George Horvath

Effects of hormone replacement therapy on QT interval.

We evaluated the electrocardiograms of 208 postmenopausal women (ages 40 to > or = 70 years) without heart disease, medications that could alter the QT interval, use of vaginal estrogens, unknown hormone replacement therapy, or electrocardiographic abnormalities both with (n = 76) and without (n = 132) hormone replacement therapy, and found no significant effects of hormone replacement therapy status on heart rate, QT interval, or the corrected QT interval. Thus, estrogen and/or progesterone effect does not explain the gender differences in myocardial repolarization.

Simultaneous Occurrence of Atrial Fibrillation and Atrial Flutter

Simultaneous Atrial Fibrillation and Flutter, Introduction: Early reports suggested that some patients with “atrial fibrillation/flutter” might have atrial fibrillation in one atrium and atrial flutter in the other. However, more recent conceptions of atrial fibrillation/flutter postulate that the pattern is due to a relatively organized (type I) form of atrial fibrillation. We report the occurrence and ECG manifestations of simultaneous atrial fibrillation and flutter in patients undergoing attempted catheter ablation of atrial flutter.

Detection of ventricular fibrillation by transvenous defibrillating leads: integrated versus dedicated bipolar sensing.

Integrated Versus Dedicated Bipolar Sensing of VF. Introduction: Dedicated bipolar sensing has been suggested to be safer than integrated bipolar sensing due to an increased incidence of failure to redetect ventricular fibrillation after an unsuccessful shock with leads that use integrated bipolar sensing. We compared sensing charactersitics during ventricular fibrillation of simultaneously recorded dedicated and integrated bipolar electrograms.

Utility of predischarge and one-month transvenous implantable defibrillator tests

We evaluated the utility of performing predischarge implantable cardioverter-defibrillator (ICD) testing and ICD testing at 1 month in detecting ICD-related problems, identifying 4 patients with an unacceptable increase in defibrillation threshold. Given that ICD testing is noninvasive and has minimal (if any) associated risk, we recommend that predischarge and 1-month follow-up ICD testing should be performed after implantation.

Reproducibility of Ventricular Fibrillation Characteristics in Patients Undergoing Implantable Cardioverter Defibrillator Implantation

Reproducibility of VF Characteristics. Introduction: The purpose of this study was to evaluate the immediate reproducibility of local electrogram characteristics recorded during repeated episodes of induced ventricular fibrillation (VF) in patients undergoing implantable cardioverter defibrillator (ICD) implantation.

Inappropriate Implantable Cardioverter-Defibrillator Therapy Due to the Detection of Premature Ventricular Complexes

Inappropriate shocks delivered by implantable cardioverter‐defibrillators may occur in 15%–40% of patients treated with these devices. This article describes a rare cause for inappropriate shocks. Two patients received inappropriate shocks due to the presence of premature ventricular complexes during attempted reconfirmation of ventricular fibrillation. Knowledge of device algorithms for detection and reconfirmation of ventricular fibrillation, designed to have a high degree of sensitivity and therefore lower specificity, allows for reprogramming to avoid further inappropriate shocks.

Electrophysiological and Anatomic Heterogeneity in Evolving Canine Myocardial Infarction

Although the heterogeneity of electrophysiological properties is increased after myocardial infarction, the degree of this heterogeneity has not been well quantitoted and its relationship to the histoslogical changes that occur after infarction has not been carefully examined. The purpose of the present study was to test the hypothesis that alterations in electrophysiological properties in healing canine infarction are related to particular histological changes. Experimental infarction was produced by left anterior descending coronary ligation. Six dogs were used as controls, six were studied 5 days following, and six were studied 8 weeks following infarction. Pacing thresholds, effective refractory periods, and activation‐recovery times were determined at 112 sites on the anterior left ventricle using a multiple electrode plaque. Conduction velocity, conduction‐heterogeneity index—a measure of conduction disturbance—and histology of the epimyocardium underlying the plaque were assessed. The effective refractory periods and activation‐recovery times were greater in both infarction groups, most prominently in the subacute group. In subacute infarction, significant postrepolarization refractoriness was present. In healed infarction, conduction velocity was decreased and the conduction‐heterogeneity index was increased compared to controls and subacute infarction. Dispersion of excitability and repolarization was associated with more extensive local scarring. Dispersion of myocardial fiber angles was associated with the conduction‐heterogeneity index. Some but not all of the electrophysiological changes noted in the animals with infarction were also seen in sham operated animals. Thus, heterogeneity in repolarization and refractoriness is greatest in the subacute phase of myocardial infarction and is associated with the extent of local cell death. In contrast, disturbances in conduction are greatest in healed infarction and associated with disarray of myocardial fibers.

Characteristics of ischemic and peri-ischemic regions during ventricular fibrillation in the canine heart.

Ischemic Ventricular Fibrillation. Introduction: Although premature heats originating in areas of ischemia have been shown to he important in initiating ventricular fibrillation (VF), the participation of the ischemic zone in maintenance of VF has not been investigated.

Twelve‐Lead QT Dispersion Is Smaller In Women Than In Men

Although gender differences in the corrected QT interval (QTc) have been described, gender differences in QT dispersion (QTd) have not been carefully evaluated. This study was designed to measure QTd in 22 young healthy women and in 20 healthy men.

Effect of Underlying Heart Disease on the Frequency Content of Ventricular Fibrillation in the Dog Heart

Although prior studies have examined the frequency content of heal electro‐gram characteristics during fibrillation, little is know about the effects of underlying heart disease on these parameters. This study was designed to compare the frequency content of local electrograms during VF in canine models of acute ischemia, subacute infarction, and chronic myocardial infarction (MI) to those in control animals to test the hypothesis that underlying heart disease can alter the basic characteristics of VF. VF was induced using burst pacing in three groups of mongrel dogs. Five dogs were evaluated 8 weeks after LAD occlusion MI, five were evaluated 5 days after experimental MI, and 5 had VF induced before (control) and immediately after LAD occlusion (ischemia). During VF, unipolar electrograms were recorded from 112 sites on the anterior LV and electrograms were evaluated 15 and 30 seconds after VF initiation in each group. Electrograms were analyzed by fast Fourier transform. No significant time dependent changes in VF characteristics were noted. The peak frequency was highest in control animals and 8‐week MI, intermediate in 5‐day MI, and lowest in acute ischemia (P < 0.01 for pairwise comparisons). In contrast, the fractional of energy within a bandwidth of 25% peak amplitude was highest in acute ischemia, (P < 0.001) and similar in the other three groups. Infarction decreased total energy by approximately 50%. In conclusion, the pressure of ischemia or infarction alters the frequency content of VF in a complex fashion. In addition to decreasing the peak frequency, the shape of the power spectral curve is altered in models of structural heart disease. These results suggest that the electrophysiological changes produced by infarction or ischemia alter the structural organization of ventricular fibrillation.