Relation between ventricular late endocardial activity during intraoperative endocardial mapping and low-amplitude signals within the terminal QRS complex on the signal-averaged surface electrocardiogram

Differential pacing from two sites to diagnose risk of ventricular arrhythmia and death

BACKGROUND

QRS abnormalities may not be apparent in sinus rhythm in electrically stable cardiomyopathy patients who can have quiescent but highly arrhythmogenic substrate. Here, we test the hypothesis that differential changes in QRS construction during right-ventricular apex pacing (RVP) as opposed to atrial pacing (AP) will identify latent substrate for ventricular arrhythmias (VA) and death.

METHODS

Forty patients with cardiomyopathy free of VA underwent baseline 114-electrode body-surface electrocardiogram during AP (100 beats per minute [bpm]) and RVP (100 and 120 bpm). The filtered-averaged QRS at each electrode was deconstructed into individual intra-QRS and post-QRS ventricular myopotentials (V_MP ). The primary outcome was VA or death. Prognostic accuracy of V_MP was validated using V1 to V6 leads in another prospective cohort of 44-cardiomyopathy patients.

RESULTS

Twenty-six patients were eligible for initial analysis. After 5 ± 2 years of follow-up, eight (31%) patients had VA (VA_Pos ) while rest were uneventful (VA_Neg ). During AP₁₀₀ , VA_Pos patients expressed more V_MP than VA_Neg patients (16 ± 1 vs 12 ± 1, P = 0.02). RVP₁₀₀ and RVP₁₂₀ in VA_Pos patients introduced an additional 5.5 ± 0.5 and 6.0 ± 0.5 V_MP (P < 0.0001 vs AP₁₀₀ ). The relative change with RVP₁₂₀ versus AP₁₀₀ in VA_Neg patients exceeded VA_Pos patients by 1.2 ± 0.5 V_MP (P = 0.03). Increment in V_MP count of <8 in lead-V5 with RVP₁₂₀ compared to AP₁₀₀ best predicted VA (area under curve 0.81, P = 0.01). In the validation cohort, primary outcome occurred in 13 (33%) patients. Native QRS features and AP₁₀₀ alone failed to predict primary outcome. Patients with increment in V_MP count of <8 in lead-V5 with RVP₁₂₀ versus AP₁₀₀ had 7.9-fold increased risk of primary outcome (95% confidence interval 1.01, 61.61; P = 0.049).

CONCLUSION

Cardiomyopathy patients at risk of VA or death perturb the QRS less than low-risk patients with differential pacing. This functional response may be useful to identify arrhythmogenic substrate.

Automated Quantification of Low-Amplitude Abnormal QRS Peaks From High-Resolution ECG Recordings Predicts Arrhythmic Events in Patients With Cardiomyopathy

BACKGROUND

Cardiomyopathy patients are at risk of sudden death, typically from scar-related abnormalities of electrical activation that promote ventricular tachyarrhythmias. Abnormal intra-QRS peaks may provide a measure of altered activation. We hypothesized that quantification of such QRS peaks (QRSp) in high-resolution ECGs would predict arrhythmic events in implantable cardioverter-defibrillator (ICD)-eligible cardiomyopathy patients.

METHODS AND RESULTS

Ninety-nine patients with ischemic or non-ischemic dilated cardiomyopathy undergoing prophylactic ICD implantation were prospectively enrolled (age 62±11 years, left ventricular ejection fraction 27±7%). High-resolution (1024 Hz) digital 12-lead ECGs were recorded during intrinsic rhythm. QRSp was quantified for each precordial lead as the total number of low-amplitude deflections that deviated from their respective naive QRS template. The primary end point of arrhythmic events was defined as appropriate ICD therapy or sustained ventricular tachyarrhythmias. After a median follow-up of 24 (15-43) months, 20 (20%) patients had arrhythmic events. Both QRSp and QRS duration were greater in those with arrhythmic events (both P<0.001) and this was consistent for QRSp for both cardiomyopathy types. In a multivariable Cox regression model that included age, left ventricular ejection fraction, QRS duration, and QRSp, only QRSp was an independent predictor of arrhythmic events (hazard ratio, 2.1; P<0.001). Receiver operating characteristic analysis revealed that a QRSp ≥2.25 identified arrhythmic events with greater sensitivity (100% versus 70%, P<0.05) and negative predictive value (100% versus 89%, P<0.05) than QRS duration ≥120 ms.

CONCLUSIONS

QRSp measured from high-resolution digital 12-lead ECGs independently predicts ventricular tachyarrhythmias in ICD-eligible cardiomyopathy patients. This novel QRS morphology index has the potential to improve sudden death risk stratification and patient selection for prophylactic ICD therapy.

Analysis of Abnormal Intra-QRS Potentials in Signal-Averaged Electrocardiograms Using a Radial Basis Function Neural Network

Abnormal intra-QRS potentials (AIQPs) are commonly observed in patients at high risk for ventricular tachycardia. We present a method for approximating a measured QRS complex using a non-linear neural network with all radial basis functions having the same smoothness. We extracted the high frequency, but low amplitude intra-QRS potentials using the approximation error to identify possible ventricular tachycardia. With a specified number of neurons, we performed an orthogonal least squares algorithm to determine the center of each Gaussian radial basis function. We found that the AIQP estimation error arising from part of the normal QRS complex could cause clinicians to misjudge patients with ventricular tachycardia. Our results also show that it is possible to correct this misjudgment by combining multiple AIQP parameters estimated using various spread parameters and numbers of neurons. Clinical trials demonstrate that higher AIQP-to-QRS ratios in the X, Y and Z leads are visible in patients with ventricular tachycardia than in normal subjects. A linear combination of 60 AIQP-to-QRS ratios can achieve 100% specificity, 90% sensitivity, and 95.8% total prediction accuracy for diagnosing ventricular tachycardia.

Analysis of unpredictable components within QRS complex using a finite-impulse-response prediction model for the diagnosis of patients with ventricular tachycardia

This study proposes a finite-impulse-response (FIR) prediction model to analyze the unpredictable intra-QRS potentials (UIQP) for identifying ventricular tachycardia patients with high-risk ventricular arrhythmias. The simulation study shows that a QRS complex including abnormal intra-QRS potentials (AIQP) has a higher UIQP and UIQP-to-QRS ratio in comparison with one without AIQP. The clinical results demonstrate that the mean UIQP-to-QRS ratios of VT patients in leads X, Y and Z were significantly larger than those of the normal subjects, and the linear and logical combination of UIQP-to-QRS ratios and ventricular late potential parameters can enhance diagnosis performance for VT patients.

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.

Detection of the fingerprint of the electrophysiological abnormalities that increase vulnerability to life-threatening ventricular arrhythmias

Reduction of sudden death requires accurate identification of patients at risk for ventricular tachycardia (VT) and effective therapies. The Multicenter Unsustained Tachycardia Trial and Multicenter Automatic Defibrillator Implantation Trials demonstrate that the implantable cardioverter defibrillator impacts favorably on the incidence of VT in patients with myocardial infarction, underscoring the need to detect the electrophysiologic abnormalities required for the development of VT. Methods used for this purpose include: Holter monitoring, ejection fraction, signal-averaged ECG, heart rate variability, T-wave alternans, baroreflex sensitivity, and programmed stimulation. Performance of each method alone has demonstrated high-negative but low-positive predictive values. Recent studies confirm that their use in combination augments performance.A second approach for improving performance has been to reexamine how well each method detects the electrophysiological derangements that lead to VT. Our recent work has focused on the signal-averaged ECG. Judging from transmural maps of ventricular activation during VT and sinus rhythm obtained from patients, late potentials fail to detect completely signals from myocardium responsible for VT. To obviate this limitation we developed an approach based on inferred epicardial potentials in the frequency domain from 190-surface ECGs using individualized heart-torso models. Torso geometry and electrode positions are measured with a 3-armed digitizer. The location of cardiac structures is determined using echocardiography. The pericardial surface is approximated by a sphere that encloses the heart. Epicardial potentials are inferred using the boundary element method with zero-order Tikhonov regularization and the Composite Residual Smoothing Operator over the QRS complex. Studies are underway to determine if analysis of bioelectrical signals enveloping arrhythmogenic tissue improves identification of patients vulnerable to VT.

The short-term and long-term reproducibility of spectral turbulence and late potential variables of the signal-averaged ECG in a population sample of healthy subjects and the impact of gender, age, and noise

Previous methods for frequency domain analysis of the signal-averaged ECG (SAECG) have had low reproducibility. The reproducibility of time domain late potential analysis and spectral turbulence analysis was evaluated with 2 immediately consecutive SAECG recordings in 121 randomly selected subjects without heart disease (short-term) and also in 47 subjects after 1 month (long-term). A test was late potential positive if 2 or more of 3 variables were outside the reference limits and spectral turbulence positive if the score was 3 or 4. The short-term reproducibility was high for the filtered QRS duration (FQRSD), root mean square amplitude of the last 40 ms (RMS40) and high frequency low amplitude signals less than 40 microV (HFLAS40) of the time domain and total QRS duration (TQRSD), power spectral density of the last 40 ms and the late potential duration of time domain analogous analyses. The Spearman rank order correlation coefficients were 0.89, 0.88, and 0.84 and 0.97, 0.91 and 0.97, respectively. The reproducibility of the spectral score variables varied, and the correlation for the low slice correlation ratio was 0.71, spectral entropy 0.61, interslice correlation mean 0.58, and interslice correlation SD 0.28. A diagnostic inconsistency between 2 tests occurred in 0 (0%) subjects in late potential analysis if FQRSD was required for positivity, and in 7 (6%) otherwise, and in 9 (7%) of spectral turbulence analysis. If the spectral variable mean peaks per slice, with a correlation of 0.89, replaced interslice correlation SD in the spectral score, diagnostic inconsistency occurred in 0 (0%) subjects. The reproducibility seemed higher in women and in younger people but significantly only for interslice correlation subjects mean and HFLAS40. The long-term reproducibility did not differ significantly from short-term for any variable. In conclusion, the reproducibility was high in all time domain and time domain analogous variables. It varied among the spectral turbulence score variables and was very low for interslice correlation SD. The reproducibility of the spectral score improved substantially if this variable was replaced by mean peaks per slice.

Ischemia-induced changes of the signal-averaged electrocardiogram: experimental investigation during percutaneous transluminal coronary angioplasty balloon-occluded coronary artery

INTRODUCTION

The influence of myocardial ischemia on the detection of an arrhythmogenic substrate with the signal-averaged ECG is unclear.

METHODS AND RESULTS

In 80 patients with single vessel coronary artery disease and a critical stenosis of the left anterior descending vessel selected after coronary angiography in whom percutaneous transluminal coronary angioplasty (PTCA) was planned, we retrospectively investigated the signal-averaged ECGs in the time domain before, during, and after occlusion of the coronary artery by the PTCA balloon. Forty patients were resuscitated from ventricular fibrillation (VF group), and 40 patients had no ventricular arrhythmia (non-VF group). Late potentials were seen at rest in 26 of 40 patients in the VF group. During ischemia, the duration of the filtered QRS complex and the duration of low-amplitude signals < 40 microV increased significantly. In another 14 patients in the VF group, late potentials were observed only during ischemia. In 4 of 26 patients in the VF group without prior infarction but with severe ischemia present at rest, successful PTCA eliminated preexistent late potentials. In the non-VF group, one patient had late potentials present at rest. In two patients with prior infarction, late potentials were provokable only during transmural ischemia.

CONCLUSION

Myocardial ischemia is able to modify detection of an arrhythmogenic substrate with the signal-averaged ECG.

Analysis of abnormal signals within the QRS complex of the high-resolution electrocardiogram

This paper presents a new, quantitative approach to measuring abnormal intra-QRS signals, using the high-resolution electrocardiogram (HRECG). These signals are conventionally known as QRS "notches and slurs." They are measured qualitatively and form the basis for the ECG identification of myocardial infarction. The HRECG is used for detection of ventricular late potentials (LP), which are linked with the presence of a reentry substrate for ventricular tachycardia (VT) after a myocardial infarction. LP's are defined as signals from areas of delayed conduction which outlast the normal QRS period. Our objective is to quantify very low-level abnormal signals that may not outlast the normal QRS period. In this work, abnormal intra-QRS potentials (AIQP) were characterized by removing the predictable, smooth part of the QRS from the original waveform. This was represented as the impulse response of an ARX parametric model, with model order selected empirically from a training data set. AIQP were estimated using the residual of the modeling procedure. Critical AIQP parameters to separate VT and non-VT subjects were obtained using discriminant functions. Results suggest that AIQP indexes are a new predictive index of the HRECG for VT. The concept of abnormal intra-QRS potentials permits the characterization of pathophysiological signals contained wholly within the normal QRS period, but related to arrhythmogenesis. The new method may have other applications, such as detection of myocardial ischemia and improved ECG identification of the site of myocardial infarction, particularly in the absence of Q waves.

Effect of subendocardial resection on sinus rhythm endocardial electrogram abnormalities

BACKGROUND

Patients with sustained ventricular tachycardia after acute myocardial infarction frequently have characteristic abnormalities of left ventricular endocardial electrical activity, including fractionated (prolonged, multicomponent, low-amplitude), split (having discrete widely separated deflections), and late (extending after the end of the QRS complex) electrograms. The exact cause and source of these electrograms are not clear.

METHODS AND RESULTS

In this study, endocardial electrograms from 18 patients were recorded with a 20-electrode array from the same area immediately before and immediately after resection of subendocardial tissue at the time of surgery for ventricular tachycardia. Electrograms could be compared before and after resection from 298 of 360 (83%) of the electrodes. Before resection, split electrograms were present in 130 (44%) and late components in 81 (27%) of the recordings. Recordings made after resection showed fewer abnormalities, including complete absence of split electrograms as well as all previously recorded late components (P < .02). Mean electrogram amplitude increased from 0.5 +/- 0.8 to 1.0 +/- 1.6 mV (P < .0001) because of removal of the attenuating effect of endocardial scar; mean duration decreased from 112 +/- 38 to 65 +/- 27 ms (P < .0001) mainly because of loss of late and split components. Overall electrogram contour was very similar aside from these changes.

CONCLUSIONS

These data show that (1) some of the signal recorded on the endocardial surface is derived from deeper tissue layers and (2) split and late electrogram components appear to be generated by cells in the superficial endocardial layers, since they are eradicated by removal of this tissue. These findings correspond well with previous histological studies of resection specimens that show bundles of surviving muscle cells separated by layers of dense scar that act as an insulator.

Phase and group-delay characteristics of signal-averaged electrocardiograms from patients with ventricular tachycardia

Fourier analysis of the signal-averaged ECG (SAECG) has previously revealed significant differences in magnitude spectra that differentiate patients with ventricular tachycardia (VT) from those without VT. To determine additional distinguishing features in the frequency domain, we analyzed phase spectra of SAECG's of sinus beats from 57 patients with VT, 65 without VT, and 20 normal controls. Unwrapped phase spectra from SAECG's of the entire cardiac cycle were calculated with respect to three fiducial points: onset of the P and Q waves, and the negative of the slope of the phase (group delay) for frequencies in the band, which accounted for 97.5% of the energy in the vector magnitude of the Frank SAECG leads. Phase spectra of SAECG's from patients with VT differed from the non-VT patients at frequencies > or = 21 Hz (p = 0.000039) for the P-wave fiducial, at frequencies > or = 60 Hz (p = 0.00085) for the Q-wave fiducial, and at frequencies < or = 62 Hz (p = 0.0035) for the 97.5% energy fiducial. Group delays in SAECG's from patients with and without VT differed from 10 to 26 Hz (p = 0.000016) for the P-wave fiducial, and from 14 to 24 Hz (p = 0.00000070) for the Q-wave fiducial. Group delays with respect to the Q-wave fiducial in the VT patients in the 14-24 Hz band were, on average, 9 ms and 5 ms longer than those of the non-VT's and normals, respectively. Thus, phase spectra of SAECG's contain previously undetected features that together with magnitude may be helpful in improving methods for stratifying the risk of VT.

Correlation of the endocardial fragmented electrogram with body surface signal-averaged electrocardiographic mapping

We compared signal-averaged electrocardiography (SAE), SAE mapping, and left ventricular catheter mapping in 60 patients with ischemic heart disease. Using the data obtained in patients with no fragmented electrograms (FE) in the left ventricle, the late potential was defined by SAE as a filtered QRS duration > 131 msec or a root mean square voltage < 16 microV for the last 40 msec of the QRS complex. SAE mapping was performed by recording the signal-averaged electrocardiogram at 48 sites on the body surface. With SAE mapping, the filtered QRS duration and the area in the last 20 msec of the QRS complex were significantly different between the patients with and without FEs. The late potential was defined by SAE mapping as a filtered QRS duration > 136 msec or an area < 28 microV.msec for the last 20 msec of the QRS complex. The sensitivity and specificity of detecting FEs were 46% and 88%, respectively, by the SAE filtered QRS criterion, while they were 66% and 88% by the root mean square criterion. In contrast, SAE mapping gave values of 66% and 92% by the filtered QRS criterion, as well as values of 100% and 92% by the area criterion. Thus, SAE mapping provided better detection of the FE and was more closely correlated with the results of catheter mapping, suggesting its potential for clinical application.

Late potentials, QTc prolongation, and prediction of arrhythmic events after myocardial infarction

In a series of 171 consecutive survivors of acute myocardial infarction, the predictive value of late potentials and QTc prolongation was prospectively assessed. QT intervals were measured in lead V2, corrected QT (QTc) was calculated using Bazett's equation (cut-off value 440 ms). Late potentials were considered to be present when all of the three signal-averaged electrocardiographic variables were abnormal (i.e. QRS > 114 ms, D40 > 38 ms, and V40 < 20 microV). Complete follow-up was obtained (mean 13 +/- 6 months, range 6-24 months). Six percent of the patients had an arrhythmic event (i.e. sustained ventricular tachycardia or sudden death). The relative risk of late potentials for arrhythmic events was 7.7 (P < 0.02). The relative risk of QTc > 440 ms was 1.1 (NS). In a multivariate analysis, the addition of QTc prolongation did not significantly improve the prognostic value of late potentials alone. It is concluded that late potentials are predictive of arrhythmic events after myocardial infarction, but the presence of concomitant QTc prolongation does not worsen the prognosis.

Signal averaging in patients with coronary artery disease: how helpful is it?

In patients who have survived acute myocardial infarction, the presence of ventricular late potentials using the high resolution signal-averaged ECG indicates areas of slow conduction and delayed activation that may potentially serve as a substrate for malignant ventricular arrhythmias. Although detection of late potentials is technique specific, signal-averaged analysis in the time or frequency domain may be a useful index for risk stratification with regard to ventricular tachycardia or sudden cardiac death. The sensitivity and specificity of late potentials for this purpose may be enhanced by combination with other variables, such as left ventricular ejection fraction and presence of complex ventricular ectopy. Therefore, the presence of ventricular late potentials in postmyocardial infarction patients, particularly in those patients with impaired left ventricular function, identifies those patients who are at high risk of malignant ventricular tachyarrhythmias. However, the strategies for prevention of serious arrhythmia complications during follow-up need to be established. The negative predictive value of late potentials is very high. Thus, the absence of late potentials indicates a low propensity to sustained ventricular tachycardia or sudden death, even in the presence of complex ventricular ectopy. Interventions may therefore not be necessary or should even be avoided. The incidence of late potentials in patients with spontaneous or induced ventricular fibrillation is lower and, if present, less pronounced than in those with sustained monomorphic ventricular tachycardia. This presumably is due to a lower degree of conduction delay, which serves as a substrate for reentry. Therefore, the ability of the signal-averaged ECG to predict a propensity to ventricular fibrillation is limited. Despite these limitations, the signal-averaged ECG may be used as a risk predictor in evaluation of patients after myocardial infarction. Unfortunately, at least as far as time domain analysis is concerned, it cannot be used as an efficacy predictor for response to pharmacologic interventions. Further studies will determine whether other modes of signal-averaged analysis can predict the response to drugs.

Current usefulness of the signal-averaged electrocardiogram

Signal-averaged electrocardiography is a relatively simple, noninvasive technique by which valuable information can be gained to help in the management of patients with cardiovascular disease. The presence of late potentials on the SAECG is a good marker for the presence of an arrhythmogenic substrate that is believed to be the source of ventricular tachycardia in patients with coronary artery disease. The value of the detection of late potentials has been studied best after myocardial infarction, when the absence of late potentials makes the occurrence of an arrhythmic event very unlikely. The positive predictive value for an arrhythmic event to occur in the presence of late potentials is low, however, comparable to the predictive value of decreased left ventricular function, complex ventricular ectopy, or abnormal autonomic tone. This appears to have its explanation in the complex pathophysiology behind the occurrence of arrhythmic events. Improved accuracy for the SAECG is achieved when the result of the test is interpreted with consideration of the presence or absence of other predictive markers. A thorough understanding of the signal-averaged electrocardiogram makes optimal clinical use of the information gained from this easily acquired test possible.

Contribution of myocardium responsible for ventricular tachycardia to abnormalities detected by analysis of signal-averaged ECGs

BACKGROUND

Current methods of signal-averaged ECG analysis interrogate the terminal 40 msec of the QRS complex and/or the ST segment and have a low positive-predictive accuracy for detecting vulnerability to sustained ventricular tachycardia (VT). The extent to which abnormalities detected during these ECG intervals are generated by myocardial tissue responsible for VT has not been well defined. The purpose of this study was to determine when, during sinus rhythm, myocardium responsible for VT is activated.

METHODS AND RESULTS

Three-dimensional ventricular activation maps were analyzed during sinus rhythm and during 10 VTs in eight patients with healed myocardial infarctions undergoing arrhythmia surgery for sustained monomorphic VT. The mechanism of VT was focal in five instances and macroreentrant in five. During sinus beats, myocardium responsible for all focal VTs activated 43 +/- 38 msec before the onset of the terminal 40-msec interval of the QRS complex. During sinus rhythm, activation of the myocardium critical to macroreentrant VT began 72 +/- 13 msec before the onset of the terminal QRS interval and in only three instances extended 2-25 msec into the terminal 40 msec of the QRS complex. Electrograms recorded during the ST segment represented late activation of epicardial sites overlying zones of infarction that were temporally and spatially remote from tissue critical to VT.

CONCLUSIONS

Current methods of signal-averaged ECG analysis limiting interrogation to the terminal QRS/ST segment exclude detection of > 95% of the signals generated by myocardium responsible for sustained VT. These results establish a pathophysiological basis for expanding signal-averaged ECG analysis to include more of the cardiac cycle.

Late potentials in a porcine model of anterior wall myocardial infarction and their relation to inducible ventricular tachycardia

In this study, normal values for signal averaged electrocardiographic parameters were assessed in healthy pigs (n = 100) and the development of late potentials after myocardial infarction (n = 41) in relation to inducible ventricular tachycardia was investigated. Normal values are: filtered QRS duration (QRS) < or = 78 msec; root mean square voltage of the averaged QRS complex (V(tot)) > or = 51 microV, and duration of terminal activity below 30 microV (D30) < or = 37 msec. The distribution of the root mean square voltage in the last 30 msec (V30) was biphasic. Two weeks after myocardial infarction, QRS was prolonged from 55 +/- 10 to 66 +/- 19 msec (P < 0.002). D30 was prolonged from 19 +/- 6 msec to 28 +/- 13 (P < 0.002). V30 was decreased from 107 +/- 135 microV to 45 +/- 77 (P < 0.02). The total voltage (V(tot)) was decreased from 195 +/- 78 to 123 +/- 61 microV (P < 0.002). In four pigs (19%) late potentials developed. Sustained ventricular tachycardia was inducible in 11 pigs (52%), ventricular fibrillation in two pigs (10%) and eight pigs (38%) were noninducible. Three of 11 inducible pigs and one of the noninducible pigs had a late potential. The incidence of late potentials and their relation to inducible sustained ventricular tachycardia is comparable to the situation in man. Therefore, this pig model is an attractive alternative to the commonly used dog models.

Late potentials in a bradycardia-dependent long QT syndrome associated with sudden death during sleep

The purpose of this study was to determine the incidence of late potentials and their relation to QT prolongation in a family with a high incidence of sudden death during sleep at a young age and bradycardia-dependent QT prolongation (n = 9) and to compare the findings with those in consanguineous family members without QT prolongation (n = 13). Six (67%) of the 9 family members with QT prolongation had late potentials on the signal-averaged electrocardiogram (ECG) compared with 1 of the 13 normal subjects (p less than 0.007). Positive predictive accuracy of the signal-averaged ECG for the detection of subjects with QT prolongation was 86%; negative predictive accuracy was 80%. During exercise testing, the QT interval normalized, whereas late potentials did not change significantly. Exercise testing did not reveal the presence of coronary artery disease as a possible cause of late potentials. It is concluded that 1) compared with family members with a normal QT interval, patients with this type of bradycardia-dependent QT prolongation have a high incidence of late potentials; 2) late potentials persist despite normalization of the QT interval at high heart rates, indicating that there is no direct relation between late potentials and QT prolongation; and 3) late potentials are not caused by coronary artery disease in these subjects. Therefore, the detection of late potentials might be a new aid in the detection and risk stratification of patients with the long QT syndrome. Late potentials possibly indicate a substrate for ventricular tachyarrhythmias in this type of bradycardia-dependent QT prolongation.