The unidentified information content of the electrocardiogram

T-wave changes of cardiac memory caused by frequent premature ventricular contractions originating from the right ventricular outflow tract

INTRODUCTION

Cardiac memory is recognized as altered T-waves when the sinus rhythm resumes after an abnormal myocardial activation period that recovers slowly over several weeks. The T-wave changes after ablation of frequent premature ventricular contractions (PVCs) as cardiac memory was not known.

OBJECTIVE

This study identified whether cardiac memory exists after successful ablation of PVCs from the right ventricular outflow tract (RVOT).

METHODS

We investigated 45 patients who underwent successful ablation of PVCs from RVOT and 10 patients who underwent unsuccessful ablation. We analyzed the amplitude of the T-wave, QT intervals, and QRST time-integral values of a 12-lead electrocardiogram before ablation and 1 day, 3 days, and 1 month after ablation.

RESULTS

In the successful ablation group, the amplitude of the T-wave and QRST time-integral values of lead II, III, aVR, aVL, and aVF significantly changed after ablation and gradually normalized within 1 month. In addition, if the number of pre-ablation PVCs was small, then the corresponding impact was also small. However, the greater the number of pre-ablation PVCs, the more prominent the changes. Significant changes were not observed in the unsuccessful ablation group.

CONCLUSION

When ablation of PVCs from RVOT was successful, primary T-wave changes because of cardiac memory and the gradual normalization of the amplitude of the T-wave were observed. No significant T-wave changes were detected after unsuccessful ablation.

A study on stability analysis of atrial repolarization variability using ARX model in sinus rhythm and atrial tachycardia ECGs

BACKGROUND

The interaction between the PTa and PP interval dynamics from the surface ECG is seldom explained. Mathematical modeling of these intervals is of interest in finding the relationship between the heart rate and repolarization variability.

OBJECTIVE

The goal of this paper is to assess the bounded input bounded output (BIBO) stability in PTa interval (PTaI) dynamics using autoregressive exogenous (ARX) model and to investigate the reason for causing instability in the atrial repolarization process.

METHODS

Twenty-five male subjects in normal sinus rhythm (NSR) and ten male subjects experiencing atrial tachycardia (AT) were included in this study. Five minute long, modified limb lead (MLL) ECGs were recorded with an EDAN SE-1010 PC ECG system. The number of minute ECGs with unstable segments (N_us) and the frequency of premature activation (PA) (i.e. atrial activation) were counted for each ECG recording and compared between AT and NSR subjects.

RESULTS

The instability in PTaI dynamics was quantified by measuring the numbers of unstable segments in ECG data for each subject. The unstable segments in the PTaI dynamics were associated with the frequency of PA. The presence of PA is not the only factor causing the instability in PTaI dynamics in NSR subjects, and it is found that the cause of instability is mainly due to the heart rate variability (HRV).

CONCLUSION

The ARX model showed better prediction of PTa interval dynamics in both groups. The frequency of PA is significantly higher in AT patients than NSR subjects. A more complex model is needed to better identify and characterize healthy heart dynamics.

Ambulatory measurement of the ECG T-wave amplitude

Ambulatory recording of the preejection period (PEP) can be used to measure changes in cardiac sympathetic nervous system (SNS) activity under naturalistic conditions. Here, we test the ECG T-wave amplitude (TWA) as an alternative measure, using 24-h ambulatory monitoring of PEP and TWA in a sample of 564 healthy adults. The TWA showed a decrease in response to mental stress and a monotonic decrease from nighttime sleep to daytime sitting and more physically active behaviors. Within-participant changes in TWA were correlated with changes in the PEP across the standardized stressors (r = .42) and the unstandardized naturalistic conditions (mean r = .35). Partialling out changes in heart rate and vagal effects attenuated these correlations, but they remained significant. Ambulatory TWA cannot replace PEP, but simultaneous recording of TWA and PEP provides a more comprehensive picture of changes in cardiac SNS activity in real-life settings.

Multipolar maps in patients with postinfarction heart failure

PURPOSE

The aim of this study was to assess the changes in body surface maps in patients with postinfarction heart failure (PIHF).

MATERIALS AND METHODS

Body surface mapping was performed in 22 patients with PIHF and 20 age-matched healthy controls, using a 64-electrode vest. A card index was made for every patient and person of the control group, containing isopotential and isointegral maps. The number and absolute value of maxima and minima were assessed for every map.

RESULTS

Only bipolar maps were recorded in the healthy control group, and multipolar maps were found in 55% of the patients with PIHF. All patients with multipolar isointegral QRST maps had also multipolar isopotential ST maps (J + 110 milliseconds); 67%, multipolar isopotential QRS peak maps; 33%, multipolar isointegral Q40 maps; 17%, multipolar isointegral QRS maps; 67%, multipolar isointegral STT maps; and 50%, multipolar isointegral ST maps. Significant differences were noticed in maxima and minima in patients with PIHF compared with healthy controls and in patients with multipolar isointegral maps (QRST and Q40) compared with those with bipolar maps. Multiple regression analysis revealed that multipolar QRST maps were significantly associated (P < .001) with maxima and minima of the isointegral maps. Isointegral multipolar QRST maps were significantly associated (P < .001) with multipolar isopotential ST maps, multipolar isointegral QRS maps, multipolar isointegral STT maps, and multipolar isointegral ST maps.

CONCLUSION

Postinfarction heart failure increases the prevalence of multipolar maps and significantly changes maxima and minima. Multipolar QRST isointegral maps are significantly associated with maxima and minima of the QRS, ST, STT, and QRST maps and with other types of multipolar maps: isointegral QRS, STT, ST and isopotential ST (J + 110 milliseconds), and QRS peak maps.

Detailed ECG analysis of atrial repolarization in humans

INTRODUCTION

Data on human atrial repolarization are scarce since the QRS complex normally obscures its ECG trace. In the present study, consecutive patients with third-degree AV block were studied to better describe the human Ta wave.

METHODS AND RESULTS

Forty patients (mean age 75 years, 17 men) were included. All anti-arrhythmic drugs were discontinued before ECG recording. Standard 12-lead ECGs were recorded, transformed to orthogonal leads and studied using signal-averaged P wave analysis. The average P wave duration was 124 +/- 16 ms. The PTa duration was 449 +/- 55 ms (corrected PTa 512 +/- 60 ms) and the Ta duration (P wave end to Ta wave end) was 323 +/- 56 ms. The polarity of the Ta wave was opposite to that of the P wave in all leads. The Ta peaks were located at 196 +/- 55 ms in Lead Y, 216 +/- 50 ms in Lead X, and 335 +/- 92 in Lead Z. No correlation was found between P wave duration and Ta duration, or between Ta peak amplitude and Ta duration. The morphology of the Ta wave was similar regardless of the interatrial conduction.

CONCLUSIONS

The Ta wave has the opposite polarity, and the duration is generally two to three times that, of the P wave. Although the Ta peak may occasionally be located in the PQ interval during normal AV conduction, it is unlikely that enough information can be obtained from analysis of this segment to differentiate normal from abnormal atrial repolarization. Hence, an algorithm for QRST cancellation during sinus rhythm is needed to further improve analysis.

The effect of mental stress on the non-dipolar components of the T wave: modulation by hypnosis

OBJECTIVE

Mental or emotional stress-induced ventricular arrhythmias and sudden cardiac death are thought to be mediated by the autonomic nervous system and ischemia. In the absence of ischemia, increased inhomogeneity of repolarization is thought to be important. We tested the hypotheses that in the absence of ischemia, mental stress may modulate repolarization by changing autonomic balance; and mental relaxation induced by hypnosis may offset the potentially adverse effects of stress on the cardiac electrophysiology.

METHODS

Twelve healthy volunteers (6 male, age 18-35, mean 25 years) experienced a series of different emotions intended to induce a wide range of autonomic response (42 test epochs) on two separate occasions, with and without hypnosis, with continuous electrocardiogram recording. Low- (LF) and HF (high-frequency) heart rate variability was measured and ventricular repolarization was assessed using the relative T-wave residua (proportion of nondipolar components of the T wave) calculated for the T-onset - T peak (TWR-peak T), T peak -T end (TWR-end T), and the whole T wave (TWR).

RESULTS

Emotionally induced changes in LF and LF/HF ratio correlated with changes in TWR, e.g., (R = 0.51, p < .001; R = 0.59, p < .0001; and R = 0.59, p < .0003, for LF/HF versus TWR, TWR-Peak T, and TWR-end T, respectively. Mental relaxation induced by hypnosis increased LF power (1,205 ms2) versus 624 ms2, p < .003 for hypnotized versus nonhypnotized state), HF power (1,619 ms2 versus 572 ms2), p < .0004), and reduced LF/HF ratio (1.0 versus 1.5, p = .052) and was associated with a marked reduction in the changes in repolarization in response to emotion, e.g., 10.7 x 10(-6) versus 5.0 x10(-6), p < .03 for TWR.

CONCLUSIONS

a) Mental stress in the absence of ischemia altered repolarization inhomogeneity via change in the autonomic balance. b) Mental relaxation induced by hypnosis greatly reduced the effect of mental stress on repolarization. c) These findings may have implications for arrhythmogenesis.

Prognostic value of heterogeneity of ventricular repolarization in survivors of acute myocardial infarction

BACKGROUND

The expansion of indications for implantation of cardioverter-defibrillators (ICD) has enhanced the need for risk stratification of patients post myocardial infarction (MI), while the improved treatment of acute MI has decreased mortality and diminished the prognostic power of traditional risk variables.

HYPOTHESIS

Increased heterogeneity of ventricular repolarization quantified by TCRT (total cosine R-to-T, angular difference between spatial QRS and T loops, decreased with increase in repolarization heterogeneity) is an independent predictor of mortality in patients post MI.

METHODS

Left ventricular ejection fraction (EF), QRS duration on signal-averaged ECG, number of ventricular ectopic beats (VE)/h, heart rate variability (HRV) triangular index, heart rate turbulence slope on 24-h Holter recording, and TCRT were analyzed in 334 survivors of acute MI followed up for 41 +/- 20 months.

RESULTS

In multivariate analysis, EF < 35% (relative risk [RR] 2.3, 95% confidence interval [CI] 1.1-4.7, p = 0.023), VE > 10/h (RR 2.2, CI 1.0-4.6, p = 0.044), HRV < 20 U (RR 2.2, CI 1.1-4.5, p = 0.032), and TCRT < -0.896 (RR 4.3, CI 2.2-8.5, p = 0.00001) were independent predictors of cardiac mortality (11%). Independent predictors of arrhythmic mortality (5%) were VE, HRV, and TCRT (RR 5.8, CI 2.1-15.6, p = 0.0004). Cardiac and arrhythmic mortality of patients with both EF <35% and TCRT < -0.896 were >60 and >30%, respectively, compared with 17 and 7% in those with only EF <35% or TCRT < -0.896.

CONCLUSION

Decreased TCRT, which reflects increased repolarization heterogeneity, is a strong and independent predictor of cardiac and arrhythmic death in patients post MI.

Simulation of QRST Integral Maps With a Membrane-Based Computer Heart Model Employing Parallel Processing

The simulation of the propagation of electrical activity in a membrane-based realistic-geometry computer model of the ventricles of the human heart, using the governing monodomain reaction-diffusion equation, is described. Each model point is represented by the phase 1 Luo-Rudy membrane model, modified to represent human action potentials. A separate longer duration action potential was used for the M cells found in the ventricular midwall. Cardiac fiber rotation across the ventricular wall was implemented via an analytic equation, resulting in a spatially varying anisotropic conductivity tensor and, consequently, anisotropic propagation. Since the model comprises approximately 12.5 million points, parallel processing on a multiprocessor computer was used to cut down on simulation time. The simulation of normal activation as well as that of ectopic beats is described. The hypothesis that in situ electrotonic coupling in the myocardium can diminish the gradients of action-potential duration across the ventricular wall was also verified in the model simulations. Finally, the sensitivity of QRST integral maps to local alterations in action-potential duration was investigated.

Measurement, interpretation and clinical potential of QT dispersion

QT dispersion was originally proposed to measure spatial dispersion of ventricular recovery times. Later, it was shown that QT dispersion does not directly reflect the dispersion of recovery times and that it results mainly from variations in the T loop morphology and the error of QT measurement. The reliability of both automatic and manual measurement of QT dispersion is low and significantly lower than that of the QT interval. The measurement error is of the order of the differences between different patient groups. The agreement between automatic and manual measurement is poor. There is little to choose between various QT dispersion indices, as well as between different lead systems for their measurement. Reported values of QT dispersion vary widely, e.g., normal values from 10 to 71 ms. Although QT dispersion is increased in cardiac patients compared with healthy subjects and prognostic value of QT dispersion has been reported, values are largely overlapping, both between healthy subjects and cardiac patients and between patients with and without adverse outcome. In reality, QT dispersion is a crude and approximate measure of abnormality of the complete course of repolarization. Probably only grossly abnormal values (e.g. > or =100 ms), outside the range of measurement error may potentially have practical value by pointing to a grossly abnormal repolarization. Efforts should be directed toward established as well as new methods for assessment and quantification of repolarization abnormalities, such as principal component analysis of the T wave, T loop descriptors, and T wave morphology and wavefront direction descriptors.

Measurement and interpretation of QT dispersion

QT dispersion was proposed as an index of the spatial inhomogeneity of ventricular recovery times. The results of studies that found significant correlation between dispersion of ventricular recovery times measured with monophasic action potentials and QT dispersion were interpreted as proof of the direct link between QT dispersion and the dispersion of ventricular recovery times. Later it was shown that QT dispersion is not a direct reflection of the spatial variation of the recovery times and cannot be used for quantification of this variation. The interlead variability of the QT intervals is a result of different projections of the spatial T-wave loop into the various electrocardiographic leads. The reliability of both manual and automatic measurement of QT dispersion is low and is often of the order of the differences of Qt dispersion between different patient groups. The measurement reliability is influenced by intrinsic factors (e.g., amplitude of the T wave) and extrinsic factors (e.g., noise, paper speed of recording, instruments for manual measurements, and type of algorithm and interalgorithmic settings for automatic measurement). There is very little to choose between the different indices of expression of QT dispersion, as well as between the different lead configurations used for its measurement. QT dispersion is not simply a result of measurement error, but a crude measure of abnormalities during the whole course of repolarization. Only grossly prolonged QT dispersion (e.g., > or =100 ms), must be interpreted simply as a sign of the abnormal course of the repolarization, and inferences about the actual dispersion of the ventricular recovery times should not be made. Newer concepts of assessment of the morphology of the T wave are already emerging and will probably be of higher clinical value.

Electrophysiological basis of QT dispersion measurements

Dispersion of ventricular repolarization is a now widely used term describing nonhomogeneous recovery of excitability or heterogeneity of ventricular repolarization. It is usually expressed as the difference or the range of various repolarization measurements obtained from a heart. Experimentally, an increased dispersion of ventricular repolarization was found to be tightly associated with increased propensity for ventricular arrhythmias, and, therefore, is considered an important arrhythmogenic mechanism. Noninvasively, this arrhythmogenic substrate was approached using multilead body surface potential mapping, but also QT interval dispersion (QTd) and similar electrocardiogram (ECG) variables from the 12-lead surface ECG. Standard QTd from the ECG correlates significantly with dispersion of repolarization measured from the myocardium. A causal relationship is, however, still unclear, and there are 2 main hypotheses to explain the electrophysiological basis of QTd. The local hypothesis explaining QTd with spatial differences in action potential duration mirrored in the various QT intervals competes with the global hypothesis explaining the variation in surface ECG measurements with different projections of a common T-wave vector. Notwithstanding the final explanation for QTd, and particularly for technical reasons, new markers like advanced T-wave loop variables may best reflect the abnormal repolarization substrate on the surface ECG.

Value of magnetocardiographic QRST integral maps in the identification of patients at risk of ventricular arrhythmias

It has been shown that regional ventricular repolarization properties can be reflected in body surface distributions of electrocardiographic QRST deflection areas (integrals). We hypothesize that these properties can be reflected also in the magnetocardiographic QRST areas and that this may be useful for predicting vulnerability to ventricular tachyarrhythmias. Magnetic field maps were obtained during sinus rhythm from 49 leads above the anterior chest in 22 healthy (asymptomatic) control subjects (group A) and in 29 patients with ventricular arrhythmias (group B). In each subject, the QRST deflection area was calculated for each lead and displayed as an integral map. The mean value of maximum was significantly larger in the control group A than in the patient group B (1,626+/-694 pTms vs. 582+/-547 pTms, P<0.0001). To quantitatively assess intragroup variability in the control group A and intergroup variability of the control and patient groups, we used the correlation coefficient r and covariance sigma. These indices showed significantly less intragroup than intergroup variation (e.g., in terms of sigma, 28.0x10(-6)+/-12.3x10(-6) vs. 3.4x10(-6)+/-12.5x10(-6), P<0.0001). Each QRST integral map was also represented as a weighted sum of 24 basis functions (eigenvectors) by means of Karhunen-Loeve transformation to calculate the contribution of the nondipolar eigenvectors (all eigenvectors beyond the third). This percentage nondipolar content of magnetocardiographic QRST integral maps was significantly higher in the patient group B than in the control group A (13.0%+/-9.1 % vs. 2.6%+/-2.0%, P<0.0001). Discriminations between control subjects and patients with ventricular arrhythmias based on magnitude of the maximum, covariance sigma, and nondipolar content were 90.2%, 90.2%, and 86.3% accurate, with a sensitivity of 89.7%, 93.1%, and 75.9%, and a specificity of 90.9%, 86.4%, and 100%. We have shown that magnitude of the maximum and indices of variability and nondipolarity of the magnetocardiographic QRST integral maps may predict arrhythmia vulnerability. This finding is in agreement with earlier studies that used body surface potential mapping and suggests that magneticfield mapping may also be a useful diagnostic tool for risk analysis.

The utility of quantitative body surface isoarea mapping for predicting ventricular tachyarrhythmias

Noninvasive techniques, such as the signal averaged ECG, have been used to assess risk of ventricular tachyarrhythmias (VT). However, these methods produce false positive and negative results. The purpose of this study was to develop body surface map algorithms which would enhance prediction of susceptibility to VT. Fifty-three patients referred for programmed electrical stimulation were enrolled in this study. All patients underwent signal averaged ECG, body surface map, programmed electrical stimulation. Group I patients had no sustained inducible VT and group II patients had either inducible sustained VT at electrophysiology study or previously documented spontaneous, sustained VT. For body surface map analysis, the difference between extrema on isoarea maps was calculated and defined as the gradient range. An abnormal body surface map was defined as a QRST gradient range < or = 109 mv.ms. The mean QRST gradient range in group II was significantly < that in group I (P < 0.05). By logistic regression analysis, the presence of coronary artery disease, a QRST gradient range < or = 109 mv.ms, an EF < 40% and a signal averaged ECG QRS duration > 114 ms predicted VT. The sensitivity, specificity, positive and negative predictive values for predicting VT susceptibility of an algorithm which combines the signal averaged ECG QRS duration and the QRST gradients were 0.93, 0.76, 0.79, and 0.91, respectively, while those for the signal averaged ECG alone were 0.52, 0.69, 0.63, and 0.59 for VT susceptibility. A combined body surface map-signal averaged ECG algorithm was more sensitive in detecting susceptibility to VT than the signal averaged ECG alone.

Human atrial repolarization: effects of sinus rate, pacing and drugs on the surface electrocardiogram

OBJECTIVES

We studied the effects of rate and some cardioactive drugs on the atrial surface electrocardiogram (ECG).

BACKGROUND

In atrioventricular block, atrial surface ECG is unmasked. The effect of rate alone permits detection of the effect of other exogenous stimulations such as drugs in the presence of rate alterations.

METHODS

High fidelity, high gain ECG leads I, II and III were recorded from 51 patients with heart block. Durations of P and Ta waves and the total PTa interval were measured from nonconducted atrial events.

RESULTS

No relationship was found between sinus cycle length and PTa, P or Ta in 31 patients. In 20 patients, progressively decreasing the atrial pacing cycle length from 853 ms to 381 ms resulted in a linear reduction of the PTa interval from 444 to 291 ms (rho = 0.76, slope = 0.24). This was largely due to shortening of Ta. A linear rate correction formula was derived: corrected PTa = PTa - 0.24 (PP - 1000). Atropine (0.02 mg/kg) shortened the PP interval (p < 0.001) and the PTa interval (p < 0.01). Propranolol (0.1 mg/kg) prolonged the PP interval (p < 0.001) but did not alter the PTa interval. Neither disopyramide (2.0 mg/kg) nor flecainide acetate (2.0 mg/kg) altered the PP interval, but both prolonged the PTa interval (p < 0.001). This was largely due to P wave lengthening after flecainide (p < 0.001) and to Ta prolongation after disopyramide (p < 0.001).

CONCLUSIONS

In heart block, PTa, P and Ta waves can be measured reliably. The effects of pacing and some antiarrhythmic drugs on the atrial myocardium are similar to those known at the ventricular level.

Algebraic decomposition of the TU wave morphology patterns

In principle, the T wave results from the differences in durations of action potentials (AP) of different ventricular regions. Based on this concept, a mathematical model has been developed that represents the TU wave morphology as a summation of four AP-like functions: TU = S1 - S2 + L1 - L2. The sigmoidal shape of AP-like curves is produced by Hill's equation V(t) = a . tn/(bn + tn). Each of the decomposition functions is characterized by two parameters: the amplitude at the beginning of QRS (Amax), and the duration at 5% of Amax (D95). The set of four decomposition functions leads to eight parameters that provide detailed characteristics of the TU wave morphology. The model was validated using 170 TU wave complexes recorded digitally in leads V2-V6 from 22 normal subjects and 12 patients with abnormal TU wave morphologies (negative, biphasic, and notched T waves). The electrocardiographic signals were sampled at 100 Hz and a best-fit procedure was used to obtain the decomposition. In all cases the coefficients of correlation between original TU patterns and their mathematical models were > or = 0.99. The mean absolute difference between the observed and modeled values of the TU patterns was similar in cases with normal and abnormal TU wave morphologies (4.65 +/- 0.41 microV vs 5.19 +/- 0.48 microV respectively) demonstrating that the model is capable of describing and categorizing various TU patterns by a set of eight numerical parameters.

Changes in electrocardiographic morphology reflect instantaneous changes in left ventricular volume and output in cardiac surgery patients

We examined the relation between changes in R-to-T wave amplitude ratios (R:T) and left ventricular (LV) performance as cardiac output was rapidly varied by inferior vena caval occlusion in 6 subjects prior to cardiopulmonary bypass. To assess the influence of contractility, paired studies before and after bypass were performed in 4 subjects. Stroke volume and cardiac output were assessed by aortic flow probe, and transesophageal echocardiographic LV area measures using the automated border-detection method were used to give LV stroke area, stroke force, maximal LV area, fractional area change, end-systolic elastance, and preload recruitable stroke force. Data were collected on computer and analyzed by linear regression. Significant changes in R:T and measured LV variables during the inferior vena caval occlusion were stroke volume (r = 0.81), LV stroke area (r = 0.77), LV stroke force (r = 0.81), maximal LV area (r = 0.78), and cardiac output (r = 0.80). However, R:T varied inconsistently in relation to fractional area change. After cardiopulmonary bypass, the linear relation between R:T with LV stroke force, LV stroke volume, and maximal LV area persisted, but at a lesser slope. Although absolute pre-inferior vena caval occlusion R:T did not correlate with end-systolic elastance or preload recruitable stroke force, the change in the slope of these linear relations correlated well with the change in end-systolic elastance after surgery (r = 0.92). Instantaneous changes in electrocardiographic morphology reflect changes in LV preload-dependent variables, whereas long-term changes in electrocardiographic morphology may also reflect changes in contractile state.

Reduction in QT interval dispersion by successful thrombolytic therapy in acute myocardial infarction. TEAM-2 Study Investigators

BACKGROUND

QT dispersion (QTd, equals maximal minus minimal QT interval) on a standard ECG has been shown to reflect regional variations in ventricular repolarization and is significantly greater in patients with than in those without arrhythmic events.

METHODS AND RESULTS

To assess the effect of thrombolytic therapy on QTd, we studied 244 patients (196 men; mean age, 57 +/- 10 years) with acute myocardial infarction (AMI) who were treated with streptokinase (n = 115) or anistreplase (n = 129) at an average of 2.6 hours after symptom onset. Angiograms at 2.4 +/- 1 hours after thrombolytic therapy showed reperfusion (TIMI grade > or = 2) in 75% of patients. QT was measured in 10 +/- 2 leads at 9 +/- 5 days after AMI by using a computerized analysis program interfaced with a digitizer. QTd, QRSd, JT (QT minus QRS), and JT dispersion (JTd, equals maximal minus minimal JT interval) were calculated with a computer. There were significant differences in QTd (96 +/- 31, 88 +/- 25, 60 +/- 22, and 52 +/- 19 milliseconds; P < or = .0001) and in JTd (97 +/- 32, 88 +/- 31, 63 +/- 23, and 58 +/- 21 milliseconds; P = .0001) but not in QRSd (25 +/- 10, 22 +/- 7, 28 +/- 9, and 24 +/- 9 milliseconds; P = .24) among perfusion grades 0, 1, 2, and 3, respectively. Similar results were obtained comparing TIMI grades 0/1 with 2/3 and 0/1/2 with 3. Patients with left anterior descending (versus right and left circumflex) coronary artery occlusion showed significantly greater QTd (70 +/- 29 versus 59 +/- 27 milliseconds, P = .003) and JTd (74 +/- 30 versus 63 +/- 27 milliseconds, P = .004). Similarly, patients with anterior (versus inferior/lateral) AMI showed significantly greater QTd (69 +/- 30 versus 59 +/- 27 milliseconds, P = .006) and JTd (73 +/- 30 versus 63 +/- 27 milliseconds, P = .007). Results did not change when Bazett's QTc or JTc was substituted for QT or JT or when ANOVA included adjustments for age, sex, drug assignment, infarct site, infarct vessel, and number of measurable leads. On ANCOVA, the relation of QTd or JTd and perfusion grade was not influenced by heart rate.

CONCLUSIONS

Successful thrombolysis is associated with less QTd and JTd in post-AMI patients. The results are equally significant when either QT or JT is used for analysis. These data support the hypothesis that QTd after AMI depends on reperfusion status as well as infarct site and size. Reduction in QTd and its corresponding risk of ventricular arrhythmia may be mechanisms of benefit of thrombolytic therapy.

Effects of right ventricular pacing on QRST isointegral maps in patients with and without myocardial infarction: body surface distribution of significant changes in QRST area compared to supraventricular complex

To assess the effects of right ventricular (RV) pacing on body surface QRST distributions, we recorded QRST isointegral maps (I-maps) during sinus rhythm and RV pacing in 25 patients with anterior myocardial infarction (MI), 19 with inferior MI, and 14 without MI. The QRST values at each lead point recorded during sinus rhythm and RV pacing with an 87-lead system were analyzed with a paired t-test in each patient. An abnormal decrease in the QRST value of the I-map was assessed by the difference map, which indicated a "-2SD area," where the QRST integral value was less than the normal range (mean - 2SD) calculated from 608 normal individuals. The I-maps were similar during the two activation sequences in patients with and without MI. However, during RV pacing, QRST values significantly decreased over the upper right anterior chest and increased over the lower left anterior chest and back. The sigma DMs (sum of QRST integral values below the normal range) for both activation sequences were strongly correlated in patients with anterior MI and with inferior MI (r = 0.91 and r = 0.92, respectively; P < 0.001). Although small but significant changes in QRST values were detected, the distribution of the "-2SD area" and the sigma DM were similar during both activation sequences in patients with prior MI. Thus, these findings demonstrate that an altered activation sequence produces small but significant changes in QRST values but that I-maps still provide information that is useful for the diagnosis of MI during RV pacing.

Correlation between various parameters derived from body surface maps and ejection fraction in patients with anterior myocardial infarction

To determine the best map parameter to predict cardiac function, various map parameters were correlated with the left ventricular ejection fraction (EF) in patients with a previous (between 3 months and 1 year) anterior myocardial infarction, but without overt congestive heart failure or ventricular dyssynergy. From 300 consecutive patients with a previous myocardial infarction, 82 patients with only an anterior infarction and who underwent cardiac catheterization and body surface mapping were selected for this study. The maps from 100 healthy subjects were used as normal controls. Body surface maps using 87 unipolar electrodes were recorded and various parameters were derived from the Q map, the QRS departure maps, the QRS isointegral (IQRS) map, and the QRST isointegral (IQRST) maps. They were compared with the angiographically determined EF. The EF was correlated with nQ (r = -0.72), four parameters derived from the QRS departure map (r ranged from -0.73 to -0.79), two parameters derived from the IQRS map (r = -0.90 and -0.86), and two parameters derived from the IQRST map (r = -0.84 and -0.85). Some parameters derived from body surface maps were found to have a very high correlation with the EF in patients who had a previous anterior myocardial infarction.

Effects of simulated left bundle branch block on QRST time-integral values of 12-lead electrocardiograms in patients with and without prior anterior wall myocardial infarction

The effects of right ventricular pacing, which simulated left bundle branch block (BBB), on QRST time-integral values of 12-lead electrocardiograms (ECGs) were examined, and the clinical usefulness of QRST values for estimating the severity of left ventricular wall motion abnormalities due to a prior anterior wall myocardial infarction (MI) in the setting of left BBB were evaluated. Digitized ECGs were recorded during normal sinus rhythm and simulated left BBB in 38 patients (24 with and 14 without prior anterior wall MI). QRST values were calculated in each lead point of 12-lead ECGs. Data from 608 normal subjects were used as control values; the mean +/- 2 SD of these values was regarded as the normal range. The parameter sigma DE was defined as the sum of the differences between the normal mean QRST value and the QRST values of a given patient in leads where the QRST value was less than the normal range. The correlation coefficient of sigma DE for the 2 activation sequences was highly significant. Although small but significant changes were seen in QRST values in leads I, II, III, aVR, aVF and V1 during simulated left BBB, left precordial leads showed no significant changes in QRST values. A criterion of sigma DE > 40 mV.ms for detecting an anterior wall MI showed a sensitivity of 88%, a specificity of 93%, and a diagnostic accuracy of 89%. The sigma DE was significantly (p < 0.001) correlated with the asynergy index calculated from left ventriculograms.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrocardiographic body surface mapping in patients with ventricular tachycardia. Assessment of utility in the identification of effective pharmacological therapy

BACKGROUND

Body surface maps of net QRST deflection areas (isointegrals) reflect regional ventricular repolarization properties. Vulnerability to ventricular tachyarrhythmias is associated with maps that feature multiple islands (extrema) of positive and negative values; such maps reflect regional disparity of ventricular recovery properties. The value of body surface mapping in prediction of the efficacy of antiarrhythmic therapy for ventricular tachyarrhythmias has not been determined.

METHODS AND RESULTS

Isointegral ECG body surface mapping was performed in 51 patients with inducible ventricular tachycardia having programmed stimulation studies at baseline and after oral quinidine therapy. The degree of nondipolarity of QRST isointegral distribution was expressed by the number of extrema and by the percentage contribution of nondipolar eigenvectors after Karhunen-Loeve transformation. QRST isointegral nondipolarity was greater in ventricular tachycardia patients than in 51 age- and sex-matched normal subjects expressed as mean number of extrema (4.1 +/- 2.8 versus 2.0 +/- 0.2, respectively), mean eigenvector-determined nondipolar content percentages (12.4 +/- 10.1% versus 4.5 +/- 4.9%), prevalence of abnormal numbers of extrema (63% versus 4%), or prevalence of abnormal nondipolar content percentages (33% versus 4%) (each p less than 0.01). Quinidine prevented ventricular tachycardia induction in 14 patients. Patients for whom quinidine was or was not effective had similar nondipolarity indexes at baseline. However, maps on quinidine differed as a function of antiarrhythmic efficacy. Although effective therapy produced no significant mean changes in nondipolarity, ineffective therapy increased the number of extrema compared with baseline (5.4 +/- 3.4 versus 3.8 +/- 2.5, respectively) (p = 0.002). Individually, 43% of patients on effective therapy had drug-induced decreases in numbers of extrema compared with 14% of those on ineffective therapy (p = 0.02). Furthermore, 29% of patients on effective therapy showed drug-induced increases in numbers of extrema compared with 62% of those on ineffective therapy (p = 0.03).

CONCLUSIONS

QRST isointegral body surface mapping shows promise as a noninvasive measure of drug efficacy in patients with ventricular tachycardia.

Evaluation of arrhythmic causes of syncope: correlation between Holter monitoring, electrophysiologic testing, and body surface potential mapping

Holter monitoring, electrocardiographic (ECG) signal-averaging, body surface potential mapping (BSPM) for PQRST isoarea maps, and electrophysiologic study (EPS) were performed in 100 patients with syncope. Coronary artery disease (CAD) was found in 46 patients and other heart disease was found in 19. EPS was diagnostic in 44 patients, while Holter monitoring suggested a diagnosis in only 21 patients. Abnormal BSPM was frequently seen (56%), especially in CAD (70%), or with inducible ventricular tachycardia (VT) (87%). Late potentials were recorded in 13 patients with CAD; five had inducible VT. In seven other patients with VT, they were either absent or bundle branch block (BBB) was found. Thirteen deaths (three sudden) occurred in our series. EPS-guided therapy resulted in a low rate of total cardiac death. In conclusion, EPS had a higher diagnostic yield than Holter monitoring regardless of cardiac pathology. ECG signal-averaging was useful in predicting VT only in patients with CAD without BBB. BSPM was abnormal in most patients with cardiac disease, but poorly predicted VT.

Dispersion of ventricular repolarization in the long QT syndrome

To identify markers of dispersion of the ventricular repolarization in the idiopathic long QT syndrome, body surface potential maps were analyzed in 40 such patients (mean age +/- standard deviation 21 +/- 11 years) and in 30 healthy control subjects (mean age 24 +/- 7 years). In each subject, 117 chest leads were recorded and maps of the integral values of the QRST interval were calculated. A multipolar distribution of the values, a marker of gross electrical inequalities of repolarization, was found only in 4 patients. To detect minor regional disparities of ventricular recovery, all the ST-T waveforms were analyzed in each subject. The ST-T waves were represented by a discrete series of potential values. The "similarity index" was computed by applying a principal component analysis, which represents (in percent) to what extent 1 fundamental pattern of ST-T reproduces all the recorded waveforms. The mean value of the similarity index was significantly lower in patients with long QT syndrome than in control subjects (49 +/- 10 vs 77 +/- 8%, p less than 0.0001). A value less than 61% (corresponding to 2 standard deviations below the mean value for controls) was found in 35 of 40 patients and in only 1 control subject (sensitivity 87%, specificity 96%). Thus, the similarity index is a more sensitive marker than the multipolar distribution of QRST integral maps in revealing electrical disparities of the ventricular recovery times.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal-averaged body surface mapping for the assessment of low-amplitude potentials. Detailed maps during early ventricular activation in normal subjects

Body surface isopotential maps around early ventricular activation were investigated in 30 normal subjects by the use of the authors' signal-averaged body surface mapping system. The number of beats averaged was 96-154 (mean, 127). Two distinct patterns were recognized in the appearance of a maximum at the onset of ventricular activation: the maximum in the first type (n = 16) was located on the right anterior chest; the maximum in the second type (n = 14) was on the central or left anterior chest. The site of the earliest ventricular activation was considered to be different in each of these types. During early ventricular activation, 25 subjects (83%) had two minima: one was on the left lateral chest and the other was on the left back. The two minima probably reflect two different receding activation fronts in the ventricles. The data in the present study are important to the understanding of the early ventricular activation process, as well as the diagnosis of heart diseases in which this process is disturbed.

Diagnostic value of QRST isointegral maps in detecting myocardial infarction complicated by bundle branch block

The clinical usefulness of QRST isointegral maps (IQRST map) for detecting myocardial infarction that was complicated by intraventricular conduction disturbances was evaluated in patients with right bundle branch block (group RBBB, 64 patients) and left bundle branch block (group LBBB, 40 patients) by comparison with the normal mean IQRST map derived from 50 normal subjects. Myocardial infarction complicated the conduction disturbances in 24 of the 64 RBBB and in 18 of the 40 LBBB patients. A correlation coefficient was used for assessing the similarity of each map pattern with the normal mean IQRST map. The difference map was made by subtracting the average normal IQRST map from each abnormal IQRST map, and those differences that were less than 2 SD from the mean were retained as a significant area. The number of leads and their sum of differences were used to represent the size of the difference map. Correlation coefficients were significantly (p less than 0.001) smaller in patients with bundle branch block complicated by myocardial infarction than in patients with conduction disturbances not complicated by myocardial infarction. A significant area emerged in the difference map in all patients with myocardial infarction complicated by conduction disturbances. The emergence of a significant area revealed high diagnostic accuracy for detecting myocardial infarction in group RBBB (89.1%). The size of a significant area in a difference map was significantly larger in cases with complicated myocardial infarction than in cases with uncomplicated myocardial infarction in either group RBBB or group LBBB (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Diagnostic value of body surface potential mapping in old anterior non-Q myocardial infarction

Body surface potential maps (BSM) were recorded from 140 chest leads in 30 healthy control subjects (C) and in 20 patients who had had an acute non-Q wave myocardial infarction (MI) 1-82 months before the study, to identify reliable indices of necrosis. In 12 MI patients the QRS complex was within normal limits on standard 12-lead ECG (group A), and in 8 patients no pathologic Q waves were present but the R waves were small and did not normally increase from V1 to V4 (group B). In each subject instantaneous potential distributions throughout the QRS interval were examined. Moreover, the potential--time integrals relating to three intervals (first 40 msec, mid-third, and last third of QRS) were calculated at each lead point and displayed as integral (I) maps. For each time interval, deviation index maps (DI), indicating the standardized differences from normal values, were calculated. An area where the integral values differed at least 2 SD from normal mean was considered abnormal. In most group A patients the inspection of instantaneous potential maps did not reveal definitively abnormal patterns. In group B patients a greater variety of patterns was found and in four cases the characteristic features of the anterior Q wave MI were observed. The DI maps of the first 40 msec of QRS provided the best diagnostic accuracy: areas of negative values 2 SD lower than normal were present in all group B patients (100%), in 8 group A patients (67%), and in 4 group C subjects (13%).(ABSTRACT TRUNCATED AT 250 WORDS)

ST-T isointegral analysis of exercise stress body surface mapping for identifying ischemic areas in patients with angina pectoris

ST-T isointegral analysis of body surface mapping was used in an attempt to localize ischemic areas on exercise tests. In 28 patients with angina pectoris and 10 healthy subjects, body surface potential was recorded with 87 leads, and ST isopotential and ST-T isointegral maps were constructed. In all 10 healthy subjects, the basic pattern of the ST-T isointegral map showed no significant change after exercise. In 23 of 28 patients with angina pectoris (82%), alterations in the ST-T isointegral map after exercise were observed. They were divided into four types (anterior, inferoposterior, lateral, and global) according to the distribution of negative values, which were well correlated with the extent of ischemic area determined by thallium myocardial scintigraphy and coronary angiography. The postexercise ST-T isointegral map was normalized after administration of nitroglycerin in four of five patients. In five patients (18%) who did not show abnormalities on the postexercise ST-T isointegral map, the magnitude of maximal ST depression was significantly smaller than that observed in the other 23 patients with angina pectoris (0.14 vs 0.23 mV on the average, p less than 0.05). It was concluded that the exercise test with ST-T isointegral mapping is a new method for noninvasive detection of location and severity of ischemic regions.

PQRST isoarea maps from patients with the Wolff-Parkinson-White syndrome: an index for global alterations of ventricular repolarization

Isoarea maps during the PQRST sequence were computed in 22 healthy subjects and 48 patients with Wolff-Parkinson-White (WPW) syndrome. Thirty-eight patients with WPW were on no medication and 10 were treated with class I, II, or III antiarrhythmic drugs. Seventeen isoarea maps were recorded before and 17 were recorded after accessory pathway ablation. One patient had intermittent preexcitation. Body surface maps from all healthy subjects were similar, although the magnitudes of the maxima and minima showed significant variability. In all patients with WPW who were on no medication and in those on class I and II agents, PQRST maps were normal. Two patients taking amiodarone had abnormal PQRST maps, as did patients early after surgery. In the patient with intermittent preexcitation, PQRST maps were very similar during normal and preexcited beats. In conclusion, our results support the theory that the PQRST time integral reflects intrinsic recovery properties of the heart and is independent of the activation sequence.

Variation in the precordial QRS transition zone in normal subjects

From body surface potential map data for 51 normal young men (with QRS axis between 0 and 90 degrees) both the spatial QRS area vector and the isoarea map of the QRS were obtained. Acting on Grant's assumption that the transition zone defined a plane perpendicular to the spatial QRS vector, we determined the angular shift in altitude and azimuth required to move the spatial vector of each individual to the position of the group mean. We then shifted the precordial map of the transition zone of each individual with the same angular correction. These resulting transition zone boundaries clustered much closer to each other, but did not move into absolute coincidence. We interpreted the nearness-to-fit to be an estimate of the degree to which the precordial QRS configurations conformed to a common simple vector or dipolar pattern.

Qualitative and quantitative analysis of characteristic body surface potential map features in anterior and inferior myocardial infarction

Body surface potential maps were recorded from 120 electrode sites in 236 normal subjects and 258 patients with initial evidence of either anterior myocardial infarction (MI) or inferior MI to identify characteristic map patterns in both groups. After time normalization, averaged map distributions were displayed at 18 equal time intervals during both QRS and ST-T waveforms from the normal, anterior MI and inferior MI groups. At each time instant, the 120-point averaged normal map was subtracted in turn from the corresponding anterior and inferior MI maps; the resulting differences at each electrode site were divided by the pooled standard deviation and the obtained values (discriminant indexes), plotted as contour lines with 1 standard deviation increments, producing discriminant maps for each bi-group comparison. The most consistent discriminant patterns in 114 patients with anterior MI were observed in early QRS in the upper left anterior chest where abnormal negative voltages reflected loss of electric potentials while reciprocal changes were noticed in the lower back; by mid-QRS, both distributions had moved jointly and vertically, the former in the lower torso on the midsternal line, the latter in the upper back. In 144 patients with inferior MI, abnormal positive distributions were observed in early QRS in the upper back, followed later by excessive negative voltages in the inferior right anterior chest; at mid-QRS, both distributions had migrated horizontally, the former proceeding toward the upper anterior torso, the latter to the lower left dorsal area.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of cardiac equivalent generator properties using a numerical expansion method

The ability of the Karhounen-Loeve (K-L) numerical expansion technique to quantitate cardiac dipolarity was assessed using data recorded from fifteen isolated perfused rabbit hearts. Potentials during the QRS complex, registered from thirty-two surface electrodes, were processed to yield percent dipolarity by a potential equation fit as well as by the K-L expansion. A total of 1174 data points was evaluated. The difference in values computed by the two methods equalled 1.02 +/- 6.99% (mean +/- 1 standard deviation); this difference, although small, was statistically significant (P less than 0.01). The correlation coefficient between the two sets of results equalled 0.53 (P less than 0.001). Thus, experimental support is given to the use of the K-L method to assess field dipolarity. Because ranges of differences were high and predictive accuracy of the regression was limited, the K-L expansion may be used with caution to detect directional shifts in equivalent cardiac generator properties.

Using body surface mapping to detect vulnerability to ventricular arrhythmias in patients with coronary artery disease

In order to examine the clinical usefulness of the vulnerability map, body surface mapping was performed in ten normal subjects and 32 patients with CAD using dipyridamole infusion to induce ventricular arrhythmias. A vulnerability map and the vulnerability index (VI) proposed by Urie et al. were constructed from QRS and T isointegral maps in the control state and QRST isointegral map after dipyridamole infusion. Premature ventricular complexes (PVCs) did not occur in normal subjects but occurred in 13 patients after dipyridamole infusion. The vulnerability index in normal subjects was significantly lower than that in patients without PVCs (8.3 +/- 1.7 vs 10.4 +/- 1.7, P less than 0.01). Patients with PVCs showed increased density of contour lines in the vulnerability map and significantly higher VI than those without PVCs (12.6 +/- 2.1 vs 10.4 +/- 1.7, P less than 0.01). This result suggests that a higher vulnerability index indicates that the condition of the cardiac muscle is at high risk of ventricular arrhythmias. It is concluded that the vulnerability map is useful for assessing whether or not the cardiac state is at high risk of ventricular arrhythmias in CAD.

Mapping of body surface potentials in patients with the idiopathic long QT syndrome

Body surface potential maps were recorded from 140 chest leads in 25 patients affected by the idiopathic long QT syndrome (LQTS) and in 25 healthy control subjects matched for age and sex. Potential time integrals of the QRST and ST-T intervals were calculated at each lead point and displayed as isointegral (ISOI) maps. The main abnormalities noted on the QRST and ST-T ISOI maps were one area of negative values larger than normal in the right anterior and inferior thorax and a complex multipeak distribution of the integral values. At least one abnormality was present in 19 (76%) of the patients with LQTS and four (16%) of the control subjects (p less than .001). Each ISOI map was also represented as a weighted sum of nine fundamental components (eigenvectors) to detect and quantitate the nondipolar content. The percent contribution of the nondipolar eigenvectors (all eigenvectors beyond the third) was significantly higher in the LQTS group than in the control group (p less than .005). Specifically, an abnormally high nondipolar content on the QRST ISOI maps was observed much more frequently for patients with LQTS than for control subjects (nine or 36% vs one or 4%), and this was also true on the ST-T ISOI maps (14 or 56% vs one or 4%). No correlation was found between the major abnormalities on body surface maps and syncopal episodes. However, the high prevalence (76%) of these alterations among the patients with LQTS and their infrequent occurrence in the control population strongly suggests that they may be useful markers for the diagnosis of atypical cases. The prominent electronegative area on the anterior thorax can be related to delayed repolarization of a portion of the anterior wall of the heart. This finding is in agreement with the hypothesis that lower than normal right cardiac sympathetic activity is the main pathogenetic mechanism of LQTS. Multipeak distribution and high nondipolar content suggest regional electrical disparities in the ventricular recovery process. This may in part account for the high susceptibility of patients with LQTS to malignant arrhythmias.

Identification of best electrocardiographic leads for diagnosing anterior and inferior myocardial infarction by statistical analysis of body surface potential maps

In view of the increasing interest in quantifying and modifying the size of myocardial infarction (MI), it is important to look for clinically practical subsets of electrocardiographic leads that allow the earliest and most accurate diagnosis of the presence and electrocardiographic type of MI. A practical approach is described, taking advantage of the increased information content of body surface potential maps over standard electrocardiographic techniques for facilitating clinical use of body surface potential maps for such a purpose. Multivariate analysis was performed on 120-lead electrocardiographic data, simultaneously recorded in 236 normal subjects, 114 patients with anterior MI and 144 patients with inferior MI, using as features instantaneous voltages on time-normalized QRS and ST-T waveforms. Leads and features for optimal separation of normal subjects from, respectively, anterior MI and inferior MI patients were selected. Features measured on leads originating from the upper left precordial area, lower midthoracic region and the back correctly identified 97% of anterior MI patients, with a specificity of 95%; in patients with inferior MI, features obtained from leads located in the lower left back, left leg, right subclavicular area, upper dorsal region and lower right chest correctly classified 94% of the group, with specificity kept at 95%. Most features were measured in early and mid-QRS, although very potent discriminators were found in the late portion of the T wave.(ABSTRACT TRUNCATED AT 250 WORDS)

Body surface potential maps in old inferior myocardial infarction. Assessment of diagnostic criteria

We assessed the accuracy of criteria for diagnosing an inferior myocardial infarction from body potential maps. Body surface potential maps were recorded from 140 lead points on the entire chest surface in three groups of subjects: group A consisted of 15 patients with an old inferior myocardial infarction and typical electrocardiographic signs of necrosis; group B consisted of 15 patients with an old inferior myocardial infarction, but without electrocardiographic signs of necrosis (inferior myocardial infarction was documented during the acute phase); group C consisted of 30 healthy controls. In each subject body surface potential distributions were examined every 2 msec of the QRS complex. Moreover, the potential-time integrals relating to three intervals (QRS, the first 20 and the first 40 msec of the QRS complex) were calculated at each lead point and transferred to diagrams representing the thoracic surface explored (isointegral maps). For each time interval, the mean isointegral map obtained from group C subjects was subtracted from the isointegral map of each patient. The value obtained at each lead point was then divided by the standard deviation of the normal values for that point; the resulting values indicating the standardized differences from normal values were transferred to another map (deviation index isointegral map, DI map). We considered a reliable index of inferior myocardial infarction an area where the time-integral values were at least 2 SD lower than normal, in the inferior half of the thorax. A number of variables relative to instantaneous potential distribution and to isointegral maps were considered. The DI maps of the first 40 msec of QRS gave the most accurate criteria; in fact, an area of negative values 2 SD lower than normal was found in all group A patients and in 11 out of 15 group B patients (sensitivity 100% in group A, 73% in group B and specificity, 83%). Thus our results indicate that body surface potential maps have greater diagnostic information content than the 12 standard electrocardiographic leads and demonstrate the usefulness of the time integral analysis of body surface potentials for diagnostic interpretation.

Determination of the spatial and intensity properties of atrial repolarization potentials in the dog

Body surface electrical potentials generated by atrial repolarization (Ta wave) normally extend from the P wave into or through the QRS complex. Thus, the Ta wave is partially obscured by the QRS complex and, conversely, the QRS complex is composed of atrial recovery as well as ventricular depolarization forces. To better study the spatial patterns and magnitudes of the Ta wave, transient atrioventricular (AV) block was induced in 15 dogs by atrial pacing using surgically implanted left or right atrial electrodes. ECG potentials were registered from 84 torso electrodes, and cycles with normal and with blocked AV conduction were segregated. In blocked cycles, the duration of the Ta wave measured 248.0 +/- 25.3 msec with right and 256.0 +/- 38.4 msec with left atrial stimulation. In all cases, the Ta wave extended into the QRS and, in 75% of cases, it extended into the S-T segment. Peak Ta magnitudes, measuring 50.7 +/- 17.9 and 51.8 +/- 21.7 uV RMS with right and left atrial stimulation, respectively, occurred during the P-R segment in all cases. The effect of Ta wave superposition on the QRS complex was assessed by subtraction of patterns in cycles with blocked AV conduction from those with intact AV conduction. Differences between directly recorded QRS waveforms and those computed by subtraction of the Ta wave were small; correlation coefficients exceeded 0.96 and differences in instantaneous RMS potential were less than 6%. Thus, the atrial recovery waveform does extend into the QRS and into the S-T segment in most cases, but the effect of this superposition of Ta on QRS waveforms is quantitatively small.

Vulnerability to ventricular arrhythmia: assessment by mapping of body surface potential

It is now well established that the vulnerability of the ventricular myocardium to repetitive dysrhythm increases in the presence of greater than normal disparity local recovery times. Local recovery is reflected in the electrocardiographic waveform as an area of the ventricular deflection (QRST time integral), and thus disparate ventricular recovery may be manifested in the body surface distribution of this quality. To assess this possibility, we obtained simultaneous 120-lead electrocardiograms from both the anterior and posterior torso in 140 subjects (ages 8 to 75) grouped as follows: group A, 97 normal subjects; group B, 16 patients resuscitated from ventricular fibrillation or sustained ventricular tachycardia; and group C, 27 patients 6 to 12 months after myocardial infarction but without clinically significant arrhythmia. In each subject, the QRST integral was evaluated for each lead and isointegral contour maps were plotted. A score was assigned to each map, based on the number of extrema; each maximum or minimum scored one point, with the exception of simultaneously occurring anterior and posterior minima on the right shoulder (frequently occurring in normal subjects), which scored together only one point. All but one group A subject had dipolar QRST integral maps (mean +/- SD score 2.11 +/- 0.2). Conversely, 10 of 16 (62.5%) group B patients had scores of 3 or more (mean 3.16 +/- 1.08; p less than .01 vs group A). Group C patients had intermediate values, with eight of 27 (29.6%) scoring 3 or more (mean 2.46 +/- 83); this was less than in group B (p less than .01), but more (p less than .05) than in group A.(ABSTRACT TRUNCATED AT 250 WORDS)

The QT-sensitive cybernetic pacemaker: a new role for an old parameter?

A critical review of the available data on QT interval is presented to delineate techniques useful to the development of a QT-sensitive cybernetic pacemaker. The reason for the development of this unit stems from the ability of QT prolongation to predict the onset of life-threatening ventricular arrhythmias in some clinical situations; the QT interval is physiologically related to the cardiac cycle length, therefore providing adequate information to drive both ventricular and atrioventricular sequential rate-responsive pacemakers. This unit might also monitor cardiac rhythm and detect the pathophysiologic precursors of advanced grades of ventricular arrhythmias. A therapeutic role, both pharmacologic and electrical, may also be possible in the future. Integration of these concepts and cooperation between interested physicians, technicians and manufactors will be necessary to produce such a unit at a low cost-benefit ratio. The potential clinical application of this pacemaker deserves attention for the prophylaxis and treatment of sudden arrhythmic death.

Change in ventricular cavity size: differential effects on QRS and T wave amplitude

Although many factors have been reported to change the R wave amplitude of the electrocardiogram (ECG), few observations have been made of the associated changes in T wave amplitude. We hypothesized that changes in R and T wave amplitude should parallel each other. To test this hypothesis, R and T wave amplitudes were measured in 15 normal subjects during increased and decreased left ventricular dimensions induced by infusion of methoxamine and by Valsalva maneuver, respectively, as well as during changes in the proximity of the left ventricle to the chest wall (i.e., shift in patient position from supine to left lateral position). Simultaneous nine-lead ECGs and two-dimensional-guided M mode echocardiograms of the left ventricle were recorded at rest and under each experimental condition. R wave amplitude increased as the left ventricular lateral wall moved closer to the V5 and V6 electrodes. Alterations in R wave amplitude seen with changes in left ventricular chamber size were primarily caused by radial movement of the left ventricle in relation to the chest wall. Proximity of the left ventricle to the chest wall was therefore a major determinant of R wave amplitude. In contrast, T wave amplitude varied directly with alterations in left ventricular chamber size but was unaffected by changes in proximity to the recording electrode on the chest wall. Left ventricular chamber size, and possibly the associated alteration in endocardial-to-epicardial surface area ratio, appeared to be the major determinants of T wave amplitude.

Spatial variation of QT intervals in normal persons and patients with acute myocardial infarction

The QT interval is a clinically important electrocardiographic measurement. This study attempted to determine 1) whether this interval was spatially distributed in a physiologically meaningful way on the torso of normal subjects, and 2) if these spatial patterns were altered in patients with acute myocardial infarction. To do so, 30 patients were studied within 72 hours of the onset of acute myocardial infarction (15 with an anterior and 15 with a posterior lesion) along with 50 normal control subjects. Electrocardiographic signals were registered from 150 torso electrodes; the QT interval in each lead was determined by a combined automated-manual method, and the durations displayed as "isointerval maps." In the normal subjects, the difference between the longest and shortest interval in each case was 59.4 +/- 12.9 ms. Long QT intervals were spatially located over the left lateral torso and short QT intervals were found over the right inferior chest. Acute infarction modified this distribution in relation to lesion location; the longest QT intervals were centrally positioned in anterior infarction and caudally located in inferior infarction. Thus, QT intervals in normal and abnormal states have distinctive spatial distributions that are consistent with known regional myocardial electrophysiology.

Evaluation of normal variations in S-T segment patterns by body surface isopotential mapping: S-T segment elevation in absence of heart disease

S-T segment elevation is commonly observed in the electrocardiogram of normal persons. To study the possible origins of such patterns, 45 normal volunteers were examined. Electrocardiographic potentials were registered from 150 torso electrodes and processed to construct isopotential maps at 2 ms intervals throughout the QRS-T interval. The maximal potentials recorded from any of the 150 electrodes were 198 +/- 76.4 and 272.1 +/- 84.2 microV at instants 40 and 80 ms into the S-T segment, respectively. Maximal voltages recorded by the six standard precordial V leads at these respective time points were 109.7 +/- 57.0 and 163.6 +/- 66.9 microV. Torso maximal potentials were significantly stronger than were those sensed by V leads; the two were significantly correlated but predictability was limited. The duration of overlap between the onset of ventricular recovery and the end of the excitation was determined from isopotential maps and ranged rom 4 to 16 ms. There was no significant correlation (p greater than 0.05) between these values and either torso or V lead potentials at either 40 or 80 ms into S-T segment. These data suggest that (1) standard precordial leads do not accurately predict maximal torso potentials during the S-T segment, and (2) the degree of overlap between repolarization and depolarization is not a major determinant of precordial voltage. Hence, the rationale for use of the term "early repolarization" to describe this clinical condition is not substantiated.

The QRST deflection area of electrograms during global alterations of ventricular repolarization

Changes of refractory period and QRST deflection area in cardiac electrograms due to localized myocardial warming and altered cycle length were determined. Localized myocardial warming consistently resulted in increased QRST deflection area which was highly correlated with reduction of refractory period. Similar reductions of refractory period by decreased cycle length were associated with insignificant changes of small QRST areas and significant reductions of larger QRST areas. The different effect of local thermal alteration of repolarization and global alteration due to cycle length changes is experimental evidence that QRST deflection area depends on differences in duration of ventricular repolarization properties. The finding of decreased deflection area with decreased cycle length is also evidence of decreased disparity of repolarization properties of normal myocardium at rapid heart rates.