Paul A. Belk

Statistical accuracy of a moving equivalent dipole method to identify sites of origin of cardiac electrical activation

While radio frequency (RF) catheter ablation (RCA) procedures for treating ventricular arrhythmias have evolved significantly over the past several years, the use of RCA has been limited to treating slow ventricular tachycardias (VTs). In this paper, we present preliminary results from computer and animal studies to evaluate the accuracy of an algorithm that uses the single equivalent moving dipole (SEMD) model in an infinite homogeneous volume conductor to guide the RF catheter to the site of origin of the arrhythmia. Our method involves measuring body surface electrocardiographic (ECG) signals generated by arrhythmic activity and by bipolar current pulses emanating from a catheter tip, and representing each of them by a SEMD model source at each instant of the cardiac cycle, thus enabling rapid repositioning of the catheter tip requiring only a few cycles of the arrhythmia. We found that the SEMD model accurately reproduced body surface ECG signals with a correlation coefficients >0.95. We used a variety of methods to estimate the uncertainty of the SEMD parameters due to measurement noise and found that at the time when the arrhythmia is mostly localized during the cardiac cycle, the estimates of the uncertainty of the spatial SEMD parameters (from ECG signals) are between 1 and 3 mm. We used pacing data from spatially separated epicardial sites in a swine model as surrogates for focal ventricular arrhythmic sources and found that the spatial SEMD estimates of the two pacing sites agreed with both their physical separation and orientation with respect to each other. In conclusion, our algorithm to estimate the SEMD parameters from body surface ECG can potentially be a useful method for rapidly positioning the catheter tip to the arrhythmic focus during an RCA procedure.

The relationship between pacing site and induction or termination of sustained monomorphic ventricular tachycardia by antitachycardia pacing.

Background: With the development of left ventricular pacing for cardiac resynchronization, there is an interest in the possibility of improving ventricular antitachycardia pacing (ATP) efficacy by pacing from the LV electrode(s).

Sinus rhythm R-wave amplitude as a predictor of ventricular fibrillation undersensing in patients with implantable cardioverter-defibrillator

BACKGROUND Ventricular fibrillation (VF) is induced during implantable cardioverter-defibrillator (ICD) implantation to ensure that the ICD will sense, detect, and defibrillate VF. ICD implant guidelines state that the amplitude of the sinus rhythm R wave recorded from the ventricular electrogram should have amplitude ≥5 mV. No study has tested the relationship between sinus rhythm R-wave amplitude and VF sensing using modern, transvenous sensing electrodes. OBJECTIVE The goal of this study was to determine whether there is a sinus rhythm R-wave amplitude cutoff that can be used to determine which patients are not at risk of VF undersensing. METHODS A retrospective analysis of induced and spontaneous VF episodes from 2 clinical trials with 2022 patients was performed. Episodes with undersensing during the initial detection of VF were identified, and the distribution of sinus rhythm R-wave amplitudes for patients with and without VF undersensing was analyzed. RESULTS Only 3% of analyzed induced VF episodes were considered to have VF undersensing, and none had clinically significant detection delays. There was no correlation between device-measured, rectified sinus rhythm R-wave amplitude and VF undersensing at the time of implantation or during follow-up, although <4% of patients had sinus rhythm R-waves with amplitude <3 mV. CONCLUSION We analyzed true bipolar sensing of induced VF or spontaneous ventricular tachycardia/VF detected in the ICD VF zone. Sensing of VF was so reliable that clinically significant undersensing did not occur. Our findings do not support any recommended minimum sinus rhythm R wave to ensure reliable sensing of VF or the necessity of inducing VF to verify sensing for rectified sinus rhythm R-waves with amplitude ≥3 mV.

Automatic Determination of Timing Intervals for Upper Limit of Vulnerability Using ICD Electrograms

Background: Implantable cardioverter defibrillator (ICD) implant testing based on the upper limit of vulnerability, or vulnerability testing, permits assessment of defibrillation safety margins without inducing ventricular fibrillation (VF) in most patients. Vulnerability testing requires that T‐wave shocks be timed at the most vulnerable intervals of the cardiac cycle, defined as intervals at which the strongest shock induces VF. Our goal was to develop and test an automated method to select these timing intervals using ICD intracardiac electrograms (EGMs).

Criteria for the observation of one‐dimensional transport in split‐gate field‐effect quantum wires

We have fabricated AlGaAs/GaAs split‐gate field‐effect quantum wires with different lengths and widths in order to study the effect of the dimensions of the confining gates on one‐dimensional (1D) transport. Two threshold voltages arise as the two‐dimensional (2D) electron gas is turned off first underneath the gates and then as the 1D electron gas is turned off in between the split‐gates through their fringing fields. In particular, our room‐temperature experiments reveal the existence of a critical width of 0.2–0.25 μm between the confining gates below which the 1D regime is not observed in 1.0‐μm‐long split‐gate quantum wires. This can be explained if the potential at the surface near the metal confining gates is influenced by the gate voltage through exchange of electrons between the metal gates and the surface states.

Criteria for one-dimensional transport in split-gate field-effect transistors

The authors have fabricated AlGaAs-GaAs split-gate field-effect transistors with different lengths and widths in order to establish geometric design criteria for one-dimensional (1D) transport. The experiments show a more negative 1D threshold voltage for zero length split-gate devices (constrictions) than for finite length split-gate devices for the same given width. The experiments reveal the existence of a critical width between the gates below which a 1D regime cannot be supported in split-gate FETs longer than 0.5 mu m.<<ETX>>

A computer simulation study of the efficacy of uniform field pacing for the prevention of ventricular tachyarrhythmia

Summary A quantitatively-reliable computer model of the human heart could be a very useful tool for the analysis and development of potential tachyarrhythmia therapies. In this article, we present a new discrete cellular automata (CA) model of the human ventricle that allows representation of important quantitative aspects of cardiac excitation waves. We use this model to analyze a hypothetical technique for preemptive antitachycardia pacing using spatially-uniform, low-energy shocks. Because of the computational efficiency and relative simplicity of the CA model, we can perform rapid simulations of a large number of pacing interventions on a statistical ensemble of two-dimensional ventricular substrates with topology and dimensions similar to the human ventricular epicardial surface. This allows us to obtain a good statistical profile of the intervention‘s efficacy and proarrhythmic potential. Our study results suggest that while spatially-uniform, low-energy preemptive pacing shocks are frequently successful at preventing tachyarrhythmia, they are often proarrhythmic. The simplicity of our CA model structure and the clear physiological interpretation of its parameters allows us to examine the mechanisms of arrhythmia prevention and spurious induction by the pacing shocks in terms of the generic properties of the waves. We believe such an approach can potentially be very useful for the preliminary analysis of a wide variety of antiarrhythmic interventions, both electrical and pharmacological.