Jonathan J. Langberg

Electrical Remodeling in Atrial Fibrillation Time Course and Mechanisms

BACKGROUNDnAtrial fibrillation is self-perpetuating, suggesting that the tachyarrhythmia causes electrophysiological changes that contribute to the progressive nature of the disease. In animal models, pacing-induced rapid atrial rates result in sustained atrial fibrillation. This is mediated by shortening of refractory periods termed electrical remodeling. The purpose of the present study was to characterize the time course of electrical remodeling and to define mechanisms of the phenomenon.nnnMETHODS AND RESULTSnClosed-chest dogs were anesthetized, pretreated with atropine and propranolol, and subjected to 7 hours of atrial pacing at 800 bpm. The effective and absolute refractory periods (ARP and ERP) were measured during and after rapid pacing, and transvenous endocardial biopsy specimens were examined using electron microscopy. Despite autonomic blockade and the absence of change in right atrial pressure, persistent atrial tachycardia caused ARP and ERP to fall by > 10%. Electrical remodeling developed quickly, with more than half of the phenomenon occurring during the first 30 minutes of high-rate pacing. Pretreatment with glibenclamide in doses sufficient to block the ATP-sensitive potassium current had no effect. Atrial electrical remodeling was blocked by verapamil and accentuated by hypercalcemia. Biopsy specimens from controls subjected to rapid pacing showed mitochondrial swelling consistent with calcium overload. Biopsies from verapamil-treated animals were normal.nnnCONCLUSIONSnAtrial electrical remodeling develops quickly, is progressive, and may be persistent. Shifts in autonomic tone, atrial stretch, or depletion of high-energy phosphates do not contribute significantly to the phenomenon. Results of the study suggest that atrial electrical remodeling is mediated by rate-induced intracellular calcium overload.

Frequency Analysis of Human Atrial Fibrillation Using the Surface Electrocardiogram and Its Response to Ibutilide

This study assesses a technique for quantifying the frequency spectrum of atrial fibrillation (AF) using the surface electrocardiogram. Electrocardiograhic recordings were obtained in 61 patients during AF. After bandpass filtering, the QRST complexes were subtracted using a template-matching algorithm. The resulting fibrillatory baseline signal was subjected to Fourier transformation and displayed as a frequency power spectrum. These frequency spectra were compared to direct measurements from the right atrium and coronary sinus in 35 patients undergoing electrophysiologic study. The clinical use of this technique was explored by correlating fibrillatory frequency with the behavior of the arrhythmia in 26 patients referred for cardioversion. The electrocardiographic frequency spectrum during AF was characterized by a single peak that varied widely between patients (range 228 to 480 beats/min). There was a strong correlation between electrocardiographic peak frequency and that measured in the right atrium and coronary sinus (r = 0.79 to 0.98, p <0.0001). Episodes of AF that terminated in < 5 minutes had a lower frequency than those that persisted > 5 minutes (324 +/- 36 vs 402 +/- 78 beats/min, p = 0.001). Chronic AF (< 3 months in duration) had a lower frequency than chronic AF (present > 3 months) (336 +/- 48 vs 408 +/- 60 beats/ min, p = 0.012). Fibrillation frequency was an accurate predictor of conversion with ibutilide. Success rate was 100% in patients with peak frequency < 360 beats/min versus 29% in patients with frequencies > or = 360 beats/min (p = 0.003). Automatic analysis of the frequency content of the fibrillatory baseline on the surface electrocardiogram accurately reflects the average rate of AF. This measurement correlates with the clinical pattern of the arrhythmia and predicts the response to administration of ibutilide.

Non-invasive assessment of fibrillatory activity in patients with paroxysmal and persistent atrial fibrillation using the Holter ECG

OBJECTIVEnAutomatic analysis of the frequency content of the fibrillatory baseline on the surface ECG accurately reflects the average rate of atrial fibrillation (AF). This frequency measurement correlates with the behavior of AF and predicts the response to administration of ibutilide, a new antiarrhythmic drug. Neither the temporal pattern of fibrillatory frequency in spontaneous paroxysmal or persistent AF, nor its response to chronic antiarrhythmic medication has been studied so far.nnnMETHODS AND RESULTSnHolter ECG recordings were made in 20 patients during AF. One minute ECG segments were selected for analysis. The frequency content of the fibrillatory baseline was then quantified using digital signal processing. After high-pass filtering, the QRST complexes were subtracted using a template matching algorithm. The resulting fibrillatory baseline signal was subjected to Fourier transformation, displayed as a frequency power spectrum and the peak frequency (f) was determined. In 11 patients (7 male, 4 female, age 62 +/- 10 years) 31 paroxysmal AF episodes were analyzed. Duration ranged from 1 min to 665 min (115 +/- 175 min). Initial mean peak f measured 5.1 +/- 0.7 Hz (range 3.9 to 6.9 Hz). There was a positive correlation between f and AF duration (R = 0.53, p = 0.002). AF of less than 15 min duration (n = 13) showed a lower f (4.8 +/- 0.6 Hz) when compared with longer lasting episodes (n = 18, 5.3 +/- 0.7 Hz, p = 0.03). In short AF episodes f was constant, whereas in longer-lasting episodes f increased to 5.8 +/- 0.5 Hz (p < 0.001) within 5 min. In 9 patients (9 male, age 58 +/- 8 years) with persistent AF oral antiarrhythmic drugs (amiodarone n = 5, sotalol n = 3, flecainide n = 1) were given prior to electrical cardioversion for prophylaxis of AF recurrence. Frequency measurements were obtained at baseline and 3 to 5 days after initiation of drug administration. At baseline mean f measured 6.9 +/- 0.4 Hz. Frequency was reduced by antiarrhythmic drugs to 5.8 +/- 0.4 Hz (p < 0.001).nnnCONCLUSIONSn(1) The duration of paroxysmal AF episodes can be predicted using spectral analysis of ECG recordings of AF episodes. (2) An increase in fibrillatory frequency is associated with AF persistence. (3) This technique can be used to monitor the response to antiarrhythmic medication.

Hands-On Defibrillation An Analysis of Electrical Current Flow Through Rescuers in Direct Contact With Patients During Biphasic External Defibrillation

Background— Brief interruptions in chest compressions reduce the efficacy of resuscitation from cardiac arrest. Interruptions of this type are inevitable during hands-off periods for shock delivery to treat ventricular tachyarrhythmias. The safety of a rescuer remaining in contact with a patient being shocked with modern defibrillation equipment has not been investigated. Methods and Results— This study measured the leakage voltage and current through mock rescuers while they were compressing the chests of 43 patients receiving external biphasic shocks. During the shock, the rescuers gloved hand was pressed onto the skin of the patients anterior chest. To simulate the worst case of an inadvertent return current pathway, a skin electrode on the rescuers thigh was connected to an electrode on the patients shoulder. In no cases were shocks perceptible to the rescuer. Peak potential differences between the rescuers wrist and thigh ranged from 0.28 to 14 V (mean 5.8±5.8 V). The average leakage current flowing through the rescuers body for each phase of the shock waveform was 283±140 μA (range 18.9 to 907 μA). This was below several recommended safety standards for leakage current. Conclusions— Rescuers performing chest compressions during biphasic external defibrillation are exposed to low levels of leakage current. The present findings support the feasibility of uninterrupted chest compressions during shock delivery, which may enhance the efficacy of defibrillation and cardiocerebral resuscitation.

Estimation of Temperature During Radiofrequency Catheter Ablation Using Impedance Measurements

Temperature monitoring during radiofrequency catheter ablation is useful but requires specialized equipment that is not generally available. Previous studies have shown that impedance characteristically decreases as the result of heating at the electrode‐tissue interface. The purpose of the current study was to determine if impedance changes during radiofrequency current application could be used to estimate endocardial temperature in patients undergoing catheter ablation. Data from 43 patients treated with a thermistor ablation catheter were retrospectively analyzed. The slope of the initial 2 seconds of the impedance curve and subsequent changes in impedance were incorporated into an equation for estimation of temperature in real‐time. The accuracy of this equation was assessed by prospectively comparing the calculated and measured temperatures in 19 patients. Of the 88% of energy applications that were suitable for analysis, the average difference between calculated and measured temperatures was 5.2 ± 5.6°C. The average error was ± 10° in 89% of applications. The results of this study suggest that impedance measurements can be used to quantify tissue temperature in real‐time during radiofrequency catheter ablation. This method is sufficiently accurate to allow titration of power output to produce temperatures in the useful range (50–80°C) while avoiding excessive heating (± 90°C).

Incidence and implications of abrasion of implantable cardioverter-defibrillator leads

Severe abrasion of implantable cardioverter-defibrillator leads is frequently found during abdominal generator replacement and occasionally results in lead system failure. Careful inspection of leads at the time of generator replacement will identify such abrasions, and, in some cases, lead repair or replacement may be indicated.

Pacing Features That Mimic Malfunction: A Review of Current Programmable and Automated Device Functions That Cause Confusion in the Clinical Setting

Modern implantable devices capable of pacing are armed with a multitude of programmable and automated features. While some features represent important advances in device safety and performance, many can also mimic device malfunction. This article discusses these features in terms of the confusion they may cause and highlights important clinical clues that aid in their recognition.

Reverse Polarity Pacing:The Hemodynamic Benefit of Anodal Currents at Lead Tips forCardiac Resynchronization Therapy

Background: Myocardial depolarization can be achieved with currents of either anodal or cathodal polarity. In contrast to conventional cathodal pacing, anodal pacing initially hyperpolarizes tissue and improves myocardial contractility in animal models.

The Relationship of Bipolar Left Ventricular Pacing Stimulus Intensity to Cardiac Depolarization and Repolarization in Humans with Cardiac Resynchronization Devices

Background: Myocardial depolarization can be altered by varying pacing stimulus output. This may have implications on response rates for cardiac resynchronization therapy (CRT). The purpose of our study was to determine the relationship of left ventricular (LV) pacing stimulus intensity to measures of depolarization and repolarization in humans with CRT devices.

Wide Complex Tachycardia Due to Automaticity in an Accessory Pathway

Patients with the Wolff‐Parkinson‐White (WPW) syndrome have preexcited tachycardia as the result of atrial arrhythmias or antidromic reentry. This article describes a patient with persistent wide complex tachycardia due to abnormal automaticity in the accessory pathway. Radiofrequency catheter ablation resulted in simultaneous elimination of accessory pathway conduction and automaticity. Accessory pathway automaticity may be an infrequent cause of preexcited tachycardia in patients with the WPW syndrome.