrial channel remains refractory. At the end of the low rate or sensorindicated interval (LRI), the atrium is paced (Ap) and an atrioventricular (AV) delay starts, which is terminated by ventricular pacing (Vp), provided there is no sensed spontaneous ventricular activity. DVIR pacing is present during the period of sensed PSVT. Atrial blanking periods are present after Ap (120 ms) and Vp (150 ms). Atrial blanking periods after atrial sensing are not shown. 1160

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Figure 5. Dual demand and Automatic Mode Switching® (AMS®) pacing response of the Telectronics Meta® DDDR model 1256D in a patient who developed atrial fibrillation. (Above) DDD mode, AMS® OFF, low rate 70 ppm, upper rate 120 ppm, and an atrioventricular (AV) delay of 200 ms. The Telectronics 9602 programmer printout demonstrates simultaneously from above: the events recording, calibrated atrial electrogram, and ECG lead IL On the events recording, the solid rectangles denote the atrial (above) and ventricular (below) blanking periods, and the open rectangles the atrial and ventricular refractory periods. Also shown are atrial (above) and ventricular (below) pacing *, sensing 0, and the sensed noise 01, which are the atrial fibrillation waves. The shortened open rectangles (240 ms) following sensed atrial noise is the retriggerable atrial refractory period (RARP) that starts after each atrial sensed event. At the end of the low rate interval an atrial stimulus is delivered, the AV delay starts and is followed by a ventricular paced event, so that pacing is DVI. In this illustration, the atrial electrogram voltage amplitudes are seen to oscillate resulting in intermittent atrial undersensing. Midway through the tracing, where the atrial electrogram is of low voltage, there is no atrial sensing in the RARP, resulting in an increase in ventricular pacing rate due to atrial sensing outside this period. In this case study, the ventricular pacing rate was always chaotic, when mode switching was turned OFF. (Relow) DDD mode, AMS® ON (200 ppm, 5 beats). Otherwise same program as above. Despite intermittent failure of atrial sensing, AMS® is maintained and the ventricular pacing is regular. lies of minute ventilation rate adaptive dual chamber pacemakers also have Automatic Mode Switching® (AMS®) as an algorithm to control the ventricular pacing rate in response to PSVT. In the first generation minute ventilation sensor model, the Meta® DDDR 1250 (H), the AMS® feature was nonprogrammable and like the dual demand algorithm, responded immediately to a single atrial impulse in the appropriate sensing window. Consequently, the dual demand function could not be

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RARPS FOR VENTRICULAR RATE CONTROL DURINC PSVT WITH DUAL CHAMBER PACEMAKERS

demonstrated. With subsequent Meta® and Tempo® DDDR models, with the AMS® feature programmed OFF, default dual demand pacing can be demonstrated in patients with PSVT at rates > 250 ppm (Fig. 5). The addition of a nonprogrammable dual demand algorithm to the dual chamber range of Telectronics pacemakers has been useful clinically. In patients with Telectronics dual chamber models (AMS® when present, programmed OFF) who develop atrial fibrillation or flutter and present for pacemaker testing, the dual demand function can be demonstrated using the ECG interpretation channel and electrograms (Fig. 5). These patients are generally not clinically compromised and rarely note palpitations. The difference between dual demand pacing and AMS®has been demonstrated in Figvire 5. With atrial fibrillation, there is frequently oscillation in the size of the atrial electrogram resulting in short periods of atrial undersensing. The AMS® algorithm can usually cover this and regular ventricular pacing is maintained. However, with dual demand pacing, a single missed atrial impulse in the RARP is sufficient to terminate dual demand pacing and for the next atrial impulse to trigger premature ventricular pacing. To some extent, this is less of a problem with the longer programmable TARPs available in the Biotronik devices. In patients with PSVT other than atrial fibrillation or flutter, dual demand pacing may not be triggered because of the fixed and short RARP of 240 ms. Atrial sensing outside this window due to a slower PSVT results in rapid ventricular rates, which can be usually corrected with an AMS® algorithm. Discussion The use of dual chamber pacing in patients with AV block and PSVT may result in inappropriate pacemaker-mediated tachyarrhythmias. The introduction of mode switching or mode conversion algorithms has been successful and has widened the indications for dual chamber pacemakers.^^'^^ To fulfil the definition of mode switching, three criteria must be fulfilled: 1. The dual chamber pacemaker must be able to detect the presence of rapid atrial rates and automatically switch from an atrial-tracking mode to an atrial nontracking mode (DDIR or VVIR). 2. Atrial monitoring must be retained during the atrial nontracking mode. 3. There must be automatic restoration of atrial-tracking upon cessation of the PSVT. Successful mode switching algorithms require accurate detection of the PSVT. There are a variety of tachyarrhythmia detection algorithms used by different pacemaker companies to initiate and terminate mode switching. ^^"^'^ PACE, Vol. 23

The dual demand feature of the Biotronik dual chamber devices conforms to the described criteria for mode switching with one major exception. In dual demand pacing, the atrial channel responds as if it is continually refractory. Since there is no microprocessor-based pacing mode change and asynchronous atrial pacing continues during dual demand pacing (DVIR), we have preferred to regard this form of pacing as "pseudomode switching." However, dual demand pacing works well in most situations to control the rapid triggered ventricular pacing in the presence of PSVT. Its ability to be activated promptly may be preferred by some patients who experience shortlived PSVT that may fail to mode switch with algorithms requiring longer switch times.^^ Although both Biotronik and Telectronics dual chamber pacemaker models demonstrate dual demand pacing, they are, nevertheless, very different algorithms. The dual demand function in the Biotronik models is programmable and can be tailored for a range of PSVT rates. The Telectronics algorithm is nonprogrammable and was primarily designed to protect the heart from rapid ventricular triggered pacing due to sensing of interference in the atrial channel. Chest wall stimulation is a useful method to evaluate dual demand pacing, and can be used to test the intervention interval.^^ For the technique to be successful, atrial sensing must be programmed unipolar with high sensitivity. Ventricular sensing in turn, should remain bipolar and within a normal sensing range (Fig. 2). A major advantage of dual demand pacing is the rapid and reliable activation by PSVT. The change from DDDR to DVIR pacing prevents rapid ventricular triggered pacing in patients with PSVT. It is also possible that asynchronous atrial pacing may terminate a reentrant PSVT, although in some instances it may initiate atrial fibrillation. Following termination of the PSVT, by whatever mechanism, DVIR pacing will provide satisfactory atrial pacing support in patients with sinus node dysfunction and chronotrophic incompetence. However, the reversion to normal atrial sensing is virtually instantaneous provided the sinus rate exceeds the programmed low rate. One of the major disadvantages of dual demand pacing with the Biotronik family of pacemakers is the linking of the intervention rate to the TARP, which introduces a number of unique problems. Programming a short TARP or AV delay is necessary to set a higher intervention rate. This may be important to prevent sensing of sinus tachycardia or to allow a higher tracking rate. To achieve this, a short PVARP becomes necessary and this may not be desirable in patients with retrograde VA conduction. For example, a TAl^ of 400 ms sets

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the intervention rate to > 150 ppm. If the AV delay is programmed to 160 ms, the resulting PVARP is 240 ms. This value will need to be shortened further if an intervention rate is set higher. Thus, in patients vi^ith retrograde VA conduction, this creates the scenario for a possible pacemaker mediated tachycardia. Considering that approximately 35%-50% of all patients with AV block requiring permanent pacemakers retain retrograde conduction, this disadvantage may be significant.^'' An alternative consideration to prevent retrograde conduction is the careful programming of the dynamic AV delay and the premature ventricular complex activated TARP extension. This will allow a longer more physiological AV delay at rest. Another disadvantage of dual dernand pacing relates to the sensitivity and specificity of the algorithm. Although the algorithm is highly sensitive, it nevertheless is relatively nonspecific. Even a single atrial sensed event in the TARP triggers dual demand pacing, and frequent atrial ectopics or ventricular ectopics with retrograde VA conduction may result in unphysiological DVIR or VVIR pacing. A similar situation was seen with the Telectronics Meta® 1250(H) model, where a single sensed atrial event caused mode switching. This necessitated the introduction of an atrial event count to trigger AMS® in later models.^^ Additionally, in patients with a slower PSVT, paroxysms of inappropriate rapid ventricular triggered

paced beats can occur when sensed events fall outside the TARP and blanking period. To overcome this, a number of additional algorithms such as SmarTracking® and selectively limiting the upper tracking limit, but not the upper sensor limit, have been introduced to work in conjunction with mode switching to cover a broader range of PSVT." These algorithms, however, are not available with Biotronik or Telectronics dual chamber pacing systems. Conclusion In patients with AV block, dual chamber pacemakers have gained widespread use with growing evidence of the beneficial effects of maintaining AV synchrony. One of the many advances in dual chamber pacemakers has been the incorporation of various algorithms, which can recognize and deal with PSVT. Dual demand pacing or "pseudo-mode switching" is one such algorithm that has been designed to prevent rapid triggered ventricular rates. The algorithm is simple and effective, but it has certain limitations. As with all pacemaker functions, it is essential to understand the algorithm to institute appropriate programming. Acknowledgments: The authors thank Dr. Nawzer Mehta Ph.D. and Mr. Gregg Harris of Biotronik Inc., USA for their continual help and constructive comments.

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RARPS FOR VENTRICULAR RATE CONTROL DURING PSVT WITH DUAL CHAMBER PACEMAKERS 23. Mayumi H, Uchida T, Shinozaki K, et al. Use of a dual chamber pacemaker with a novel fallback algoritbm as an effective treatment for sick sinus syndrome associated vfith transient supraventricular tachycardias. PACE 1993; 16:9921000. 24. Den Dulk K, Dijkman B, Pieterse M, et al. Initial experience with mode switching in a dual sensor, dual chamber pacemaker in patients with paroxysmal atrial tachyarrhythmias. PACE 1994; 17:1900-1907.

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25. Osyshcher I, Katz A, Bondy C. Initial experience with a new algorithm for automatic mode switching from DDDR to DDIR mode. PACE 1994; 17:1908-1912. 26. Provenier F, Jordaens L, Verstraeten T, et al. The automatic mode switching in successive generations of minute ventilation sensing dual chamber rate responsive pacemakers. PACE 1994; 17:1913-1919. 27. Barold SS, Zipes DP. Heart disease. In E Braunwald (ed.); A Textbook of Cardiovascular Medicine, 5th Edition. WB Saunders and Company, Philadelphia, PA, 1996, p. 718.

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