Bruce G. Hook

Implantable cardioverter-defibrillator therapy in the absence of significant symptoms. Rhythm diagnosis and management aided by stored electrogram analysis.

BackgroundThis report describes the value of stored ventricular electrogram analysis in the diagnosis and management of patients experiencing minimal or no symptoms before implantable cardioverterdefibrillator (ICD) therapy Methods and ResultsThe study population included 48 patients who received the Cadence Tiered Therapy Defibrillator System, an investigational third-generation ICD with ventricular electrogram storage capabilities. Criteria for arrhythmia diagnosis were based on analysis of the electrogram rate, RR interval variability, and morphology. Twenty-nine of the 48 patients (60%) experienced at least one episode of antitachycardia pacing or shock (one shock or more in 25 of 29 patients) that was preceded by minimal or no symptoms during a mean follow-up of 15.1±7.8 months. There were 194 tachycardia episodes registered by the device, including 101 for which ventricular electrograms were stored and available for analysis. Of the 101 stored electrograms, 74 were classified as ventricular tachycardia (VT), 24 as non-VT rhythms (atrial fibrillation, 13; supraventricular tachycardia, six; rate-sensing lead disruption, four, T wave oversensing, one), and only three as indeterminate rhythms. Based on the electrogram analysis, changes in tachycardia detection criteria and/or antiarrhythmic drug regimens were implemented and were associated with a reduction in the number of device responses for non-VT rhythms from 24 during the initial study period to three during 11.0±7.2 months of additional follow-up. ConclusionICD responses in the absence of symptoms are relatively common in third-generation devices with antitachycardia pacing capabilities. Despite potential limitations such as the effect of bundle branch block on the electrogram morphology during supraventricular tachycardia, the availability of electrogram storage capabilities allowed a presumptive diagnosis of the events precipitating asymptomatic device responses. Device reprogramming based on analysis of stored electrograms was associated with a dramatic reduction in the incidence of ICD responses for non-VT rhythms.

Value of ventricular electrogram recordings in the diagnosis of arrhythmias precipitating electrical device shock therapy

An antitachycardia pacemaker-cardioverter-defibrillator that is capable of storing ventricular electrograms before and after delivery of device shock therapy was implanted in 16 patients. Three of the patients experienced out-of-hospital device shock therapy preceded by minimal symptoms. Although limitations of electrogram analysis exist and are discussed, careful analysis and registration of electrograms during all supraventricular and ventricular rhythms observed during in-hospital testing served as an important reference for subsequent arrhythmia diagnosis. By analyzing the electrogram rate and RR interval stability and configuration, a definitive diagnosis was established in all three patients (atrial fibrillation, polymorphic ventricular tachycardia and rate-sensing lead disruption, respectively). Thus, the ability to store ventricular electrograms before shock therapy represents a major advance in the management of patients who receive an electrical device to treat ventricular tachyarrhythmia.

ICD Data Storage: Value in Arrhythmia Management

Sophisticated diagnos tic information is provided by the latest generation of implantable defibriliators. The success of therapy and the type of therapy successful in terminating ventricular arrhythmias is provided by interrogating the ICD device. In addition, R to R interval information can be retrieved. In selected devices, either locai bipolar electrograms from the rate sensing leads or wide bipolar electrograms from the energy delivering leads provide visual confirmation of the presence of ventricular tachyarrhythmic events loading to ther apy. The value and limilations of this sophisticated diagnostic information in providing insight into the electrical events triggering therapy and the events triggering ventricular arrhythmias are discussed.

Unique sensing errors in third-generation implantable cardioverter-defibrillators

OBJECTIVES Third-generation cardioverter-defibrillators appear to be susceptible to unique sensing errors. This study was performed to determine the incidence and types of sensing errors in combination therapy implantable devices. BACKGROUND One of the advantages offered by third-generation implantable cardioverter-defibrillators is the combination of bradycardia and antitachycardia pacing and cardioversion-defibrillation capabilities in a single device. The potential for unique sensing errors, those caused by the conflicts presented by combining bradycardia and tachycardia sensing and therapy algorithms in the same device, has not been previously addressed. METHODS To determine the incidence of important sensing errors, 61 patients with a combination therapy device (Cadence [Ventritex] and PCD [Medtronic]) were studied for a 25-month period. In addition to surface electrocardiographic recordings during implantation and routine device testing, real-time and stored electrograms recorded from the rate-sensing leads (Cadence) and real-time marker channel recordings (PCD) were reviewed to diagnose sensing errors that resulted in symptoms, device inefficacy or delivery of inappropriate therapy. After recognition, specific reprogramming steps were performed in an attempt to avoid recurrent sensing errors. RESULTS A total of 13 sensing errors were diagnosed in 12 patients (19.7%); the incidence was similar in both devices. Five distinct categories of sensing errors were identified. After device reprogramming, only one recurrent error occurred in 98 patient-months of follow-up. CONCLUSIONS Important sensing errors occur in approximately 20% of patients with third-generation combination therapy cardioverter-defibrillators. Prompt diagnosis of sensing errors can lead to specific reprogramming steps to avoid recurrent errors.

Importance of abortive shock capability with electrogram storage in cardioverter-defibrillator devices

OBJECTIVES This study evaluates the ability of a third-generation cardioverter-defibrillator to abort energy delivery and the importance of electrogram storage in analyzing the aborted events. BACKGROUND In the Cadence Tiered Therapy Defibrillator, when a tachycardia satisfies detection criteria for cardioversion or defibrillation therapy, high voltage capacitors begin charging. The Cadence defibrillator continues monitoring the rhythm during charging and if the rate decreases to below the rate triggering therapy, charging is terminated. This event is registered as an aborted shock. The defibrillator also has the ability to store intracardiac electrogram recordings of the electrical events that precipitate device therapy or aborted shocks. METHODS During a mean follow-up interval of 10 +/- 7 months, 55 aborted events were registered by the Cadence defibrillator in 18 of the 49 patients who received it. Thirty-two stored ventricular electrograms of events leading to aborted shocks were available for analysis in 15 patients. RESULTS Intracardiac electrogram recordings demonstrated the probable electrical events leading to these aborted shocks included nonsustained ventricular tachycardia (n = 10), nonsustained rapid polymorphic ventricular tachycardia/ventricular fibrillation (n = 2), atrial fibrillation (n = 5), supraventricular tachycardia (n = 2) and electrical noise (n = 13). Eleven patients had a therapeutic intervention initiated as a consequence of the diagnostic information provided by analysis of intracardiac electrogram recordings. Four of the 15 patients had no changes made. During a follow-up period of 9 +/- 5 months after therapy was altered, no patient had subsequent aborted shocks. Five patients have had seven appropriate shocks for sustained ventricular tachycardias. CONCLUSIONS The ability of Cadence defibrillator to continue tachycardia sensing during capacitor charging and to abort shock therapy for self-terminating events prevented unnecessary shocks in 18 (37%) of the 49 patients. Intracardiac electrogram recordings were critical for instituting appropriate therapy that may have prevented unnecessary device charging and inappropriate discharges.

Prediction of Sustained Ventricular Tachycardia Inducible by Programmed Stimulation in Patients With Coronary Artery Disease Utility of Clinical Variables

BACKGROUND Cardiologists often use clinical variables to determine the need for electrophysiological studies to stratify patients for risk of sudden death. It is not clear whether this is rational in patients with coronary artery disease, left ventricular dysfunction, and nonsustained ventricular tachycardia. METHODS AND RESULTS We analyzed the first 1721 patients enrolled in the Multicenter UnSustained Tachycardia Trial to determine whether clinical variables could predict which patients would have inducible sustained monomorphic ventricular tachycardia. The rate of inducibility of sustained ventricular tachycardia was significantly higher in patients with a history of myocardial infarction and in men compared with women. There was a progressively increased rate of inducibility with increasing numbers of diseased coronary arteries. There was a significantly lower rate of inducibility in patients with prior coronary artery bypass surgery and in patients who also had noncoronary cardiac disease. The rate of inducibility was higher in patients of white race, patients with recent (</=6 weeks) angina, left ventricular dyskinesis, and in patients with greater numbers of fixed thallium defects. Inducibility was more likely in patients who had a prior myocardial infarction complicated by congestive heart failure, ventricular tachycardia, or fibrillation </=48 hours after the onset of infarction. Although these associations are statistically significant, the accuracy of the clinical variables in discriminating between patients with and those without inducible ventricular tachycardia is only modest (receiver operator characteristic area <0.70). CONCLUSIONS Multiple clinical variables are independently associated with inducible sustained ventricular tachycardia. However, they have limited utility to guide clinical decisions regarding the use of electrophysiological testing for risk stratification in this patient population.

Comparison of resetting and entrainment of uniform sustained ventricular tachycardia. Further insights into the characteristics of the excitable gap.

BACKGROUND Resetting and entrainment have both been used to characterize the electrophysiological properties of the reentrant circuit in ventricular tachycardia. Several entrainment studies have suggested that the circuit has decremental properties, because the return cycle increases at faster pacing rates. Resetting, however, demonstrates a fully excitable gap in the majority of tachycardias. METHODS AND RESULTS The response to resetting and overdrive pacing was analyzed in 18 ventricular tachycardias. Resetting demonstrated some duration of a fully excitable gap in 14 of 18 tachycardias. Overdrive pacing was performed at several cycle lengths with an incremental number of stimuli (1-15 beats) such that the first beat that interacted with the tachycardia (the nth beat) could be identified. The return cycles measured during resetting and the nth beat of pacing were identical (r = 0.99). At relatively long paced cycle lengths, paced beats after the nth beat resulted in a constant return cycle in most tachycardias with a fully excitable gap. At rapid paced cycle lengths, an increase in the return cycle from the nth to the nth + 1 beat was associated with progressive prolongation in the return cycle with each incremental paced beat until a longer equilibrium return cycle was reached or the tachycardia terminated in response to pacing. CONCLUSIONS We propose that the responses to resetting and overdrive pacing with or without entrainment appear to provide conflicting information about the characteristics of the circuit because they in fact measure entirely different electrophysiological parameters. The nth beat of pacing foreshortens the excitable gap to the extent that it arrives prematurely. Subsequent paced beats interact with an altered tachycardia circuit that has had less time to recover excitability. Resetting is the interaction of a single paced beat with the tachycardia and, as such, provides a more accurate assessment of the characteristics of the unaltered tachycardia circuit.

Paced Beats Following Single Nonsensed Complexes in a “Codependent” Cardioverter Defibrillator and Bradycardia Pacing System: Potential for Ventricular Tachycardia Induction

Patients with implantable defibrillotors often require bradycardia pacemakers. Adverse interactions between separate defibrillator and bradycardia pacing units hove occurred. Including failure in detect ventricular fibrillation due to persistent bradycardia pacing during the arrhythmia. A device with combined bradycardia pacing and antitachycardia therapy capability may obviate adverse device interactions. We describe a previously unrecognized phenomenon that may occur in a combined device when the algorithms for sensing bradycardia and tachycardia are “codependent”; that is, the circuitry for brady and tachyarrhythmia detection relies on the same automatic gain sense amplifier. Three of 37 patients in whom the device was implanted had ventricular tachycardia initiated when bradycardia pacing stimuli were delivered by the device after probable nonsensed sinus beats. In each case, nonsensed beats appeared to have a markedly diminished amplitude, occurred after ventricular premature depolarizations that produced large amplitude electrograms, and had an electrogram morphology that matched that of sinus rhythm. In each case, the bradycardia pacing interval was at least 1,200 msec (range 1.200 to 1,714 msec). In two of the three patients, large amplitude ventricular premature depolarizations or nonsustained ventricular tachycardia caused an adjustment of the gain control that potentiated the failure to sense the subsequent lower amplitude signal. In all three patients, the induced arrhythmia was rapidly terminated by pacing or cardioversion. Decreasing the bradycardia pacing interval by 110–514 msec has prevented recurrence during short‐term follow‐up. Our findings suggest that codependent bradycardia and antitachycardia devices may have their own unique potential difficulties in adapting to rapid changes in rate and signal amplitude.

Effect of rate and coupling interval on endocardial R wave amplitude variability in permanent ventricular sensing lead systems.

OBJECTIVES We have observed sensing errors in third generation implantable cardioverter-defibrillators that appear to be caused by variation in the R wave amplitude during sinus rhythm, particularly after premature beats. The purpose of this study was to quantify spontaneous R wave variability during sinus rhythm and to determine whether abrupt changes in cycle length further augment R wave amplitude variability. BACKGROUND Pacemaker sensing algorithms presume a relatively constant R wave signal to establish a sensing threshold. The concept of a fixed sensing threshold is not as applicable in third-generation cardioverter-defibrillators, which depend on automatic gain amplifiers to rapidly detect ventricular fibrillation. These devices may be susceptible to undersensing during sinus rhythm if significant variability in R wave signal characteristics occurs. METHODS Twelve patients with combination bradycardia pacing cardioverter-defibrillators were studied. The device used (Cadence, Ventritex) allowed recording of real time, telemetered electrograms from the sensing lead system. Measurements were made of the maximal range of the R wave amplitude during sinus rhythm and in response to abrupt changes in heart rate produced by premature atrial and ventricular stimuli. RESULTS The maximal range in R wave amplitude during sinus rhythm was 1.7 +/- 1.3 mV, or 23.7 +/- 19.2% of the mean R wave amplitude. The R wave amplitude variability increased with abrupt changes in cycle length, with a range of 2.8 +/- 1.5 mV, or 38.8 +/- 18.3% of the mean R wave amplitude (p < 0.05 compared with sinus rhythm). In most patients, R wave amplitude and coupling interval demonstrated an inverse proportional relation. CONCLUSIONS There is substantial variability in the R wave amplitude during sinus rhythm measured by permanent ventricular sensing lead systems, and this variability is further augmented by abrupt changes in cycle length. This phenomenon may explain the occurrence of undersensing of sinus rhythm in implantable cardioverter-defibrillators with automatic gain sense amplifiers.

Effectiveness of Noninvasive Programmed Stimulation for Initiating Ventricular Tachyarrhythmias in Patients with Third‐Generation Implantable Cardioverter Definrillators

Previous generations of implantable cardioverter defibrillators (ICDs) required invasive electrophysiological testing to assess defibrillator function. Newer third‐generation ICDs include the capability for performing noninvasive programmed stimulation (NIPS) and may reduce the need for invasive studies to assess tachycardia recognition and antitachycardia therapy algorithms. The effectiveness of ICD‐based NIPS for the induction of ventricular arrhythmias has not, however, been formally assessed. Third‐generation ICDs were implanted in 79 patients, who underwent a total of 166 postoperative defibrillator tests. NIPS with rapid ventricular pacing was performed in all patients in an attempt to induce ventricular fibrillation. In patients with prior sustained uniform ventricular tachycardia, programmed stimulation with up to three extrastimuli was performed in order to attempt to initiate the clinical ventricular tachcardia. Ventricular fibrillation was induced with NIPS in 146 of 166 studies (88%). Ventricular tachycardia was initiated with NIPS in 104 of 123 studies (85%). The type of defibrillator and the use of endocardial or epicardial rate sensing/ pacing leads did not influence the efficacy of NIPS. NIPS with third‐generation ICDs is generally effective at inducing ventricular fibrillation and clinically relevant ventricular tachycardias, and reduces the need to perform invasive electrophysiological testing following device implantation. In a minority of patients temporary transvenous pacing catheters must still be used to facilitate arrhythmia induction.