The management of paroxysmal tachycardias using the Cybertach-60

Pacing therapy for atrial tachyarrhythmias

The use of pacing techniques for the treatment of atrial tachyarrhythmias has been advocated for more than 30 years. Although it has played a beneficial role in the management of paroxysmal supraventricular tachycardia (PSVT) in drug-refractory patients, tachycardia acceleration and development of atrial fibrillation has been the major drawback. With the availability of radiofrequency catheter ablation therapy, the use of implantable antitachycardia devices for PSVT is currently negligible. From retrospective and small control studies it has been shown that atrial or dual-chamber pacing in patients with sick sinus syndrome has been associated with a lower incidence of paroxysmal atrial flutter or fibrillation than in those who received a ventricular pacemaker. Furthermore, recent studies have reported the potential benefit of reducing frequency of paroxysmal atrial flutter and fibrillation with multisite atrial pacing. As a result, there is a resurgence of research interest in antitachycardia pacing for prevention of atrial tachyarrhythmias. This paper briefly describes the basic aspects of antitachycardia pacing, reviews the data on the use of implantable antitachycardia devices for PSVT and the selection of patients, and assesses the current status of research on atrial pacing for prevention of paroxysmal atrial flutter and fibrillation.

Description and evaluation of the Res-Q Arrhythmia Control Device

The Res-Q Arrhythmia Control Device (Intermedics Inc.) is one of the latest entries into the growing implantable cardioverter defibrillator (ICD) market. Dysrhythmias are classified according to their zone of detection, with a bradycardia zone, up to 3 tachycardia zones, and a fibrillation zone. Detection criterion, therapies, and redetection criterion within each zone are independently programmable, tailoring the setup to each individual's needs. In a hierarchical manner, this allows efficacy, urgency, and patient comfort to be appropriately balanced. Tachycardia therapy options include antitachycardia pacing (ATP), low-energy cardioversion, and high-energy shock, while VVI pacing provides bradycardia therapy. ATP programming is extremely flexible. Biphasic waveform and a maximum output of 700 V have yielded a high rate of successful implantation. Unique features include the multiprogrammable sensing autogain, which tracks evoked T waves during pacing, as well as the use of the pulse generator to perform implant testing. Major strengths include programming flexibility and individualized therapy for multiple dysrhythmias. The major shortcoming relates to a lack of stored electrograms. Although long-term follow-up is not yet available, the Res-Q appears to be a capable challenger to a peer group of advanced generation ICDs.

Antitachycardia pacing and low-energy cardioversion for ventricular tachycardia termination: a clinical perspective

When incorporated into tiered therapy implantable cardioverter defibrillators (ICDs), antitachycardia pacing (ATP) techniques have proved useful for termination of sustained monomorphic ventricular tachycardias (VT) and have the advantages of rapid delivery, absence of patient discomfort, and minimal battery drain. The efficacy of low-energy cardioversion (LEC) is similar to that of pacing techniques for VT termination, but LEC has the disadvantages of patient discomfort, atrial proarrhythmia, and greater battery drain compared with ATP. Acceleration of VT occurs with similar frequency with each technique. Neither technique should be used without back-up defibrillation capability in an ICD. VT termination algorithms are currently empiric and require repetitive arrhythmia induction and trials of ATP or LEC. Future studies of the risk and benefits of each technique are likely to define optimal programming strategies in tiered therapy ICDs.

A prospective randomized repeat-crossover comparison of antitachycardia pacing with low-energy cardioversion

BACKGROUND

Multiprogrammable antiarrhythmia devices can treat monomorphic ventricular tachycardia (VT) with autodecremental overdrive pacing and/or with low-energy cardioversion. These two methods provide the opportunity to decrease patient discomfort typically experienced with high-energy pulses. Although both therapies are known to be effective, controversy persists over their relative safety and efficacy.

METHODS AND RESULTS

The purpose of this study was to examine the safety and efficacy of autodecremental overdrive pacing and low-energy cardioversion in reproducibly terminating monomorphic VT in 24 patients with multiprogrammable antiarrhythmia devices. The protocol required that identical ECG morphology VT be reproducibly induced four times to assess the outcome of antitachycardia pacing and cardioversion twice for each patient in a randomized fashion. Each episode of VT was induced via the implanted device. Autodecremental overdrive pacing initially began with seven stimuli at 97% of the VT cycle length, decrementing by 10 msec per stimulus to a minimum coupling interval of 200 msec. If ineffective, autodecremental overdrive pacing was allowed to iterate three more times for a total of four pacing interventions. With each iteration, one stimulus was added to the pacing train. Similarly, with low-energy cardioversion, up to four therapeutic attempts were made, beginning with a 0.2-J pulse. If ineffective, pulse energy was increased to 0.4, 1.0, and finally 2.0 J. All interventions were automatic without human interference. VT (cycle length, 306 +/- 42 msec) was repeatedly terminated in 15 of 24 patients (63%) by autodecremental overdrive pacing and in 18 of 24 patients (75%) by low-energy cardioversion (p = 0.53). Eight of the 24 patients (33%) had their VT terminated repeatedly by both therapies. VT accelerated to faster VT or ventricular fibrillation by autodecremental overdrive pacing in four of 24 patients (17%) and by low-energy cardioversion in five of 24 (21%) (p = 0.88). Only one of the 24 patients (4%) accelerated with both therapies. No patient was unaffected by either therapy.

CONCLUSIONS

In the manner programmed, autodecremental overdrive pacing and low-energy cardioversion have similar efficacy and acceleration rates. Response to one therapy does not predict response to the other.

Long-term follow up of patients treated with a software based antitachycardia pacemaker

INTRODUCTION

Over the past decade, several advances have been made in the management of tachycardias by pacing techniques, but limited data are available on the long-term outcome of patients treated with antitachycardia pacemakers.

PATIENTS AND METHODS

An antitachycardia pacemaker, the Intermedics Intertach, was implanted in 22 (17 female) patients with supraventricular tachycardia over a five year period. All were selected after detailed evaluation and testing of a temporary antitachycardia pacemaker system showed that their arrhythmia could be stopped promptly, reliably, and under different physiological conditions.

RESULTS

The 22 patients have been followed up for a mean period of 57.3 (range 19-76) months. All except one of the patients has had frequent episodes of tachycardia reliably ended by the pacemaker. Complications have occurred in seven patients, necessitating removal of the pacing system in four. Of the 18 patients who continue to have pacemakers, seven are being treated with beta blockers or verapamil; no other antiarrhythmic drugs are being taken.

CONCLUSIONS

Antitachycardia pacing is an acceptable long-term option for carefully selected patients with supraventricular tachycardia, but even after extensive testing a substantial number of the patients may continue to require drug treatment. Furthermore, the widespread use of curative techniques for supraventricular arrhythmias (catheter ablation and surgery) has decreased the need for this palliative treatment.

Long-Term Therapy of Antitachycardia Pacing for Supraventricular Tachycardia

Long-term antitachycardia pacing therapy with the InterTach 262-12 and 262-16 was evaluated in 32 consecutive patients (mean age 50 +/- 13 years) with recurrent, drug refractory supraventricular tachycardia. AV nodal reentrant tachycardia was present in 20 patients, Wolff-Parkinson-White syndrome in ten patients, and a reentrant tachycardia due to Mahaim fibers in one patient. During follow-up of 39 +/- 17 months, 250 persistent tachycardia episodes occurred in 22 patients. By adjusting detection and termination mode, recurrent supraventricular tachycardia could be controlled in 19 of 32 patients (60%) by antitachycardia pacing alone. Concomitant antiarrhythmic drug therapy was required in ten of 32 patients (30%). During follow-up antitachycardia pacing became ineffective in three patients (10%). Thus, chronic antitachycardia pacing proved to be safe and effective in selected patients with drug refractory supraventricular tachycardia and could significantly improve quality of life by rapid termination of recurrent supraventricular tachycardia episodes.

Atrial antitachycardia pacing in children and young adults

Atrial antitachycardia pacing was tested in 23 children and young adults. The majority of these patients had had operative repair of congenital cardiac defects and had both bradycardia and tachycardia. Pacemakers were usually implanted by the transvenous technique using bipolar leads. In each patient it was possible to find a tachycardia termination algorithm that successfully converted the tachycardia. In some patients very complex algorithms were necessary. In each patient it was also possible to find an algorithm that successfully differentiated the abnormal tachycardia from sinus tachycardia. Twelve patients required no antiarrhythmic drugs after pacemaker implantation, while 10 patients required one drug and one patient required two drugs. Eight of 23 patients had symptomatic tachycardias that required reprogramming the pacemaker to a different tachycardia termination sequence. Seven patients required reoperations, five for adapter problems and two for infection or erosion. Cardiac function improved in 15 of the 23 patients. Antitachycardia pacing is a viable option for management of tachycardias in children and young adults.

Long-term efficacy of an antitachycardia pacemaker and implantable defibrillator combination

Antitachycardia pacemakers and implantable cardioverter defibrillators (ICD) were implanted in 14 patients to control recurrent hemodynamically stable ventricular tachycardia (VT). All patients underwent extensive preimplant testing in the electrophysiology laboratory documenting that in each patient at least 50 episodes of VT could be reliably terminated by an external model of the antitachycardia pacemaker. The burst scanning mode of antitachycardia pacing was used in all patients. ICDs were implanted solely as a back up should acceleration of VT occur, and all had high nonprogrammable rate cutoffs (mean 191 +/- 12 beats/min). During a mean follow-up of 25 +/- 6 months, 6,029 episodes of VT were treated in the 14 patients. Only 103 ICD discharges were required (approximately one discharge per 60 episodes of VT). Ten of the 14 patients received discharges from their ICDs. No deaths have occurred. All devices remain active and in the automatic mode. Thus, an antitachycardia pacemaker and ICD combination can safely and effectively terminate VT in highly selected patients who are subjected to extensive preimplant testing. In such patients, the vast majority of episodes of VT can be terminated with antitachycardia pacing, and only rarely is a discharge required from the ICD.

Differentiation of ventricular tachyarrhythmias

Implantable devices capable of several modes of therapy will require differentiation of various ventricular tachyarrhythmias. Three methods of arrhythmia analysis, magnitude-squared coherence, ventricular rate, and irregularity of cycle length were performed for 45 episodes of induced ventricular tachyarrhythmia in 15 patients. Differentiation of monomorphic ventricular tachycardia from polymorphic ventricular tachycardia and ventricular fibrillation was possible by mean magnitude-squared coherence, less possible by rate, and not possible by beat-to-beat irregularity. Faster monomorphic ventricular tachycardia overlapped with rates of polymorphic ventricular tachycardia and ventricular fibrillation. Differentiation of polymorphic ventricular tachycardia and ventricular fibrillation was not possible by rate or irregularity. A progressive decrease in mean magnitude-squared coherence from monomorphic ventricular tachycardia to polymorphic ventricular tachycardia to ventricular fibrillation strengthens previous observations that coherence is a measure of rhythm "organization."

Rate augmentation and atrial arrhythmias in DDDR pacing

Dual chamber, rate-modulated pacemakers provide the capability of augmenting the heart rate of patients with chronotropic incompetence but also may cause atrial arrhythmias because of high rate, competitive atrial pacing. We studied ten patients with two consecutive 24-hour Holter monitors during which they were alternately programmed to either DDD or DDDR pacing in random order. Maximum heart rates (max HR) were measured at every 15-minute interval during each 24-hour period. DDDR pacing showed rate augmentation, 80 +/- 7 average max HR when compared with DDD pacing, average max HR 76 +/- 5. These results were even more striking when waking hours (7 am to 10 pm) were compared: average max HR 86 +/- 7 DDDR versus 78 +/- 4 average max HR DDD. Several patients showed marked rate augmentation. Seven of ten patients preferred DDDR pacing over DDD pacing. In the entire population, DDDR pacing did not result in an increased number of atrial arrhythmias (1.25 atrial events/24 hour) when compared to DDD pacing (1.75 atrial events/24 hour). We conclude that DDDR pacing provides heart rate augmentation during daily life in a clinical population while not resulting in a significant increase in atrial arrhythmias.

Clinical experience with the Intertach 262-12 pulse generator in patients with recurrent supraventricular and ventricular tachycardia

An antitachycardia pulse generator, the Intermedics Intertach 262-12 was implanted in 16 patients (14 patients with supraventricular tachycardia of various origins and two patients with recurrent ventricular tachycardia), who were not responsive to various antiarrhythmic drug regimens. The follow-up was from 6-49 months (mean 30.9 +/- 13.8). Five patients had a follow-up of over 3 years. The device was used in all patients. One patient with ventricular tachycardia died from a nonarrhythmic cause. Loss of responsiveness to burst pacing was observed in 1/14 patients with supraventricular tachycardia and nontolerance of antitachycardia pacing in one patient. Overall clinical success of pacing was observed in 13/16 patients = 81%. The pacemaker proved to be a versatile system with reliable tachycardia detection and termination functions.

Clinical experience with a new software-based antitachycardia pacemaker for recurrent supraventricular and ventricular tachycardias

The Intermedics Intertach 262-12 tachycardia reversion pulse generator was implanted in 14 patients (six male, eight female, mean age at implantation 45 +/- 16 years) with recurrent symptomatic tachycardias. Six patients had atrioventricular (AV) nodal reentrant tachycardia, three patients had orthodromic tachycardia with Wolff-Parkinson-White syndrome, two had circus movement tachycardia via a concealed bypass tract, two had ventricular tachycardia, one patient had atrial flutter. Mean duration of symptoms before implantation was 8 +/- 4 years and mean number of antiarrhythmic drug trials was 3.5 +/- 1. The primary tachycardia response made consisted of autodecremental pacing in one patient, burst pacing in two patients, and adaptive scanning of the initial delay or burst cycle length in eleven patients. The secondary tachycardia response mode consisted of autodecremental pacing in four patients, burst pacing in three patients and burst scanning in four patients. Tachycardia response was automatic in all but one patient with ventricular tachycardia. During a follow-up period of 30.5 +/- 10.6 months, one patient with ventricular tachycardia died from a nonarrhythmic cause. Reinterventions were necessary due to electrode fracture in one patient and due to pacemaker software defect in another one. Two patients underwent surgical cure of their arrhythmia: one patient with atrial flutter and one patient with AV nodal reentry tachycardia, 24 months and 11 months postpacemaker implantation, respectively. Four patients required digitalis to prevent pacing induced atrial fibrillation. Other proarrhythmic effects were not encountered. The pacemaker proved to be a versatile system with reliable tachycardia detection and termination functions. It provided a valuable adjunctive therapy in these selected patients.

Long-term results of antitachycardia pacing in patients with supraventricular tachycardia

Between 1979 and 1984 the Cybertach-60, (Intermedics, Inc. Model 262-01), a programmable, automatic antitachycardia pacemaker was implanted in 11 patients who had drug-refractory supraventricular tachycardia (SVT). The patients have been followed for a total of 64-108 (mean 84 months). All patients were symptomatic and had failed two or more drugs and six patients had required prior DC cardioversion. The mechanism of supraventricular tachycardia was atrioventricular (AV) nodal reentry in six patients, AV reentry in four patients, and atrial tachycardia in one patient. Preoperatively all patients had reliable termination of the tachycardia without induction of atrial fibrillation by pacing methods available to Cybertach-60. Postimplant, Cybertach-60 reliably terminated all episodes of tachycardia without ancillary drug therapy. Nevertheless, at long-term follow-up antitachycardia pacing was effective and safe in the minority (36%), with only four patients out of eleven still using a pacemaker for supraventricular tachycardia. One of these four patients required additional drug therapy. In one of the patients, the Cybertach-60 was replaced after 78 months by a more advanced device, (Intertach, Intermedics, Inc.) because of a depleted Cybertach-60 battery. In seven patients who no longer use antitachycardia pacing for termination of tachycardia, one patient developed atrial fibrillation during tachycardia termination (at 58 months postimplant). Three patients experienced induction of tachycardia or atrial fibrillation by the pacemaker due to undersensing of sinus P waves (at 36, 48, and 51 months).(ABSTRACT TRUNCATED AT 250 WORDS)

Automatic methods for detection of tachyarrhythmias by antitachycardia devices

Electrical devices play an increasingly important role in the control of tachyarrhythmias. Antitachycardia pacing and automatic defibrillation have been severely limited by the poor specificity of tachycardia discrimination in commercially available devices. Although absolute heart rate has been the principal means of automatic diagnosis, several new detection algorithms and methods are being investigated. Multiple electrode timing comparison, signal processing and pattern recognition are employed in these newer techniques. Although each offers some improvement over present technology, none is capable of identifying all arrhythmias. The methods employing comparison of atrial and ventricular rates, without additional criteria, are unable to detect ventricular tachycardia in the presence of 1:1 retrograde conduction. Electrographic analysis techniques require very stable electrodes and may not tolerate normal morphologic variations. A combination of two or more approaches may ultimately be required. All techniques will require that certain critical variables be programmable to allow for individualization in each clinical situation. Soft-ware-controllable devices and those capable of sensing from both the atria and the ventricles will provide the sophistication necessary for the implementation of complex tachycardia detection algorithms. This report reviews automatic tachycardia detection techniques in current use and under investigation.

Electrical devices for treatment of arrhythmias

Electrical devices can be used for preventing and terminating tachycardia and for achieving hemodynamic improvement during a continuing tachycardia. Conventional approaches to tachycardia prevention include pacing at physiologic rates to prevent brady-cardia-related tachycardia or tachycardias associated with prolonged QT-interval syndromes. More exotic techniques, such as those involving stimulation during the refractory period, are undergoing investigation. Some tachycardias cannot be easily terminated or recur incessantly. Hemodynamics can be improved by pacing methods that result in a narrower QRS complex by coupled pacing and, in supraventricular tachycardias, by pacing rapidly enough to create atrioventricular block. Most clinical tachycardias are caused by reentry. Careful analysis of the timing of individual stimuli that successfully terminate tachycardias indicate that critical relations exist in the conduction velocity, refractoriness and physical properties and dimensions of the reentry circuit and the remaining myocardium. Elucidating these relations has permitted inferences into the mechanisms by which pacing terminates or accelerates tachycardias. A vast number of pacing patterns have evolved for use in tachycardia termination. None of these appear to be foolproof. There is widespread and justified concern about the risk of acceleration of tachycardia when antitachycardia pacing is used in the ventricle. Experience indicates that only a few patients are suitable for termination of ventricular tachycardia by pacing, but these carefully selected patients may do well. Both the results and the potential for widespread use may be better with pacing for termination of supraventricular tachycardia. Life-threatening tachycardias or fibrillation can be terminated by direct-current countershock. Although many technical problems remain, implantable cardioverter-defibrillators, possibly combined with antitachycardia pacemakers, will play an increasing role in the management or serious arrhythmias.

Nonpharmacologic treatment of life-threatening cardiac arrhythmias

Numerous nonpharmacologic modalities have been introduced for the management of patients with life-threatening arrhythmias. These include cardiac pacing, insertion of an automatic internal cardiac defibrillator (AICD), cardiac electrosurgery, and catheter ablative techniques. Each modality is effective; AICD shows particular promise because it has demonstrated remarkable efficacy in decreasing the incidence of sudden cardiac death in patients with malignant ventricular arrhythmias. Each modality also has its limitations or contraindications. Nonpharmacologic antiarrhythmic therapy represents an important advance against the serious public health problem of sudden cardiac death.

Double sensing by atrial automatic tachycardia-terminating pulse generator

This report describes two cases of double sensing of the P and QRS signals by an implanted atrial bipolar automatic burst tachycardia-terminating pulse generator (Intermedics CyberTach 60) used for the treatment of supraventricular tachycardia. Double sensing occurred during normal sinus rhythm at a rate slower than the tachycardia detection criterion and caused inappropriate delivery of burst stimulation. These observations underscore the importance of the far-field QRS signal in the detection of supraventricular tachycardia by automatic pulse generators sensing the atrial electrogram.

Implantable pacers for tachycardia termination: stimulation techniques and long-term efficacy

The long-term efficacy of pacing for termination of supraventricular tachycardia (SVT) and ventricular tachycardia (VT) was reviewed. Increasingly complex and sophisticated antitachycardia pacing stimulation patterns have evolved, and are outlined. Although excellent results are reported with simple patterns, it may be that the more complex algorithms increase the percentage of tachycardia patients who may be candidates for implantation of a device. In the papers reviewed, there were 460 patients, 268 with SVT, and 192 with VT. Results were judged to be good-excellent in 96.5% of both VT and SVT groups.

Long-term management of ventricular tachycardia by implantable automatic burst tachycardia-terminating pacemakers

This report describes the long-term follow-up of two patients who received implantable automatic burst tachycardia-terminating ventricular pacemakers for the treatment of drug-refractory sustained ventricular tachycardia. After implantation, both pulse generators continued to terminate ventricular tachycardia without any major complications. In one patient, after three years, many episodes of ventricular tachycardia were slower than the tachycardia-detection criterion rate of 137 per minute; ventricular tachycardia was then terminated by chest wall stimulation that activated the burst function of the pacemaker. In this particular patient, the pulse generator was removed after four and one-half years and replaced with a DDD system because of the pacemaker syndrome and attacks of ventricular tachycardia, often at a rate of about 100/minute. In the second patient, the pacemaker continued to terminate ventricular tachycardia for over five and one-half years as determined by the repeated activation of the flag (memory) function of the pacemaker indicating detection of tachycardia by the pulse generator and resultant delivery of burst pacing.

Automatic burst extrastimulus pacemaker to treat recurrent ventricular tachycardia in a patient with mitral valve prolapse: more than 2,000 documented successful tachycardia terminations

An automatic burst pacemaker (Cybertach-60) was used to treat drug-resistant recurrent ventricular tachycardia in a patient with mitral valve prolapse. The arrhythmia was associated with multiple syncopal episodes and aborted sudden death. During a 23 month follow-up period the patient has remained asymptomatic and more than 2,000 episodes of ventricular tachycardia have been successfully terminated by the pacemaker.

Antitachycardia pacing: a comparison of burst overdrive, self-searching and adaptive table scanning programs

Eight patients with the Siemens Elema "Tachylog" generator implanted for management of paroxysmal reentrant tachycardia were studied to assess the safety and efficiency of three antitachycardia programs. The programs investigated were burst overdrive, self-searching, and adaptive table scanning. There were five males and three females aged 19-62 years. Seven had Wolff-Parkinson-White syndrome, and one had dual atrioventricular nodal pathways. Four had right atrial electrodes and four had right ventricular electrodes. Patients were studied lying, standing, and exercising in all three modes, and the appropriate long-term programs were chosen. The generator remained in a program for 1 month, it was interrogated and the memory was read, and then it was reprogrammed to a different antitachycardia mode. Burst overdrive was unsuitable for long-term use in four patients, producing atrial fibrillation in one and ventricular arrhythmias in three. In this group, self-searching and adaptive table scanning were safe and equally effective (mean number of attempts/tachycardia 6.97 and 6.3, respectively). In the four patients in whom all three programs could be used, burst overdrive proved to be most efficient, the mean number of attempts/tachycardia were 2.4 (cf 9.6 and 9.0 for self-searching and adaptive table scanning). Thus, burst overdrive was only suitable for long-term use in 50% of our patients, but when safe it was more efficient than the other two programs, especially in those with narrow termination windows on exercise.

Clinical intracardiac electrophysiologic testing: technique, diagnostic indications, and therapeutic uses

Clinical cardiac electrophysiologic testing has evolved rapidly since 1968, when the technique was first described. In an electrophysiologic study, electrode catheters are positioned within the heart to record electrical activity from the atrium, atrioventricular conduction tissue, and ventricle. Programmed stimulation is then performed, which involves pacing of the atrium or ventricle and introducing critically timed premature stimuli during sinus rhythm or paced rhythm. The use of programmed stimulation in conjunction with intracardiac recordings in electrophysiologic studies has facilitated the diagnosis of mechanisms of arrhythmias and the assessment of therapy. Electrophysiologic testing is useful in selected patients with sinus node dysfunction, conduction system disorders, supraventricular tachycardia, ventricular tachycardia, or ventricular fibrillation and in survivors of out-of-hospital cardiac arrest and patients with symptomatic but unsubstantiated rhythm disturbances. Therapeutic approaches that can be assessed by electrophysiologic testing include serial drug testing to determine the effectiveness of antiarrhythmic agents, antitachycardia pacing, the implantable defibrillator, transcatheter ablation, and electrophysiologically guided surgical procedures. In this review, we discuss the methods of electrophysiologic testing, its clinical applications in diagnosing the various cardiac rhythm disturbances, and its use in assessing various therapeutic modalities.

Techniques for improving cardiac performance with implantable devices

A variety of pacing techniques are available to improve cardiac performance in patients who have bradyarrhythmias. These approaches include the preservation of AV synchrony and rate responsiveness, whereby pacing rate is varied according to some physiologic marker such as the atrial rate. New rate responsive pacemakers utilize other sensors to govern pacing rate and these units may monitor QT interval duration, respiratory rate, activity, venous oxygen saturation, temperature, or pH. Additional sensors are presently being investigated and prototype stroke volume and pressure monitoring devices should be available in the near future. All of these approaches require clinical evaluation and their eventual widespread application must be preceded by thorough studies of benefit, risk, and cost.