Marc S. Fuller

QT interval dispersion: dispersion of ventricular repolarization or dispersion of QT interval?

The QT interval (QTI) has long been useful as a clinical index of the duration of ventricular repolarization, particularly as a marker of prolonged repolarization and its well-established association with arrhythmogenic cardiac states. Likewise, inhomogeneity (dispersion) of repolarization has been linked definitively to increased susceptibility to reentrant arrhythmias. Recent studies have reported the use of QTI dispersion as a meaningful clinical index to identify patients at risk, but the interpretation of the measurement has been controversial. A Langendorff-perfused, isolated canine heart suspended in a torso-shaped, electrolytic tank filled with NaCl-sucrose solution was used to investigate the relationship between body surface QTIs and ventricular repolarization measured directly from the cardiac surface by using activation-recovery intervals, which have been documented to reflect the duration of local action potentials as well as local refractory periods. The data showed poor correlation between cardiac surface activation-recovery intervals and QTIs, as well as the insensitivity of QTIs to regional repolarization shortening in the presence of prolonged repolarization elsewhere. Furthermore, the data confirmed that torso tank QTI dispersion does not reflect directly the full range of measured ventricular repolarization inhomogeneity. It is concluded that body surface QTI dispersion is not a reliable index of repolarization dispersion.

Estimates of repolarization and its dispersion from electrocardiographic measurements: Direct epicardial assessment in the canine heart

This study investigates a technique to estimate dispersion based on the root mean square (RMS) signal of multiple electrocardiographic leads. Activation and recovery times were measured from 64 sites on the epicardium of canine hearts using acute in situ or Langendorff perfused isolated heart preparations. Repolarization and its dispersion were altered by varying cycle length, myocardial temperature, or ventricular pacing site. Mean and dispersion of activation and recovery times, and activation-recovery interval (ARI) were calculated for each beat. The waveform was then calculated from all leads. Estimates of mean and dispersion of activation and recovery times and mean ARI were derived using only inflection points from the RMS waveform. QT intervals were also measured and QT dispersion was determined. Estimates determined from the RMS waveform provided accurate estimates of repolarization and were, in particular, a better measure of repolarization dispersion than QT dispersion.

Determination of local myocardial electrical activation for activation sequence mapping. A statistical approach.

Electrical activation sequence mapping requires accurate identification of local activation, but because extracellular recordings do not exclusively reflect local events, complex electrograms may be difficult to interpret. In such cases, the assignment of local activation is subject to error that could affect interpretation of the resulting activation maps. The purpose of this investigation was to develop an approach that would provide quantitative indexes of error in the determination of local activation. An electrode array with 64 closely spaced unipolar electrodes was used to record from the left ventricular surface during open heart surgery. Electrograms with multiple deflections were recorded from four patients with scarred myocardium; two other patients with normal myocardial function served as controls. Each of 784 deflections was scored on the basis of three features: evidence for propagation, the configuration of the bipolar signal, and the effect of changing from the chest to an average reference. Local activation was considered probable if evidence for all three features was present and improbable if none of the three features was present. Deflections that were ambiguous with respect to this standard were excluded. Of over 30 test variables analyzed, the three with the greatest power to discriminate signals due to local activation from those due to distant activity were 1) a linear combination of the extracellular potential plus the ratio of the second derivative and the extracellular potential, 2) the second derivative, and 3) the minimum (greatest negative) first derivative. For each of these variables, the threshold value providing the greatest performance was identified by the maximum quality of efficiency, an index of agreement. This statistical approach provides an objective basis for determining local activation and provides a quantitative assessment of error that could enhance interpretation of electrical activation sequence maps.

Detection and localization of prolonged epicardial electrograms with 64-lead body surface signal-averaged electrocardiography.

BackgroundProlonged, fractionated ventricular electrograms often are detectable after myocardial infarction and are a marker for an arrhythmia-prone state. QRS late potentials detected on the body surface with signal-averaged electrocardiography (SAECG) are thought to arise from the diseased tissue that generates prolonged ventricular electrograms and as such are also a marker for arrhythmias. A limitation of the current SAECG technique is that recordings are obtained from only three bipolar lead pairs. Because late potentials probably arise from multiple small sources in the heart, more extensive sampling of the body surface may contribute additional information to the SAECG. The present study investigates the additional sensitivity of SAECG using 64 body surface leads in detecting prolonged epicardial electrograms and examines its use in determining the epicardial location of prolonged electrograms. Methods and ResultsDogs were studied before and 5–10 days after either lateral left ventricular (n = 13) or right ventricular (n =8) myocardial infarction. Greater prolongation of signal-averaged QRS duration was detected with 64-lead SAECG (postinfarction QRS duration, 100.3±16.3 msec) than with three-lead SAECG (postinfarction QRS duration, 89.4±10.1, p = 0.0005). Nineteen of the 21 dogs (90%) had prolonged epicardial electrograms detected over the infarct. The correlation between epicardial electrogram duration and signal-averaged QRS duration calculated from individual leads was much better for 64-lead SAECG (r =0.88, p < 0.0001) than for three-lead SAECG (r = 0.53, p =0.01), and the difference was most marked in cases with longer electrogram durations (more than 100 msec). Local late potential maxima on the thorax after lateral left ventricular infarction were located to the left and inferior compared with those after right ventricular infarction (p =0.006). ConclusionsSAECG with more extensive recording from the body surface using 64 leads detects greater QRS prolongation than three-lead SAECG, and the longer QRS durations detected correspond to the duration of prolonged epicardial electrograms. Body surface location of late potentials corresponds to the epicardial location of the prolonged electrograms. This application of body surface mapping techniques to SAECG may permit more sensitive detection of arrhythmiaprone states and may aid in identifying arrhythmia sources.

Mechanisms of the Spatial Distribution of QT Intervals on the Epicardial and Body Surfaces

Spatial Distribution of the QT Interval. Introduction: The role of QT dispersion as a predictor of arrhythmia vulnerability has not been consistently confirmed in the literature. Therefore, it is important to identify the electrophysiologic mechanisms that affect QT duration and distribution. We compared the spatial distributions of QT intervals (QTI) with potential distributions on cardiac and body surfaces and with recovery times on the cardiac surface. We hypothesized that the measure of QTI is affected by the presence of the zero potential line in the potential distribution, as well as the sequence of recovery. We also investigated use of the STT area as a possible indicator of recovery times on the cardiac surface.

Noninvasive Indices of Repolarization and Its Dispersion

In experimental studies using Langendorff perfused, isolated canine hearts immersed in a torso-shaped electrolytic tank we studied repolarization and its dispersion using direct epicardial measurements and newly derived, noninvasive body surface indices. Activation recovery intervals (ARIs) measured from 64 epicardial sites based on differences between activation times (ATs) and recovery times (RTs) provided direct measures of repolarization. The indirect, torso surface indices were derived from inflections of the root-mean-square (RMS) voltage of the torso tank surface electrocardiograms recorded simultaneously with the epicardial data. For cycle lengths ranging from 300 to 900 ms, and electrolyte temperatures ranging from 32 degrees C to 40 degrees C we calculated mean, variance, and range of ATs, RTs, and ARIs from the epicardium. From epicardial and torso surface RMS waveforms, we used times of R and T peaks and their differences to estimate mean ATs, RTs, and ARIs, respectively. The RMS T wave width as determined from the second derivative inflections on either side of the T peak served as an estimate of the dispersion of RTs. In parallel studies, we showed that the direct measures of repolarization and its dispersion were reflected in RMS waveforms generated from the epicardial electrograms themselves. In this study, we confirm that the torso and epicardial RMS waveforms reflect comparable information for estimating repolarization and its dispersion. Furthermore, the derived measures provide a method to assess mean ARIs and dispersion of RTs on a beat-to-beat basis and during abnormal (ectopic ventricular) activation sequences.

Estimating ECG distributions from small numbers of leads

The utility of body surface potential mapping to improve interpretation of electrocardiographic information lies in the presentation of thoracic surface distributions to characterize underlying electrophysiology less ambiguously than that afforded by conventional electrocardiography. Localized cardiac disease or abnormal electrophysiology presents itself electrocardiographically on the body surface in a manner in which pattern plays an important role for identifying or characterizing these abnormalities. Thus, in myocardial infarction, transient myocardial ischemia, Wolff-Parkinson-White syndrome, or ventricular ectopy, observation of electrocardiographic potential patterns, their extrema, and their magnitudes permits localization and quantization of the abnormal activity. Conventional electrocardiography assesses pattern information incompletely and does not use information of distribution extrema locations or magnitudes. Thus, increases or decreases in the magnitudes of electrocardiographic features (ST-segment potential displacement, amplitude, or morphology of Q, R, S, or T waves) associated with changes in cardiac sources (ischemia, infarction, conduction abnormalities, etc.) as measured from fixed leads have a high likelihood of being misinterpreted if the distribution itself is changing. In this study, the authors demonstrate the utility of estimating distributions from small numbers of optimally selected leads, including conventional leads, to reduce uncertainty in the interpretation of electrocardiographic information. This issue is highly relevant when thresholds are used to detect significance of potential levels (exercise testing, detection of myocardial infarction, and continuous monitoring to assess ST-segment changes). Significance of this work lies in improved detection and characterization of abnormal electrophysiology using conventional or enhanced leadsets and methods to estimate thoracic potential distributions.

Evaluation of novel measurement methods for detecting heterogeneous repolarization

There exists a well-documented link between heterogeneity of cardiac recovery characteristics and vulnerability to arrhythmia; however, electrocardiographic detection of this heterogeneity remains problematic. The only modalities suitable for measuring variation of repolarization are electrophysiologic in nature, with action potential duration in single cells the most direct method and QT intervals from the body surface electrocardiogram the most common clinical approach. The authors have shown previously, however, that the QT interval is a poor measure of regional change in repolarization, especially when shortening occurs. Here, the authors discuss an experimental preparation based on an isolated canine heart suspended in a human-shaped, instrumented, electrolytic tank and describe a method of applying cold to create local, transient changes in recovery characteristics. The authors have simultaneously recorded epicardial and torso tank surface potentials before, during, and after intervention, and from them have generated isopotential and isointegral maps and computed activation-recovery intervals (ARIs). In all cases, epicardial potentials revealed changes in recovery associated with localized heating and cooling. The changes were visible from tank surface potential distributions in some, but not all, cases. The results also suggest that epicardial ARIs are sensitive to changes in recovery and that, at least for a subset of tank surface leads, ARIs can be used to create noninvasive indices of disparity of repolarization characteristics.

Criteria for local myocardial electrical activation: effects of electrogram characteristics

The ability of several variables to distinguish unipolar deflections due to local activation from those due to nonlocal activity is studied. A model of polyphasic deflections based on atrial recordings during reentrant tachycardia was used to facilitate distinction of local and distant activity by methods independent of the test variables. The performances of variables were assessed by comparing areas under receiver operating characteristic curves. Optimal thresholds of test variables were identified by maximizing statistics which corrected for the pretest probability of local activation. It was found that the greatest negative first derivative of the unipolar potential discriminated between local and distant ventricular signals, but performed less well than the ratio of the first derivative to the potential for distinguishing between local atrial signals and distant ventricular signals. A linear combination of the potential and the ratio of the first derivative and the potential performed well for all groups of signals studied. Optimal criteria for detecting local activation are discussed.<<ETX>>

Optimal lead selection for detection of ST segment shifts.

A comparison was made to determine the ability of optimal sets of 2–6 unipolar leads and a normal Holter lead set to estimate ST potential distributions changes induced by balloon inflation during angioplasty. The performance of these lead nets was compared to measurements observed in recorded 32‐lead body surface maps. Unipolar lead potentials were estimated using a linear, least mean squared error estimator of the total body surface map. The correlation between maximum ST potential change in the body surface map and that predicted by the unipolar lead sets ranged from 0.84–0.93. The correlation between maximum ST segment change measured from the body surface map and measured from the Holter leads was 0.29. Therefore, shifts in ST segment potentials can accurately be estimated from a small number of unipolar leads. In contrast, current bipolar ambulatory recording techniques may introduce significant bias to such estimates.