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Biomedical subjects

D J Callans

Publications and source records attributed to D J Callans.

At least 19 recordsLinked to original sources
PubMed1996

Ventricular tachycardia: progress in acute and chronic patient care.

Patients with ventricular tachycardia (VT) typically have significant structural heart disease and therefore benefit from all advances made in the past 50 years in cardiovascular care. Once sinus rhythm is restored, subsequent treatment goals include detecting the primary causes of VT and treating acute recurrences, determining the extent of structural heart disease and assessing the need for treatment of heart failure and ischemia, and prescribing chronic antiarrhythmic therapy. Implantable-device therapy has revolutionized the treatment of patients with VT.

Acute Disease↗
PubMed1995

Circadian variation of ventricular arrhythmia recurrences after cardioverter-defibrillator implantation in patients with healed myocardial infarcts.

Past studies using Holter monitoring and retrospective reviews of death certificates have documented peak occurrence of sudden death and nonsustained ventricular tachycardia (VT) in the morning hours. We used the Ventritex Cadence device (Ventritex, Sunnyvale, California) which documents the date and time of all stored arrhythmias leading to device therapy to evaluate the circadian pattern of sustained ventricular arrhythmia recurrence. Mean follow-up after defibrillator implantation was 628 +/- 285 days. All 390 patients had at least 1 episode (range 1 to 43) of sustained VT documented from analysis of the stored electrograms associated with an arrhythmia event. Stored electrograms were available for review and analysis in 3,041 device detections; 349 stored events were excluded because they did not fulfill the diagnostic criteria for VT or failed to document the onset of the ventricular arrhythmia at the beginning of the recorded event of the arrhythmia episode. Criteria for the diagnosis of VT or ventricular fibrillation were met in 2,692 arrhythmia episodes occurring in 390 patients. There was circadian variation for ventricular arrhythmia recurrence for the whole patient group with the data fit to the sinusoidal density function: f(t) = 126 - 51 x cos (-57 + 2 pi t/24) - 25 x sin (63 + 2 pi t/12) (p < 0.0001). Ventricular arrhythmia occurrence rate was lowest between 2:00 and 3:00 A.M., and highest between 10:00 and 11:00 A.M. In addition, the same circadian pattern was demonstrated regardless of patient age, gender, left ventricular ejection fraction (< 35% or > or = 35%), and VT cycle length (< 300 or > or = 300 ms).(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗
PubMed1995

Characterization of return cycle responses predictive of successful pacing-mediated termination of ventricular tachycardia.

OBJECTIVES: The purpose of this study was to characterize response patterns during overdrive pacing that predict successful termination of ventricular tachycardia. BACKGROUND: Overdrive pacing during ventricular tachycardia typically results in entrainment at slow pacing rates and in termination or acceleration at faster rates. The factors that determine the critical paced cycle length that results in tachycardia termination have not been extensively studied. METHODS: Ventricular tachycardias in 14 patients with coronary artery disease were studied with overdrive pacing at several cycle lengths. Return cycles were measured after each additional paced beat at each paced cycle length. The return cycle responses during pacing trials that resulted in tachycardia termination and those that resulted in entrainment were compared. RESULTS: Three return cycle responses were identified: flat, plateau and increasing. Twenty trials of overdrive pacing resulted in tachycardia termination; all were characterized by an increase in the return cycle with the delivery of each successive beat in the pacing drive until the tachycardia terminated (increasing response). Thirty-four pacing trials resulted in entrainment and not termination; these were characterized either by a constant return cycle (flat response) or an initial increase in return cycle followed by a longer, constant return cycle (plateau response) with the delivery of additional paced beats. The longest paced cycle length that resulted in tachycardia termination correlated with the relative refractory period of the circuit, defined as the tachycardia cycle length minus the fully excitable gap (r2 = 0.764, p = 0.0001). Tachycardia termination was not observed unless the paced cycle length was shorter than the relative refractory period of the circuit. CONCLUSIONS: The critical paced cycle length that causes termination of ventricular tachycardia depends on the relative refractory period of the circuit because this factor determines whether the nth + 1 beat of the pacing drive will encounter partially recovered tissue. These data provide insights into the mechanism of pacing-mediated tachycardia termination and entrainment and are applicable to the development of improved antitachycardia pacing algorithms.

Aged↗
PubMed1995

Comparison of initial detection and redetection of ventricular fibrillation in a transvenous defibrillator system with automatic gain control.

OBJECTIVES: The purpose of this study was to prospectively evaluate postshock redetection of ventricular fibrillation by a system that coupled an implantable cardioverter-defibrillator with an automatic gain control sense amplifier and a transvenous lead system. BACKGROUND: Redetection of ventricular fibrillation after an unsuccessful first shock has not been systematically evaluated. Previous studies have suggested that sensing performance of some lead systems may be adversely affected by the delivery of subthreshold shocks. METHODS: The time required for both initial detection and redetection of ventricular fibrillation was compared in 22 patients. These times were estimated by subtracting the capacitor charge time from the total event time. RESULTS: A total of 113 successful and 57 unsuccessful initial shocks were delivered during induced ventricular fibrillation. The mean +/- SD initial time to detection of ventricular fibrillation was 5.5 +/- 1.7 s (range 2.4 to 10.8); the time to redetection ranged from 1.5 to 18.5 s (mean 4.5 +/- 2.8, p = NS vs. detection time). Abnormal redetection episodes, defined as a redetection time > 10.2 s (i.e., > 2 SD above the mean redetection time), were observed in 4 (18%) of 22 patients. CONCLUSIONS: Redetection of ventricular fibrillation after a subthreshold first shock may be delayed. Device testing with intentional delivery of subthreshold shocks to verify successful postshock redetection of ventricular fibrillation should be performed routinely in all patients.

Algorithms↗
PubMed1995

Postoperative lead-related complications in patients with nonthoracotomy defibrillation lead systems.

OBJECTIVES: This study sought to document postoperative complications attributable to nonthoracotomy defibrillation lead systems in a large cohort. BACKGROUND: The incidence of postoperative complications specifically associated with nonthoracotomy defibrillation lead systems is unknown. METHODS: Postoperative lead-related complications were evaluated in 170 patients with a nonthoracotomy defibrillation lead system who were followed up for a mean (+/- SD) of 17 +/- 12 months. Each system incorporated one or more intravascular leads. In 117 patients (69%), the system incorporated a subcutaneous defibrillation patch. All implantations were performed in an operating room by cardiothoracic surgeons. Defibrillation thresholds were measured at implantation, before hospital discharge (mean 3 +/- 2 days) and at 4 to 18 weeks after implantation. Patients were evaluated every 2 to 3 months after implantation or as indicated by clinical exigency. RESULTS: Twenty-seven patients (15.9%) were diagnosed with a lead-related complication that either extended the initial hospital period or led to a second hospital admission. Complications included endocardial lead or subcutaneous defibrillation patch dislodgment in eight patients (4.7%), which was diagnosed between 2 and 345 days after implantation; endocardial or subcutaneous patch lead fracture in six (3.5%), which was diagnosed between 53 and 600 days after implantation; subcutaneous patch mesh fracture in one, which was diagnosed at 150 days after implantation; subclavian vein thrombosis in three (1.8%), which was diagnosed at 2 to 50 days after implantation; and unacceptably elevated defibrillation threshold (within 5 J of maximal device output) in nine (5.3%), which was documented at one of the two postimplantation evaluations in eight patients or at the time of failure to terminate a spontaneous ventricular tachycardia in one. Seventeen of the 27 patients required reoperation for correction of their complication. In addition, system infection requiring complete explantation occurred in seven other patients (4.1%) at an interval from implantation ranging from 14 to 120 days. CONCLUSIONS: Postoperative complications related to a nonthoracotomy defibrillation lead system were common and frequently required reoperation for correction. The rate of system explantation due to infection was also significant. Postoperative defibrillation testing and vigilant outpatient follow-up evaluation are necessary to ensure normal lead function.

Binomial Distribution↗
PubMed1995

Efficacy of antitachycardia pacing in patients presenting with cardiac arrest.

The efficacy of antitachycardia pacing (ATP) incorporated into implantable cardioverter defibrillators (ICDs) was assessed in 29 consecutive survivors of cardiac arrest, not attributable to acute myocardial infarction, ischemia, or drug and electrolyte effects. The cohort included 25 men and 4 women with a mean age of 65 years and a mean left ventricular ejection fraction of 29%. Seventeen patients had coronary artery disease, 11 had nonischemic dilated cardiomyopathy, and 1 had long QT syndrome. Programmed stimulation yielded monomorphic ventricular tachycardia (VT) in 17 patients, polymorphic VT in 6, and no inducible VT in 6. During a mean follow-up of 22 months, a total of 91 episodes of monomorphic VT occurred, 73 of which were successfully pace terminated (83%). Monomorphic VT amenable to pace termination recurred only in the group that had this arrhythmia inducible. The recurrent arrhythmias in the 12 patients having either no inducible VT or polymorphic VT were all rapid VTs, having a cycle length < 220 ms; and therefore, not amenable to pace termination. These results suggest that ATP incorporated into ICDs is useful in survivors of cardiac arrest and may significantly reduce the number of shocks that these patients would otherwise receive. Programmed stimulation may also help to define those patients who would receive the maximum benefit from ATP.

Adult↗
PubMed1995

Benefits and lessons learned from stored electrogram information in implantable defibrillators.

Implantable defibrillators have evolved from simple event counters to sophisticated diagnostic monitoring units capable of storing electrocardiographic information surrounding arrhythmia events and device therapy. In this review, the nature and characteristics of these stored electrocardiographic recordings are discussed and examples displayed. Potential benefits and limitation of stored electrogram analysis are described with respect to both clinical utility and the ability to enhance our understanding of ventricular arrhythmogenesis. Finally, future developments to improve data storage, retrieval, and analysis are identified.

Defibrillators, Implantable↗
PubMed1995

Postshock sensing performance in transvenous defibrillation lead systems: analysis of detection and redetection of ventricular fibrillation.

INTRODUCTION: The sensing performance of transvenous lead systems may be adversely affected by the delivery of high-energy shocks. This may be due to the proximity of the sensing and energy-delivery electrodes on transvenous leads. METHODS AND RESULTS: The time required for detection of ventricular fibrillation and redetection after a failed first shock was compared in 93 patients with five different lead system-pulse generator combinations: Cadence--Endotak 60 series, Ventak P--Endotak 60 series, Jewel--Transvene, Cadence--TVL, and Cadence--Transvene. A total of 418 successful and 204 failed first shocks were delivered during induced ventricular fibrillation. Redetection times (RED) were consistently shorter than detection times (DET) in the Jewel-Transvene (RED minus DET: -1.9 +/- 0.8 sec, P < 0.0001), the Cadence-TVL (-1.6 +/- 1.0 sec, P < 0.0001), and the Cadence-Transvene combinations (-2.0 +/- 0.9 sec, P < 0.0004). Redetection times were not significantly different than detection times in the Cadence-Endotak combination (0.9 +/- 3.1 sec; P = 0.09). Redetection times were significantly longer than detection times in the Ventak-Endotak combination (1.2 +/- 2.3 sec; P = 0.034). Prolonged individual redetection episodes (> 8.2 sec) were observed in the Cadence-Endotak (7 [10%] of 73 episodes) and the Ventak-Endotak (4 [10%] of 39 episodes), but not in the Jewel-Transvene, the Cadence-TVL, and the Cadence-Transvene combinations. CONCLUSIONS: Redetection of ventricular fibrillation may be delayed in some transvenous lead-pulse generator combinations. Successful redetection of ventricular fibrillation following a failed first shock should be demonstrated prior to hospital discharge of patients with implantable defibrillators.

Adolescent↗
PubMed1995

Insights into the electrophysiology of atrial arrhythmias gained by the catheter ablation experience: "learning while burning, Part II".

Although the development and wide-scale application of catheter ablative techniques has drastically changed the practice of electrophysiology, catheter ablation does not preclude physiologic investigation. On the contrary, given the precise and directed nature of this technique and the increased attention to detailed cardiac mapping that it requires, catheter ablation may be viewed as a tool to provide unique information about arrhythmia substrates. In this article, the insights provided by the catheter ablation experience into the pathophysiology of the focal atrial arrhythmias, atrial tachycardia, sinus node reentrant tachycardia, and inappropriate sinus tachycardia will be reviewed. Atrial arrhythmias were initially difficult to treat with ablative therapy, particularly because they can occur anywhere within the atria and the experience with mapping for surgical ablation was quite limited. A number of novel approaches to atrial mapping have been developed in response to this challenge, and presently, catheter ablation provides effective therapy for the majority of patients with focal atrial arrhythmias. In addition, deliberate attempts at "learning while burning" have already begun to enhance our understanding of the interaction of the structural and electrophysiologic aspects of the substrate for atrial arrhythmias.

Arrhythmias, Cardiac↗
PubMed1994

Effectiveness of noninvasive programmed stimulation for initiating ventricular tachyarrhythmias in patients with third-generation implantable cardioverter defibrillators.

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.

Adult↗
PubMed1994

Insights into the electrophysiology of ventricular tachycardia gained by the catheter ablation experience: "learning while burning".

The success of catheter ablation has significantly improved the treatment of patients with cardiac arrhythmias and has established electrophysiology as an increasingly interventional subspecialty. Some members of the electrophysiology community have expressed concern that this success has been purchased at the cost of undermining what had been our primary concern: understanding the anatomic and physiologic basis of arrhythmia syndromes. In many laboratories, endpoints such as case load and primary success have eclipsed physiologic investigation. Despite these trends, however, catheter ablation is not inherently at odds with investigation and education. On the contrary, because the lesions delivered with current techniques are much more discrete than the effects of antiarrhythmic agents or surgical ablation, catheter ablation can be used as a research tool directed toward a more precise understanding of arrhythmia substrates. Conscious attempts at "learning while burning" have already provided important and unique information about arrhythmia pathogenesis.

Cardiomyopathy, Dilated↗
PubMed1993

Electrophysiologic recovery in postischemic, stunned myocardium despite persistent systolic dysfunction.

Previous investigators have hypothesized that myocardial "stunning" may result either from a primary impairment in excitation or from electromechanical dissociation. Thrombolytic therapy and angioplasty have increased the importance of understanding the electrophysiologic effects of brief ischemia followed by reperfusion. We investigated the electrophysiologic properties of mechanically dysfunctional stunned myocardium in 18 dogs anesthetized with pentobarbital (30 mg/kg, intravenously administered). After thoracotomy, the proximal anterior descending coronary artery was occluded for 15 minutes, which was followed by 20 minutes of reperfusion. At baseline, peak ischemia, and 20 minutes of reperfusion, local electrogram durations, activation times, and refractory periods were measured from 12 standardized sites within the ischemic and border zones. Echocardiographic percentage of systolic wall thickening confirmed normal preischemic and markedly reduced postischemic function in the investigated region. Despite the marked electrophysiologic abnormalities observed in the ischemic zone during ischemia, mean electrogram duration, calculated conduction velocity, and mean effective refractory period after 20 minutes of reperfusion had returned almost to baseline values 39.2 +/- 11.5 msec versus 37.2 +/- 12.1 msec, 0.65 +/- 0.15 m/sec versus 0.68 +/- 0.15 m/sec, and 134 +/- 14 msec versus 131 +/- 8 msec, respectively. Corresponding mean values within the ischemic border zone were similarly close to baseline values after reperfusion. There was no significant difference in local heterogeneity (coefficient of variation) within the ischemic or border zone after reperfusion versus baseline values. Although the postischemic electrophysiologic status returned to normal, systolic thinning and dyskinesis persisted in the region of measurement. The contractile dysfunction that results from reperfusion-induced injury can thus occur in the setting of apparent excitation-contraction uncoupling.

Analysis of Variance↗
PubMed1993

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.

Analysis of Variance↗
PubMed1993

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.

Algorithms↗
PubMed1993

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.

Aged↗