J.M. Wharton

Cardiac potential and potential gradient fields generated by single, combined, and sequential shocks during ventricular defibrillation.

Background Potential gradient field determination may be a helpful means of describing the effects of defibrillation shocks; however, potential gradient field requirements for defibnrllation with different electrode configurations have not been established. Methods and Results To evaluate the field requirements for defibrillation, potential fields during defibrillation shocks and the following ventricular activations were recorded with 74 epicardial electrodes in 12 open-chest dogs with the use of a computerized mapping system. Shock electrodes (2.64 cm2) were attached to the lateral right atrium (R), lateral left ventricular base (L), and left ventricular apex (V). Four electrode configurations were tested: single shocks of 14-msec duration given to two single anode-single cathode configurations, R:V and L: V, and to one dual anode-single cathode configuration, (R+L):V; and sequential 7-msec shocks separated by 1 msec given to R:V and L:V (R:V → L:V). Defibrillation threshold (DFT) current was significantly lower for R:V → L:V than for the other configurations and markedly higher for L:V. Despite these differences, the minimum potential gradients measured at DEFI were not significantly different (approximately 6–7 V/cm for each electrode configuration). Potential gradient fields generated by the electrode configurations were markedly uneven, with a 15–27-fold change from lowest to highest gradient, with the greatest decrease in gradient occurring near the shock electrodes. Although gradient fields varied with the electrode configuration, all configurations produced weak fields along the right ventricular base. Early sites of epicardial activation after all unsuccessful shocks occurred in areas in which the field was weak; 87% occurred at sites with gradients less than 15 V/cm. Ventricular tachycardia originating in high gradient areas near shock electrodes followed 11 of 67 successful shocks. Conclusions These data suggest that 1) defibrillation fields created by small epicardial electrodes are very uneven; 2) achievement of a certain minimum potential gradient over both ventricles is necessary for ventricular defibrillation; 3) the difference in shock strengths required to achieve this minimum gradient over both ventricles may explain the differences in DIFTs for various electrode configurations; and 4) high gradient areas in the uneven fields can induce ectopic activation after successful shocks.

Effect of Ebstein's anomaly on short- and long-term outcome of surgically treated patients with Wolff-Parkinson-White syndrome.

BackgroundEbsteins anomaly is the most commonly occurring congenital abnormality associated with the Wolif-Parkinson-White (WPW) syndrome. However, the effects of Ebsteins anomaly on the risks and benefits of surgical ablation of accessory pathways in patients with WPW syndrome are unknown Methods and ResultsThis study compared the long-term outcome of 38 WPW patients with Ebsteins anomaly undergoing accessory pathway ablation to a reference population of 384 similarly treated patients without the anomaly. Ebsteins anomaly was mild in 21 patients (55%) and moderate-to-severe in 17 patients (45%). Sixteen patients (42%) required tricuspid valve surgery, and 23 (61%) had an atrial septal defect or patent foramen ovale repaired. Baseline clinical characteristics and preoperative clinical arrhythmias were similar in both groups. Ten-year survival was 92.4% and 91.2% for patients with and without Ebsteins anomaly, respectively (p = NS). During a mean follow-up of 6.2±3.8 and 5.3±3.6 years, 82% of patients with and 90% without Ebsteins anomaly had either clinically insignificant or no arrhythmias, and 18% versus 10% reported symptoms suggesting arrhythmias lasting longer than 1 minute, respectively. Atrial fibrillation was reduced postoperatively to 9% (p < 0.00l) in patients with and to 4% (p < 0.00l) in those without the anomaly. Fewer hospitalizations were reported postoperatively by 90% versus 96% of patients with and without Ebsteins anomaly; 9.4% versus 6.0% of patients were disabled at follow-up, respectively (p = NS) ConclusionsPatients with Ebsteins anomaly are improved significantly after accessory pathway ablation. The presence of this anomaly should not preclude accessory pathway ablation in these patients.

Activation in unipolar cardiac electrograms: a frequency analysis

Several detectors of local activations in unipolar cardiac electrograms are discussed. The detectors are based on the frequency content of the waveforms. For this study, myocardial regions with no local electrical activity were created with cryoablation in canine ventricles, so that the characteristics of electrograms reflecting local activation could be compared with those with only distant electrical activity. For each electrogram, representations, of the original signal were created using the output of bandpass filters; for each representation, the value of the maximum amplitude was taken as a measurement of the frequency content of the electrogram in that frequency band. The content of each frequency band and the first derivative of the signal were tested as discriminators between local and distant electrical activity. Combinations of frequency bands were also tested using a logistic regression technique. It was found that a detector based on multivariate analysis of different frequency components of a signal may be more effective than single-band filtering in discriminating between local and distant electrical activity in the heart, especially when those components have very different magnitudes.<<ETX>>

Experimental results pertinent to the modelling of defibrillation

Recent experimental results pertinent to the modeling of defibrillation are presented. The extracellular potential gradient created by a large electric stimulus predicts whether or not several different types of electrophysiologic response will occur for a given stimulus waveform. Stimulation of fully recovered myocardium occurs for a truncated exponential monophase waveform when the potential gradient is greater than 0.64 V/cm along myofibers or 1.84 V/cm across fibers. A potential gradient of approximately 6 V/cm is needed throughout for all or almost all of the ventricular myocardium to defibrillate. More than approximately 60 V/cm causes a temporary conduction block. Thus, defibrillation electrode configurations should create cardiac fields with the highest gradient no more than ten times the lowest gradient. Values for stimulation, defibrillation, and conduction block are probably different for other waveforms.<<ETX>>

Digital filters for activation detection in unipolar cardiac electrograms

A limitation on the use of cardiac activation mapping is the inability to distinguish reliably between electrical activity occurring next to a sensing electrode and the effects of distant wavefronts. The authors modeled electrograms without local electrical activity by recordings in lesions created in canine epicardium by freezing, where, by definition, local electrical activity is not possible. Electrograms with local electrical activity were recorded in healthy myocardium. The authors present discriminators based on the frequency characteristics of unipolar electrograms. Different frequency bands were tested as discriminators and compared to the typically used first-derivative discriminator.<<ETX>>

Use of coherence in activation detection during ventricular fibrillation

The use of cardiac activation mapping to study ventricular fibrillation (VF) is discussed. The locations of isochrones in maps are based on decisions on whether and when segments of the electrogram have local electrical activation. The authors compare two quantitative criteria for making decisions about local activity in electrograms. The criteria are based on the comparison of electrograms recorded in those excitable zones with electrograms recorded in normal myocardium. One uses the first derivative of the electrogram (only one electrogram affects the decision); the second uses the first derivative of the primarily local component of the waveform.<<ETX>>

The mechanism of ventricular defibrillation

A summary is presented of findings about the mechanisms by which a shock succeeds or fails in halting ventricular fibrillation. It is shown that earliest activations following a subthreshold defibrillation shock occur in regions of low potential gradient generated by the shock. Activation fronts after subthreshold shocks are not continuations of fronts present just before the shock. An upper limit exists to the strength of shocks that induce fibrillation during the vulnerable period of regular rhythm and correlates with the defibrillation threshold. The response to shocks during regular rhythm just below the upper limit of vulnerability is similar to the response to subthreshold defibrillation shocks. To defibrillate, a shock must not only halt the activation fronts of fibrillation, but it must also not give rise to new activation fronts that reinduce fibrillation.<<ETX>>

Multiuser clinical electrophysiology database using low-cost PCs in a Novell Netware Environment

A multiuser database has been developed by the Duke University Medical Centers clinical electrophysiology (EP) group that runs on low-cost IBM compatible PCs running in a Novell Netware Environment and is written in the FoxPro programming language. The authors describe the functioning of the Duke EP database and illustrate how networked PCs in combination with the powerful database development systems now available can be employed by end-users to solve problems quickly and economically. The functions provided by the Duke EP database include storage of patient information, procedure report generation, quality assurance monitoring, intrahospital billing, clinic scheduling, and implantable cardiac defibrillator monitoring. The system incorporates features such as pop-up menus for data entry, step-by-step instructions, and an online help system to make it easy to use by nontechnical personnel. The effectiveness and cost of the system are discussed.<<ETX>>

Spatial filtering of unipolar electrograms

In order to be able to derive some quantitative criteria to assist in making decisions about the presence of local activations, zones of canine myocardium incapable of undergoing electrical activation (EA) were created. The criteria are based on the comparison of electrograms recorded in those inexcitable zones with electrograms recorded in normal myocardium. Two criteria for making decisions about local EA in unipolar electrograms are compared. The first is based on the first time derivative (time filtering) of the electrogram and the second is based on th first spatial derivative (spatial filtering) between two electrograms recorded in neighboring electrodes. It is shown that the spatial derivative performs much better than the time derivative in both supraventricular rhythm and ventricular fibrillation.<<ETX>>