High-frequency electrocardiography: an evaluation of lead placement and measurements

Electrocardiographic arrhythmia risk testing

Among the most compelling challenges facing cardiologists today is identification of which patients are at highest risk for sudden death. Automatic implantable cardioverter-defibrillators are now indicated in many of these patients, yet the role of noninvasive risk stratification in classifying patients at high risk is not well defined. The purpose of this review is to evaluate the various electrocardiographic (ECG) techniques that appear to have potential in assessment of risk for arrhythmia. The resting ECG (premature ventricular contractions, QRS duration, damage scores, QT dispersion, and ST segment and T wave abnormalities), T wave alternans, late potentials identified on signal-averaged ECGs, and heart rate variability are explored. Unequivocal evidence to support the widespread use of any single noninvasive technique is lacking; further research in this area is needed. It is likely that a combination of risk evaluation techniques will have the greatest predictive power in enabling identification of patients most likely to benefit from device therapy.

Late potential analysis: is a mathematically-derived X,Y,Z lead system comparable to a true orthogonal X,Y,Z lead system?

BACKGROUND

Analysis of ventricular late potentials (LP) with signal-averaged ECG (SAECG) using three bipolar, orthogonal X,Y, Z leads is a validated method of risk-stratification in patients prone to ventricular tachycardia. The aim of this study was to validate a ECG system, which allows LP analysis using X,Y, Z leads mathematically derived from the standard 12-lead ECG.

METHODS AND RESULTS

In 36 patients (age 56 +/- 12 years, coronary artery disease 71%, LVEF 46 +/- 14%) with known or suspected ventricular tachyarrhythmia, two consecutive SAECGs were recorded, one with mathematically derived and another one with true X,Y, Z leads. Time domain measurements with these different lead systems were compared using linear regression analysis and "Bland-Altman" plots. Correlation was good (r = 0.92) for the filtered QRS complex duration, but poor for the terminal QRS amplitude (RMS) and duration (LAS) criteria (r = 0.66 and 0.61, respectively; P < 0.0001). Defining LPS as present if at least two of the three time domain criteria were abnormal, the result matched in 28 (78%), but differed in 8 (22%) patients.

CONCLUSION

SAECG using X,Y, Z leads mathematically derived from the standard 12-lead ECG compared to true bipolar X,Y, Z leads show a close correlation in filtered QRS duration, but can differ considerably in the other time domain measurements, resulting in different interpretation of LP analysis in 22%. Therefore, SAECG registration should currently be performed with true X,Y, Z leads, until the accuracy of other approaches is validated.

Immediate and day-to-day reproducibility of the signal-averaged electrocardiogram in patients with coronary artery disease

The purpose of this study was to prospectively evaluate the immediate (5 minutes) and short-term (1 day) reproducibility of the signal-averaged electrocardiogram (SAECG) in patients with coronary artery disease. A total of 59 consecutive patients with coronary artery disease (50 male, 9 female, mean age 59 +/- 13 years) were included. Analysis was performed using a commercially available system with high-pass filters of 25, 40, and 80 Hz without any change in electrode position. The following time-domain parameters were measured: (1) total filtered QRS duration; (2) duration of high-frequency low-amplitude signals (HFLA); and (3) root-mean-square voltage of the terminal 40 ms of the QRS complex (RMS40). Correlation between pairs of measurements was excellent, ranging from 0.91-0.99 for QRS and HFLA duration, and from 0.83-0.98 for RMS40 value. However, the range of differences was wide for all parameters, especially at 25 and 80 Hz, revealing marked individual variations in selected cases. The immediate reproducibility of an initially normal SAECG was 95% at 25 Hz, 100% at 40 Hz, and 81% at 80 Hz; figures for an initially abnormal SAECG were 89%, 91%, and 93%, respectively, and the coefficient of agreement kappa was highest at 40 Hz than at 25 or 80 Hz. Discordant results were most frequently observed in patients with borderline values, or in cases with nonsimilar residual noise levels. In conclusion, the immediate and day-to-day reproducibility of the SAECG is excellent in patients with chronic stable coronary artery disease. However, individual variations are observed in selected cases, leading to modification in interpretation of the result. For this reason, individual reproducibility should be considered before evaluating the effect of any therapeutic intervention on the SAECG, especially in patients with borderline values of the quantitative parameters of the SAECG.

Variation in late potentials and the reproducibility of their measurement

The usefulness of a test depends on its reproducibility. This determines how closely the test result indicates the actual pathophysiologic state, how well it will predict that state in the future, and if interventions or further pathologic changes are reflected by the test. There is a variation in the parameters of the signal-averaged ECG, more so with spectral than with time domain measurements. These must be accounted for when estimating risk. If one presumes that risk is proportional to the extent of abnormality, then the variation in measurements simply means that only borderline cases can potentially be miscategorized. More important, the lack of reproducibility of measurements made from the signal-averaged ECG indicates that changes noted in an individual after an intervention, such as a surgical intervention, must be viewed with a jaundiced eye. Group changes are perhaps meaningful, and indicate a physiologic effect, but clinical decisions cannot be made unless the changes observed in an individual patient exceed the confidence limits of expected variation. There has been debate as to the usefulness of measurements made from the signal-averaged ECG in predicting antiarrhythmic drug effects (the effect of drugs is discussed elsewhere in this symposium). Here an analogy must be made to the suppression of asymptomatic ventricular ectopy. First, we cannot make a statement that there has been a drug effect unless the parameter measured changes beyond the confidence limits of normal variation or reproducibility. Second, we cannot translate a change in a measurement into a change in risk for arrhythmic events without subjecting that hypothesized relationship to a long-term placebo-controlled clinical trial, albeit acute electrophysiologic trials correlating changes in the signal-averaged ECG to ventricular tachycardia induction provide some insight. And perhaps the relationship must be tested independently for each drug assessed. In the same regard, there is much excitement about the benefits of thrombolytic therapy, but when diagnosing benefit to the individual patient we have to remember the lack of reproducibility of the measurements and also keep in mind that an improved signal-averaged ECG cannot be translated into an improved prognosis without long-term controlled studies. In summarizing the variation and reproducibility of measurements made from the signal-averaged ECG we avoided providing more than a sense of the extent of variation expected because precise confidence intervals depend on the particular techniques used to make the measurements.(ABSTRACT TRUNCATED AT 400 WORDS)

Reproducibility of the signal-averaged electrocardiogram

Variation in measurements made from signal-averaged electrocardiograms was examined. Averaging 200 beats in 18 normal subjects, pairs of high-frequency QRS and low-amplitude signal durations correlated at immediate, short-term, and long-term intervals. The percent high-frequency (60 Hz to 120 Hz) voltages in the late potential region had modest correlations. However, 95% confidence intervals of differences in paired measurements were as much as 7% for high-frequency QRS, 20% for a low-amplitude signal, and 53% for percent high-frequency, similar at all time intervals. With electrocardiograms averaged to 0.3 microV noise, high-frequency QRS and low-amplitude signals prolonged, but variation was similar to that of 200 beat pairs. In contrast, low noise reduced percent high-frequency and lessened variation to 29%. Therefore variation in signal-averaged electrocardiographic measurements was considerable (high-frequency voltage greater than durations). Noise did not appreciably influence variation in durations but was critical to consistent voltage measurements in the frequency domain.

Body surface mapping of high-frequency components in the terminal portion during QRS complex for the prediction of ventricular tachycardia in patients with previous myocardial infarction

To study the clinical significance of terminal QRS high-frequency components for the prediction of ventricular tachycardia, an 87-lead body surface signal-averaged mapping was performed in 21 healthy subjects (control) and in 41 patients with previous myocardial infarction (anterior, 20; inferior, 21). Mapping data were analyzed and averaged (129.7 +/- 26.5 beats) for 160 seconds, and the signal-averaged beat was filtered with a bidirectional bandwidth (80-250 Hz) digital filter. J-point was determined from the 87-lead RMS voltage of nonfiltered QRS. For each lead, we calculated the sum of the absolute value of filtered QRS from 20 msec ahead of the J-point to the J-point (A-20). The body surface distribution of A-20 was expressed as A-20 map. The maxima in A-20 maps were mainly located on the upper sternal region in healthy subjects, on the left anterior chest in patients with previous anterior myocardial infarction, and on the central anterior chest in patients with previous inferior myocardial infarction. In the patients in both the group with anterior myocardial infarction and the group with inferior myocardial infarction, the value of maximum was significantly greater than in the subjects in the control group (0.181 +/- 0.086 and 0.138 +/- 0.048, respectively, vs. 0.075 +/- 0.031 mV.msec; p less than 0.01). In patients with myocardial infarction (n = 41), the value of maximum was significantly greater with ventricular tachycardia (n = 11) than without ventricular tachycardia (n = 30) (0.240 +/- 0.076 vs. 0.130 +/- 0.043 mV.msec; p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thrombolysis on the 12-lead signal-averaged ECG in the early postinfarction period

Signal-averaged ECG has been used to identify patients at risk for ventricular tachycardia and sudden death after myocardial infarction. The goals of this prospective study were to examine the effects of reperfusion achieved with thrombolytic therapy on the 12-lead signal-averaged ECG and on ventricular arrhythmias in the early period after acute myocardial infarction (AMI). A total of 190 consecutive patients with AMI who fulfilled the inclusion criteria were enrolled. Thrombolysis was attempted in 80 patients and was considered successful in 57 (group I) and unsuccessful in 23 (group II); 110 patients were not treated with thrombolytic agents (group III). Signal averaging of 12 ECG leads was performed within 2 days in all patients and between 7 and 10 days after admission in 163 patients. The filtered QRS complex duration (QRSD) was significantly shorter in group I compared to group III in 7 of 12 ECG leads at 2 days and in 10 of 12 leads at 7 to 10 days. The root mean square voltage of the terminal 40 msec of the QRS complex (RMS40) did not change between the two signal-averaged ECG recordings in group I, whereas it became lower in three ECG leads in group II and in seven ECG leads in group III. There was no correlation between infarct site and significant changes in infarct-related signal-averaged ECG leads. The occurrence of complex ventricular arrhythmias was not significantly different among the three groups. We conclude that successful reperfusion, compared with failed and nonattempted reperfusion, is associated with fewer abnormalities in the 12-lead signal-averaged ECG in the early period after AMI.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of thrombolysis on the evolution of late potentials within 10 days of infarction

Patients with late potentials in the signal averaged electrocardiogram are more at risk of lethal arrhythmias in the period after acute myocardial infarction. To test the effects of thrombolysis on the incidence and evolution of late potentials, 158 consecutive patients were prospectively studied during the first 10 days after acute myocardial infarction. The study population consisted of two groups: 93 control patients treated conservatively and 65 patients treated with intravenous thrombolysis. Recordings of signal averaged electrocardiogram were obtained within two days and 7-10 days after infarction. The incidence of late potentials in the first two days after infarction was not significantly different in the thrombolytic and control groups (14% v 11.8%). By 7-10 days the incidence of late potentials among patients who underwent thrombolysis remained unchanged (14%); however, it increased significantly in the control group (11.8% to 22.5%). Thus thrombolysis seems to reduce the evolution of late potentials within 10 days of infarction. Because the risk of fatal arrhythmias is higher in patients with late potentials this study may partly explain the reduced mortality after thrombolysis.

Electrophysiological and anatomical substrates for late potential recorded by signal averaging in seven-day-old myocardial infarction in dogs

A filtered QRS (fQRS) was recorded by signal averaging in 7-day-old myocardial infarction (MI) in dogs to detect late potential (LP). The criteria for the LP included a duration of fQRS (D) greater than or equal to 60 msec and a voltage in the last 15 msec (V15) less than or equal to 10 microV. These parameters were determined from the control data from 15 dogs without infarction (D: 45 to 60 msec and V15: 12.0 to 83.6 microV). On the seventh day of infarction, the D had increased from 53.5 +/- 4.7 to 62.2 +/- 9.6 msec (P less than 0.05) and the V15 decreased from 38.6 +/- 19.5 to 18.4 +/- 16.0 microV (P less than 0.01). Of 23 dogs, 14 met the LP criteria (group A) and 9 did not (group B). Sustained ventricular tachycardia (SVT) was induced in 12 group A dogs and in none of the group B dogs. The delayed epicardial activation (DEA) was recorded after the end of QRS at 5.1 +/- 4.7 sites in group A dogs and 1.3 +/- 1.8 sites in group B dogs (P less than 0.05). The maximum value of epicardial activation time was more prolonged in group A than in group B (70.0 +/- 28.3 vs 44.4 +/- 9.8 msec, P less than 0.01). The area of MI was more extensive in dogs with DEA than those without (24.9 +/- 5.8% vs 10.3 +/- 9.0% of the total left ventricular weight, P less than 0.01). In 72 of 90 sites with DEA, the thickness of the surviving epicardial muscle was less than or equal to 1 mm. The sensitivity and specificity of the criteria for LP in detecting DEA were 71.4% and 55.6%, and 100% and 81.8% for predicting inducibility of SVT. It was thus concluded that LP, reflected the DEA, was identified from infarct areas of slow conduction within a reentry circuit of SVT.