Electrophysiology Corner

A Blind Comparison of Four Non-Invasive Twelve-Lead Electrocardiogram Algorithms for Predicting Susceptibility to Ventricular Ta

Todd J. Cohen, MD, Cindy Quan, Bassiema Ibrahim, MD, Simcha Pollack, PhD, Weilun Quan, PhD
Todd J. Cohen, MD, Cindy Quan, Bassiema Ibrahim, MD, Simcha Pollack, PhD, Weilun Quan, PhD
QT dispersion (QTD) has been proposed as an indicator of risk for ventricular tachyarrhythmias (VT) in different clinical settings (long QT syndrome,2,3 congestive heart failure4 and post-myocardial infarction5–9). However, a major limitation in the use of QTD is the intra- and inter-observer variability of the measurement,10–13 as well as its relatively low predictive power compared with other clinical indexes (late potentials, T-wave alternans and left ventricular ejection fraction).14 Previously, Cohen and colleagues demonstrated the utility of combining QTD with QRS duration in predicting susceptibility to VT.14 However, this technique was limited by the measurement of QT intervals from different cardiac cycles using a standard, 3-channel, 12-lead electrocardiogram (ECG). In addition, patients with bundle branch block were not excluded in the study and the contribution of QRS to the overall detection performance may have been obscured. The main objective of the present study was to examine the utility of combining QRS duration with QTD, measured from either 12-lead or 3-lead ECG, for predicting susceptibility to VT and to quantitatively compare their predictive performances. METHODS One-hundred and twenty-eight consecutive patients were referred for an electrophysiological study (EPS) to assess for VT. Fifteen patients with bundle branch block were excluded. The remaining 113 patients (65 men and 48 women; mean age ± SD, 66 ± 14 years), regardless of their cardiac substrates and initial presentations to electrophysiological study, were enrolled in this study as shown in Table 1. All patients underwent a baseline simultaneous 12-lead ECG using a Prucka electrophysiology recording system (Prucka Inc., Houston, Texas) in the electrophysiology laboratory prior to programmed electrical stimulation off antiarrhythmic drugs. The Prucka system uses 12 channels to record a 12-lead ECG simultaneously; therefore, ECG variables can be measured in a single cardiac cycle. In addition, the recorded ECGs are stored in a digital format via computer hard disk and can be played back and displayed on a computer screen for review and analysis in a variety of gains and sweep speeds. All ECG variables were manually measured from the computer screen directly using an internal electronic caliper provided by the Prucka recording system. The electronic caliper is a software tool for measuring intervals on the computer screen directly without the need to print out paper strips. ECG measurements included QRS duration (QRS), QTD (QTmax-QTmin) in all 12 leads (QTD12) or in 3 quasi-orthogonal leads (leads I, aVF, V1) (QTD3). QTD3 + QRS and QTD12 + QRS were calculated based on QTD12, QTD3 and QRS measurements. All the ECG variables were measured in a single cardiac cycle with high gain (8 times normal) and high speed (100 mm/second) to facilitate more precise discrimination. QRS duration was measured from the lead with the earliest onset to the lead with the latest offset from all 12 leads. The end of the T-wave was measured where it intersected the isoelectric TP baseline. In cases of low T-wave amplitude, gain was further increased (16 times or 32 times normal) so that the end of the T-wave could be visually identified. In the presence of a U-wave, the end of the T-wave was obtained from the nadir between the T- and U-wave peaks. In the presence of biphasic T-waves, the intersection of the late stage of the T-wave either above or below the isoelectric TP baseline was used as the end of the T-wave. All measurements were performed by a single reader (WQ) who was blinded to the clinical characteristics of the patient. Programmed electrical stimulation was performed using a standard protocol of up to three extra stimuli from two right ventricular sites (apex, outflow tract or septum using two different drive cycle lengths). Pacing stimuli were two milliseconds in duration and twice the diastolic threshold. VT susceptibility was defined as spontaneous and/or inducible sustained VT. Sustained VT was defined as VT of >= 30 seconds in duration or requiring cardioversion because of hemodynamic instability. To measure the predictive power of the addition of QRS to QTD, the area under the curve (AUC) of the receiver operating characteristic (ROC) curve was computed for each of the four standard ECG algorithms as a predictor of VT and for QTD with QRS as combined predictors of VT. Receiver operator characteristic curves were determined. The areas below the ROC curves were simply calculated using the trapezoidal rule.18 To minimize the estimation error of the area, the maximum (Table 2) possible number of points were used to construct the ROC curves. A comparison of the areas of the four receiver operator characteristic curves was utilized in order to help define the optimal VT detection algorithm. All data are presented as means ± SEM unless otherwise noted and statistical comparisons of the categorical variables between inducible and non-inducible patients were done by the Chi-square analysis (with p
References
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