Atrial sensing to augment ventricular tachycardia detection by the automatic implantable cardioverter defibrillator: a utility study

Randomized controlled study of detection enhancements versus rate-only detection to prevent inappropriate therapy in a dual-chamber implantable cardioverter-defibrillator

OBJECTIVES

The purpose of this study was to compare rate-only detection to enhanced detection in a dual-chamber implantable cardioverter-defibrillator (ICD), to discriminate ventricular tachycardia from supraventricular tachycardia.

BACKGROUND

ICDs are highly effective in treating ventricular tachycardia (VT) or ventricular fibrillation (VF). However, they frequently deliver inappropriate therapy during supraventricular tachycardia (SVT).

METHODS

We conducted a randomized clinical trial of detection enhancements in a dual-chamber ICD compared to control (rate-only) detection to discriminate VT from SVT. Detection enhancements included a specific standardized protocol identical for all patients for programming rate stability, sudden onset, atrial-to-ventricular relationship (sudden onset = 9% and rate stability = 10 ms; V > A "on"), and "sustained rate duration" (3 minutes). The primary endpoint was the time to first inappropriate therapy classified by a blinded events committee.

RESULTS

One hundred forty-nine patients had a history of sustained VT or VF. Mean age (+/- SD) was 60 +/- 13 years; 83% were male, and mean ejection fraction was 35 +/- 15%. Control (n = 70) and "enhanced" (n = 79) groups did not differ with regard to age, sex, ejection fraction, or primary arrhythmia. The proportion of patients free of inappropriate therapy over time was significantly higher in the enhanced versus the control group (hazard ratio = 0.47, P = .011). High-energy shocks were reduced from 0.58 +/- 4.23 shocks/patient/month in the control group to 0.04 +/- 0.15 shocks/patient/month in the enhanced group (P = .0425). No patient programmed per protocol failed to receive therapy for VT detected by the ICD (422 VT episodes).

CONCLUSIONS

Standardized programming in a dual-chamber ICD leads to a significant and clinically important reduction in inappropriate therapies compared to rate-only detection and does not compromise safety with respect to appropriate treatment of VT.

Safety and Efficacy of a Dual Chamber Implantable Cardioverter Defibrillator Capable of Slow Ventricular Tachycardia Discrimination:

New developments in dual chamber implantable cardioverter defibrillators (ICD) have increased the specificity of therapy delivery. This study was performed to examine the performance of an algorithm, focusing on its ability to distinguish slow ventricular tachycardia (VT) from sinus rhythm or supraventricular tachyarrhythmias. The patient population included 77 men and 13 women, 63 +/- 11 years old, treated with ICDs after episodes of spontaneous or inducible ventricular tachyarrhythmias. They were randomized to programming of the ICD to a lower limit of VT detection at 128 beats/min (group I, n = 44), versus 153 beats/min II (group II, n = 46). The primary endpoint of the study consisted of comparing the specificity and sensitivity of the algorithm between the two groups of patients. Over a 10.1 +/- 3.5 months follow-up, 325 episodes were detected in the Tachy zone in group I, versus 106 in group II. The sensitivity and specificity of the algorithm in group I were 98.8% and 94.4%, respectively, versus 100% and 89% in group II (NS). A single episode of VT at a rate of 132 beats/min was diagnosed as SVT in group I. The sensitivity and specificity of the algorithm for tachycardias <153 beats/min were 97.4% and 94.5%, respectively. Overall VT therapy efficacy was 100% in both groups. The performance of this algorithm in the slow VT zone supports the programming of a long Tachy detection interval to document slow events, and allows to treat slow VT, if necessary, without significant risk of inappropriate interventions for sinus tachycardia.

Ventricular tachycardias above the initially programmed tachycardia detection interval in patients with implantable cardioverter-defibrillators: incidence, prediction and significance

OBJECTIVES

This retrospective study was performed to provide data on ventricular tachycardias (VT) with a cycle length longer than the initially programmed tachycardia detection interval (TDI) in patients with implantable cardioverter defibrillators (ICDs).

BACKGROUND

It has been clinical practice to program a safety margin of 30 to 60 ms between the slowest spontaneous or inducible VT and the TDI.

METHODS

Baseline characteristics of 659 consecutive patients with ICDs were prospectively; follow-up information was retrospectively collected.

RESULTS

During a mean follow-up of 31+/-23 months, 377 patients (57.2%) had at least one recurrent VT or ventricular fibrillation; 47 patients (7.1%) suffered 61 VTs above the TDI. The risk of a VT above the TDI ranged between 2.7% and 3.5% per year during the first four years after ICD implantation. The difference between the cycle length of the slowest VT before ICD implantation, spontaneous or induced, and the first VT above TDI was 108+/-58 ms. Fifty-four VTs (88.5%) above the TDI were associated with significant clinical symptoms (angina or palpitation 63.9%, heart failure 6.6% and syncope 8.2%). Six patients (9.8%) had to be resuscitated. Kaplan-Meyer analysis identified New York Heart Association class II or III (p = 0.021), ejection fraction < 0.40 (p = 0.027), spontaneous (p<0.001) or inducible (p<0.001) monomorphic VTs and the use of class III antiarrhythmic drugs (amiodarone, p<0.001; sotalol, p = 0.004) as risk predictors of VTs above the TDI. The risk of recurrent VTs above TDI was 11.8%, 12.5% and 26.6% during the first, second and third year after first VT above TDI, respectively.

CONCLUSIONS

The risk of VTs above the TDI is significantly increased in some patients, and many VTs above TDI cause significant clinical symptoms. A larger safety margin between spontaneous or inducible VTs and the TDI seems to be necessary in selected patients. This is in conflict with an increased risk of inadequate episodes and demands highly specific and sensitive detection algorithms in these patients.

Clinical experience with the new detection algorithms for atrial fibrillation of a defibrillator with dual chamber sensing and pacing

INTRODUCTION

A major drawback of therapy with an implantable defibrillator is the nonspecificity of detection. Theoretically, adding atrial sensing information to a decision algorithm could improve specificity of detection.

METHODS AND RESULTS

This open-label nonrandomized study compares the detection algorithm of the Ventak AV and the Ventak Mini implantable defibrillators. The Ventak AV (n = 39) uses dual chamber detection as opposed to single chamber detection (with rate stability) in the Ventak Mini (n = 55). Programmed zone configurations, rate thresholds, and stability criteria were identical in all patients. In the Ventak AV group, 235 ventricular tachyarrhythmias were adequately detected and treated by the device. In the Mini group, 699 episodes of ventricular fibrillation/tachycardia occurred. All but six of the latter episodes were correctly identified and treated: one patient with incessant ventricular tachycardia had five episodes not terminated by the device, another episode occurred in a patient with a device/lead defect. In the Ventak AV group, 33 episodes of sinus tachycardia and 166 episodes of atrial fibrillation/flutter activated the device; inappropriate therapy was applied to 41% of atrial fibrillation/flutter episodes. In the Ventak Mini group, 226 supraventricular tachyarrhythmias activated the device, eight of which were sinus tachycardia and 218 were atrial fibrillation or flutter; of the atrial fibrillation/ flutter episodes 24% were treated inappropriately (fewer vs Ventak AV, P < 0.001).

CONCLUSION

The new detection algorithm incorporated in the Ventak AV did not inadvertently withhold therapy for ventricular tachyarrhythmias, but at the same time the number of inappropriate therapies for atrial fibrillation was not decreased in comparison to a single chamber device.

Performance of basic ventricular tachycardia detection algorithms in implantable cardioverter defibrillators: implications for device programming

Around 20% of patients with third generation implantable cardioverter defibrillators receive inappropriate therapy, usually triggered by atrial fibrillation. This is because the criteria used for ventricular tachycardia detection by current implantable cardioverter defibrillators are based on the analysis of a sequence of RR intervals and may be inappropriately satisfied by supraventricular tachyarrhythmias. Algorithms for ventricular tachycardia detection were challenged against the full RR interval sequences from 482 spontaneous episodes of atrial fibrillation and 260 spontaneous episodes of ventricular tachycardia to determine their ability to discriminate between the arrhythmias. The sensitivities and specificities of the algorithms were calculated over a wide range of programmable parameters. For a given window length and detection interval, the most stringent algorithms, that required all beats to be classified as "fast", were more specific than those allowing a proportion of "normal" intervals, even after adjustment for differing sensitivity. These differences were less marked for faster tachycardias. Specificity increased with the detection window length to a limit of approximately 18 beats. We conclude that ventricular tachycardia is detected with the highest specificity if all beats in an analyzed sequence are required to be "fast," even after lengthening of the tachycardia detection interval to maintain sensitivity. Further improvement in algorithm performance may require the incorporation of criteria such as tachycardia onset and stability.

Antiarrhythmic therapy--future trends and forecast for the 21st century

This article discusses recent changes in antiarrhythmic therapy, with a focus on nonpharmacologic therapy (electrode catheter ablation, implantable cardioverter-defibrillators [ICDs]), and puts them into perspective for the coming years. The treatment of supraventricular tachycardias and tachycardia involving accessory pathways is likely to remain the domain of catheter ablation. With promising new techniques under investigation, the spectrum of arrhythmias that can be cured will probably be expanded. Treatment of life-threatening ventricular arrhythmias is likely to remain the domain of the ICD in the foreseeable future. With the safety net of the ICD in place, new antiarrhythmic drugs or other forms of antiarrhythmic therapy can be developed and tested.

Inappropriate discharge of an implantable cardioverter defibrillator during atrial flutter and intermittent ventricular antibradycardia pacing

INTRODUCTION

Inappropriate discharges of an implantable cardioverter defibrillator (ICD) are troublesome to the patient and sometimes a difficult task for the physician trying to identify and treat the cause.

METHODS AND RESULTS

For the first time, we report a mechanism of inappropriate ICD discharges during episodes of atrial flutter with a slow ventricular response and intermittent antibradycardia pacing. The episodes occurred in two patients and were triggered by the unique sensing algorithm of the Ventritex Cadence V-100 in combination with the tripolar CPI Endotak 072 transvenous defibrillation lead, which provides integrated bipolar sensing.

CONCLUSION

Besides treatment of the underlying arrhythmia, reprogramming of the device, an electrode position far away from the atria, and true bipolar sensing will enhance the performance of ICD systems with respect to the episodes described here. In addition, more flexible sensing algorithms may, in the future, prevent this overall rare complication.

Ventriculoatrial conduction capability and prevalence of 1:1 retrograde conduction during inducible sustained monomorphic ventricular tachycardia in 305 implantable cardioverter defibrillator recipients

Despite the advent of dual chamber ICDs, differentiation of VT (SMVT) with 1:1 VA conduction will remain a challenge. In this study, VA conduction capability and prevalence of inducible sustained monomorphic (SM) VT with 1:1 VA conduction was assessed in 305 ICD recipients. SMVT with a mean cycle length (CL) of 304 +/- 61 ms was induced in 161 (53%) patients. Twenty-six percent of the patients maintained 1:1 VA conduction to CL < or = 400 ms during incremental ventricular pacing, regardless of presenting tachyarrhythmia or presence of inducible SMVT. Among ten patients who had inducible SMVT with possible 1:1 VA conduction (based on SMVT CL comparable to the shortest CL associated with 1:1 retrograde conduction during ventricular pacing), all seven with available intracardiac tracings had documented 1:1 VA conduction during the induced SMVT--representing 4.4% of the patients with inducible SMVT (95% CI 1.2%-7.6%), and 2.3% of the entire ICD cohort (95% CI 0.6%-4.0%). We conclude that about one-fifth of ICD recipients possess 1:1 VA conduction to CL < or = 400 ms and that inducible SMVT with 1:1 VA conduction can be demonstrated in a small but nonnegligible proportion of ICD recipients. These data are relevant to the design of tachyarrhythmia-discrimination algorithms for dual chamber ICDs.

[New algorithms for discrimination between supraventricular and ventricular tachyarrhythmias in patients with implantable cardioverter/defibrillator]

The current therapy with implantable cardioverter/defibrillators (ICD) is lacking specificity. The technical concept of arrhythmia detection, as a single channel ventricular rate threshold does not provide a clear differentiation between supraventricular and ventricular rhythms. Nearly 25% of all ICD-therapies are related to a false positive detection of supraventricular arrhythmias.The use of alternative, non rate based detection algorithms is limited due to the reduced long-term stability of the sensors and due to a high battery drain. Based on the current concept of arrhythmia detection, the use of additional timing algorithms as the variability of consecutive RR-cycle length ("Rate-Stability") and the detection of a sudden acceleration in the ventricular rate ("Rate-Onset") provide a clinical relevant increase in specificity in ICD-therapy. A further class of detection algorithms uses the morphologic assessment of intracardiac electrograms. Despite the fact that these morphology based algorithms have shown a high sensitivity and specificity in the discrimination between ventricular and non-ventricular rhythms, only one algorithm was implemented in ICD-generators and the clinical importance in ICD-therapy was very small. In 1995 a new morphology based detection algorithm was introduced in ICD-therapy. The "width criterion" is based on the measurment of the duration of the intracardiac signal. In 1995 and 1996 the first two series of a dual chamber ICD system were introduced in clinical ICD-therapy. These devices provide a two-channel atrio-ventricular arrhythmia detection in connection with a DDD antibradycardia pacing therapy. The use of DDD-ICD systems is expected to be a great step forward to enhance diagnostic specificty. A number of rate and timing detection algorithms and algorithms comparing the assoziation of atrial and ventricular signals may improve the discrimination between supraventricular and ventricular tachyarrhythmias. The future implementation of additional morphology based detection algorithms in DDD-ICD systems may solve the problem of the limited detection specificity in clinical ICD-therapy.

Automatic arrhythmia identification using analysis of the atrioventricular association. Application to a new generation of implantable defibrillators. Participating Centers of the Automatic Recognition of Arrhythmia Study Group

BACKGROUND

Atrioventricular association is a key criterion for arrhythmia diagnosis. Its use in a defibrillator should significantly reduce the incidence of inappropriate shocks. Therefore, we evaluated the diagnostic accuracy of an algorithm that uses dual-chamber sensing and analysis of atrioventricular association to discriminate ventricular from supraventricular arrhythmias in a prototype of an implantable defibrillator.

METHODS AND RESULTS

The algorithm performed a stepwise analysis of arrhythmias. The rhythm was first classified on the basis of cycle lengths. Each episode was then classified as supraventricular or ventricular in origin on the basis of the stability of cycle lengths and atrioventricular association. This algorithm was evaluated in 156 episodes of induced sustained tachycardias. Eighty-nine tachycardias were taken from the Ann Arbor electrogram library; the others were recorded in 50 patients during electrophysiological studies. The atrial and ventricular signals were stored on an external recorder and then injected into an external prototype of a defibrillator system. The algorithm correctly diagnosed 96% of ventricular tachycardia episodes, 100% of ventricular fibrillation episodes, and 92% of double-tachycardia episodes. The mean detection time for ventricular tachycardia was 2.6 +/- 0.8 seconds, and for ventricular fibrillation, it was 2.1 +/- 0.4 seconds. The positive predictive values for the diagnoses of atrial fibrillation and atrial flutter were 92% and 86%, respectively. For ventricular tachycardia and ventricular fibrillation, the values were 95% and 100%, respectively.

CONCLUSIONS

Analysis of atrioventricular association promotes reliable differentiation between ventricular and supraventricular tachycardias and should enhance the diagnostic capabilities of implantable defibrillators.

Augmented two-channel arrhythmia detection: an efficient diagnostic method for implantable devices

ICDs are highly effective in preventing sudden cardiac death. However, inappropriate device shocks caused by false-positive diagnoses are estimated to happen in 20% of all patients. The need for implantable electrical devices to detect with precision arrhythmias requiring therapy has spawned a variety of proposals for better means of tachycardia identification. To address this problem, the augmented two-channel arrhythmia detection (A2CAD) algorithm, a real-time scheme utilizing timing and morphology from both the atrial and ventricular channels, is introduced. The algorithm uses rate detection as a first stage and augments this with morphological signal analysis in rhythms that confound the rate only diagnoses. The software executes in real-time (online), and has been tested on 60 passages of two-channel intracardiac signals. The following arrhythmias constituted the test set: 10 AF and/or atrial flutter; 15 SVT; 16 VT; 10 ventricular flutter or VF; 5 sinus tachycardia; and 4 cases of AF concurrent with VF. Results from 60 patient cases indicate 57 (95%) of 60 success rate for A2CAD, validating its potential for implementation in future implantable devices.

Nonpharmacological therapy for malignant ventricular arrhythmias: implantable defibrillator trials

Implantable cardioverter-defibrillators (ICDs) are an important nonpharmacological option in the treatment of malignant ventricular arrhythmias. Technological advances in current devices permit nonthoracotomy implantation with transvenous lead systems using biphasic shocks. Decreasing device size has resulted in pectoral implantation. Battery longevity is still short in comparison with that of pacemakers. Lead failure rates as well as pacing thresholds are significantly higher than those for cardiac pacing lead systems. Other complications of ICD systems include infection, perforation, and thrombosis. The long-term performance of nonthoracotomy lead systems for ICD devices has now been extensively studied. Sudden death recurrence rates for these systems are less than 2% in 3 years and less than 5% at 5 years. Clinical trials with both monophasic and biphasic systems show a high degree of prevention of sudden death. Comparison of ICD outcome with that of drug therapy in three large retrospective studies and two small prospective randomized trials favors improved survival and sudden death prevention with device therapy. However, these studies need corroboration from large prospective trials. Two large prospective trials, CIDS and the AVID study, are now in progress to address this issue.

Differentiation of sinus tachycardia from ventricular tachycardia with 1:1 ventriculoatrial conduction in dual chamber implantable cardioverter defibrillators: feasibility of a criterion based on the atrioventricular interval

Tachycardia discrimination in future implantable cardioverter defibrillators (ICDs) is likely to be enhanced by the addition of an atrial sensing/pacing lead. However, differentiation of sinus tachycardia (ST) from ventricular tachycardia (VT) with 1:1 VA conduction will remain problematic. We assessed the use of the AV interval as a potential criterion for correctly differentiating ST from VT. Incremental V pacing at the right ventricular (RV) apex served as a "VT" model in each of 41 patients with 1:1 VA conduction to pacing cycle lengths < or = 450 msec. High right atrial and RV apical electrograms during normal sinus rhythm (NSR) and during incremental V pacing were digitized (simulating ICD sensing). From these signals, AV interval versus pacing cycle length plots were computer generated to identify crossover cycle lengths, each defined as the cycle length at which the AV interval during V pacing equals the AV interval during NSR. At cycle lengths longer than the crossover value, the AV interval during "VT" exceeds the AV interval during NSR. In contrast, the AV interval during ST is physiologically shorter than the AV interval during NSR. Thus, ST can be readily differentiated from "VT" over a range of cycle lengths greater than the crossover value. The overall mean calculated crossover cycle length was 371 +/- 52 msec. In 11 patients paced multiple times, each crossover cycle length was reproducible (mean coefficient of variation was 1.2% +/- 0.9% per patient). AV intervals measured at the RV apex were also analyzed with incremental V pacing during catecholamine stimulation (isoproterenol, n = 5) and during alternate site "VT" (RV outflow tract [n = 8] and left ventricle [n = 2]). In all these cases, the new "VT" plots of AV interval versus pacing cycle length coincided with or fell to the left of those obtained during control RV apical pacing and recording (i.e., these AV interval values crossed the NSR baseline at cycle lengths < or = the crossover cycle length). Thus, the cycle length range for recognizable differentiation of ST from "VT" remained valid. The data suggest that the described AV interval criterion relying on the crossover cycle length: (1) is a promising approach to improve differentiation of ST from relatively slow VTs with 1:1 VA conduction, and (2) can readily be automated in future dual chamber ICDs, given its computational simplicity.

Underdetection of ventricular tachycardia by algorithms to enhance specificity in a tiered-therapy cardioverter-defibrillator

OBJECTIVES

The goal of this study was to determine the incidence and clinical significance of underdetection in 125 patients treated with a tiered-therapy cardioverter-defibrillator, the Medtronic PCD.

BACKGROUND

Underdetection, distinct from undersensing, is a unique, potential complication of new algorithms that enhance specificity in tiered-therapy cardioverter-defibrillators. These algorithms may delay or prevent recognition of ventricular tachycardia even though electrograms are sensed accurately and RR intervals meet the programmed interval criterion.

METHODS

Underdetection was defined as delay in detection > 5 s at electrophysiologic study or symptomatic delay or detection failure at follow-up of 15 +/- 8 months.

RESULTS

We identified six specific mechanisms of underdetection caused by algorithms to discriminate sustained ventricular tachycardia from sinus tachycardia, atrial fibrillation, ventricular fibrillation and nonsustained ventricular tachycardia. Underdetection caused detection delays in 13 (1.9%) of 677 induced ventricular tachyarrhythmia episodes in 12 patients (9.6%). During follow-up, underdetection occurred in 7 (9.9%) of 71 patients in whom ventricular tachycardia therapies were programmed. Failure to detect ventricular tachycardia occurred in 6 (0.6%) of 988 spontaneous ventricular tachycardia episodes in four patients (5.6%); 2 episodes required external cardioversion. After defibrillator reprogramming, underdetection did not occur.

CONCLUSIONS

Algorithms to enhance specificity cause underdetection of ventricular tachycardia in a significant minority of patients with tiered-therapy cardioverter-defibrillators. Optimal programming can minimize underdetection.

Discrimination of ventricular tachycardia from sinus tachycardia and atrial fibrillation in a tiered-therapy cardioverter-defibrillator

OBJECTIVES

This study was conducted to evaluate criteria for discrimination of ventricular tachycardia from atrial fibrillation and sinus tachycardia in a tiered-therapy cardioverter-defibrillator (Medtronic PCD).

BACKGROUND

Interval stability algorithms discriminate ventricular tachycardia from atrial fibrillation. Onset algorithms discriminate ventricular tachycardia from sinus tachycardia. Neither has been validated clinically.

METHODS

The stability criterion requires that a ventricular tachycardia interval not vary from any of the three previous intervals by more than the programmable stability value. The onset criterion detects initiation of ventricular tachycardia only if the ratio of an interval to the mean of four previous intervals is less than a programmed onset ratio and either the second or third preceding interval exceeds the ventricular tachycardia detection interval. We evaluated these criteria in 100 patients at electrophysiologic study and exercise testing (65 patients) and during a mean (+/- SD) follow-up of 16.2 +/- 7.9 months. The PCDs were programmed to tiered therapy in 54 patients. In the remaining 46 patients, the PCD's memory for detected ventricular tachycardia was used to study the specificity of the chosen onset criterion for rejecting sinus tachycardia. We used stored intervals preceding appropriate (n = 99) and inappropriate (n = 54) detections to test a new onset criterion that was less sensitive to a single index interval.

RESULTS

Programmed stability of 40 ms decreased detection of induced atrial fibrillation by 95% (20 patients), paroxysmal atrial fibrillation by 95% (6 patients) and chronic atrial fibrillation by 99% (9 patients); all episodes of spontaneous (n = 877) and induced (n = 339) ventricular tachycardia were detected. A programmed onset ratio of 87% rejected sinus acceleration (98%) but caused underdetection of 0.5% of ventricular tachycardias. The onset criterion permitted inappropriate detection of premature ventricular complexes during sinus tachycardia, but the new criterion reduced these inappropriate detections by 98%.

CONCLUSIONS

The PCD's onset and stability criteria reduced inappropriate detection of atrial fibrillation and sinus acceleration while detecting 99.5% of ventricular tachycardias.

MATIC--an intracardiac tachycardia classification system

The use of an additional atrial sensing electrode together with a morphology recognition algorithm provides a significant improvement in classification performance over the current rate based algorithms used in implantable cardioverter defibrillator (ICD) devices. The classification system, called morphology and timing intracardiac classifier (MATIC), follows a classification process similar to that used by cardiologists. Timing between the atrial and ventricular channels is examined using a decision tree and forms the primary criterion for arrhythmia classification. A neural network based morphology classifier is used for cases such as ventricular tachycardia with 1:1 retrograde conduction where timing alone cannot make a reliable decision. MATIC achieves 99.6% correct classification on a database of intracardiac electrogram (ICEG) signals containing 12,483 QRS complexes recorded from 67 patients during electrophysiological studies. Arrhythmias in this database include sinus tachycardia, normal sinus rhythm, normal sinus rhythm with bundle branch block, sinus tachycardia with bundle branch block, atrial fibrillation (AF), various supraventricular tachycardias, ventricular tachycardia, ventricular tachycardia with 1:1 retrograde conduction, and ventricular fibrillation. Within these arrhythmias, there were numerous ventricular ectopic beats, fusion beats, noise, and other artifacts. MATIC addresses the classification problem from start to finish, inputs being raw intracardiac electrogram signals and the outputs being the recommended ICD therapy. Results achieved with MATIC were compared with a classifier used in the Telectronics Guardian ATP 4210, which achieved 75.9% correct classification on the same database. MATIC is simple and efficient, making it suitable for use in a low power implantable device.

Clinical performance of the implantable cardioverter defibrillator: electrocardiographic documentation of 101 spontaneous discharges

Records of 105 patients, who received an automatic implantable cardioverter defibrillator (AICD), were studied to investigate the causes of spontaneous AICD discharges and to correlate the symptoms with the arrhythmias triggering AICD discharges. During a follow-up period of 13 +/- 8 months, 46/105 (44%) patients had 566 spontaneous AICD discharges. A total of 101 discharges were documented with Holter monitoring in 23 patients. In this study group, there were 8 (8%) AICD discharges for 5 episodes of ventricular fibrillation, and 68 (67%) discharges for 63 episodes of sustained ventricular tachycardia. Patients lost consciousness in all episodes of ventricular fibrillation, but were symptomatic prior to only 36 (53%) discharges in ventricular tachycardia. Nonsustained ventricular tachycardia persisting for a period of 7.5 +/- 2 seconds resulted in 20 AICD discharges; patients were symptomatic prior to 13 (65%) discharges. Supraventricular tachycardias triggered three discharges. One patient had two spurious discharges during sinus rhythm. In conclusion, most of the spontaneous AICD discharges were appropriate for the detected rhythms, but only clinically appropriate for the management of arrhythmias in 75% of the cases. A significant portion of the patients with sustained or nonsustained ventricular tachycardias triggering AICD discharges were asymptomatic prior to discharge, which requires further assessment of the physiology of the arrhythmia as a component of the detection algorithm.

Tachycardia recognition algorithms for implantable systems

The detection algorithm of current antitachycardia pacing systems incorporates derivatives of heart rate as the only objective parameter used for recognition. Present pacing systems use one or more rate-related parameters: (1) rate cutoff, (2) sudden onset, (3) rate stability, and (4) sustained high rate. Recently, the development of antitachycardia systems has focused on combining the four detection criteria to form multiple recognition algorithms. Nevertheless, inability to differentiate between supraventricular and ventricular tachycardias at compatible rates remains a limitation. Incorporating atrial sensing capability, in addition to ventricular sensing capability, an alternative strategy that potentially could enhance diagnostic accuracy, is discussed. The incorporation of time and frequency analysis, arterial pressure, and stroke volume are also examined.

Continuous right ventricular volume assessment by catheter measurement of impedance for antitachycardia system control

Current implantable defibrillators are unable to differentiate between hemodynamically stable and unstable arrhythmias. This may result in unnecessary high energy shocks during arrhythmias that are better managed with other interventions. This study assessed the efficacy of the impedance catheter in sensing relative volumetric changes in the right ventricle as a measure of the hemodynamic status during an arrhythmia. During electrophysiological testing, 37 arrhythmias were induced in 12 patients aged 28-74 years. Rhythms recorded were: (A) hemodynamically stable tachyarrhythmias (supraventricular tachycardia and sustained monomorphic ventricular tachycardia)--21 episodes; and (B) hemodynamically unstable ventricular arrhythmias causing syncope (hypotensive ventricular tachycardia and ventricular fibrillation)--16 episodes. During unstable arrhythmias, stroke impedance (32 +/- 17%), arterial systolic pressure (40 +/- 11%), and right ventricular pulse pressure (15 +/- 20%), expressed as percentages of corresponding sinus rhythm values, were significantly lower than in stable arrhythmias (84 +/- 26%, 72 +/- 8%, and 111 +/- 37%, respectively); P less than 0.001. There was a good correlation between stroke impedance and mean arterial pressure during arrhythmia (r = 0.84). Impedance sensing is a practical method for distinguishing between hemodynamically stable and unstable arrhythmias. Implementation of hemodynamic sensing into the algorithm of future antitachycardia systems may improve the management of arrhythmias by adding options for selective pace termination or cardioversion.