G. van Herpen

Diagnostic interpretation of electrocardiograms in population-based research : Computer program research physicians, or cardiologists?

We assessed the performance of diagnostic electrocardiogram (ECG) interpretation by the computer program MEANS and by research physicians, compared to cardiologists, in a physician-based study. To establish a strategy for ECG interpretation in health surveys, we also studied the diagnostic capacity of three scenarios: use of the computer program alone (A), computer program and cardiologist (B), and computer program, research physician, and cardiologist (C). A stratified random sample of 381 ECGs was drawn from ECGs collected in the Rotterdam Study (n = 3057), which were interpreted both by a trained research physician using a form for structured clinical evaluation and by MEANS. All ECGs were interpreted independently by two cardiologists; if they disagreed (n = 175) the ECG was judged by a third cardiologist. Five ECG diagnoses were considered: anterior and inferior myocardial infarction (MI), left and right bundle branch block (LBBB and RBBB), and left ventricular hypertrophy (LVH). Overall, sensitivities and specificities of MEANS and the research physicians were high. The sensitivity of MEANS ranged from 73.8% to 92.9% and of the research physician ranged from 71.8% to 96.9%. The specificity of MEANS ranged from 97.5% to 99.8% and of the research physician from 96.3% to 99.6%. To diagnose LVH, LBBB, and RBBB, use of the computer program alone gives satisfactory results. Preferably, all positive findings of anterior and inferior MI by the program should be verified by a cardiologist. We conclude that diagnostic ECG interpretation by computer can be very helpful in population-based research, being at least as good as ECG interpretation by a trained research physician, but much more efficient and therefore less expensive.

Reproducibility of computerized ecg measurements and coding in a nonhospitalized elderly population

The standard 12-lead electrocardiogram (ECG) is used in many epidemiologic studies to diagnose and predict cardiovascular disease. In view of this, knowledge about the reproducibility of ECG measurements and coding is essential. Minute-to-minute, day-to-day, and year-to-year variability of ECG measurements, composite scores, and Minnesota Code classification were assessed by use of a computer program, in 101 nonhospitalized elderly men and women. Interval ECG measurements were more reproducible than amplitude measurements. The best reproducibility was found for the overall QTc interval (coefficient of variation 3.1%, 4.0%, and 5.2% for the minute-to-minute, day-to-day, and year-to-year groups, respectively) and the poorest was found for the Cardiac Infarction Injury Score (coefficient of variation 67.1%, 78.5%, and 94.3%, respectively). Minnesota Code discrepancies occurred in 16%, 19%, and 22% of the ECGs in the minute-to-minute, day-to-day, and year-to-year groups, respectively. Reproducibility within specific code categories was much better. Overall, variability tended to increase with time. In the routine setting, electrode positioning had relatively little effect on total variability.

Use of the standard 12-lead ECG to simulate electrode displacements

Placement of the precordial electrodes for recording a 12-lead electrocardiogram (ECG) is subject to variation. Previous research has shown that displacement, especially in the longitudinal direction, can lead to changes in diagnosis. In practice, both the displacement and the effects of displacement on an individual ECG are unknown. To assess this effect for a given ECG, the authors developed a method to simulate ECGs at different displacements using only the recorded ECG. The material consisted of 746 body surface potential maps (BSPMs) containing 232 cases without abnormalities, 277 with myocardial infarction (MI), and 237 with left ventricular hypertrophy. By interpolating BSPMs, ECGs from closely spaced electrode positions could be derived. Taking electrode positioning errors that may be encountered in practice, 40 ECGs at different electrode displacements (displaced ECGs) for each BSPM were derived. Using half of the BSPMs, for each displacement, a transformation matrix that transforms the ECG at the standard 12-lead electrode positions (standard ECG) to the displaced ECG was determined. Using the other half of the BSPMs, each displaced ECG was compared with the ECG yielded by the corresponding transformation matrix (transformed ECG). For each comparison, the differences were assessed between the two sets of ECG signals and between the diagnostic computer classifications of the two sets. Signal differences were expressed as mean absolute amplitude differences over the QRS. Computer interpretation of MI and left ventricular hypertrophy was graded in five levels of certainty (no, consider, possible, probable, definite). For instance, for the largest longitudinal displacement studied of about one intercostal space, the 96th percentile mean absolute amplitude difference over the test set was 204 microV. The percentage of cases showing a change in MI classification of more than two certainty levels was 2.7% for this displacement. When comparing the standard ECG with the displaced ECG, these figures were 434 microV and 8.3%, respectively. It is concluded that ECGs from displaced electrodes can be well simulated by transforming the standard ECG, both for the ECG signal and diagnostic classifications.

The U wave explained as an intrinsic part of repolarization

In the ECG the U wave follows the T, which is considered to reflect ventricular repolarization. Several hypotheses about the genesis of the U wave have been put forward, but a satisfactory explanation is still outstanding. We present a simple digital model of the left ventricle that simulates the formation of the U wave on the basis of known electrophysiological processes responsible for the electrical sources in the myocardium, and of the physical laws, embodied in the lead vector concept, which link the potentials in or on the body to these sources. The repolarization waves constructed by the model reproduce the natural aspects of a T wave followed by a U wave. The creation of a U wave appears to be conditional on small voltage differences between the tail ends of the action potentials assigned to the myocardial cells. No fundamental demarcation exists between U wave and preceding Twave

Improved spatial sampling of ECG potentials on the body surface by repositioning electrodes from the standard 12-lead ECG

We propose a simple, practical approach to increase the diagnostic information content of the standard 12-lead ECG by repositioning selected chest leads. We used a set of 746 120-lead body-surface potential maps (BSPMs). Coefficients to reconstruct all BSPM leads from the standard ECG were derived by linear regression. Similarly, BSPMs were reconstructed when two of the leads V3-V6 were replaced by electrodes at other positions on the anterior part of the thorax. Repositioning lead V4 at two intercostal spaces below V2 and lead V6 at two intercostal spaces above V2 or at three intercostal spaces above V4 showed increased reconstruction performance as compared with the standard electrode positions in most parts of the anterior thorax, including regions that are known to contain important diagnostic information that is less well represented by the standard ECG. Also, this approach obviates the need to determine the precise location of V4, which may be difficult in women.

A method to reduce the effect of electrode position variations on automated ECG interpretation.

To reduce the effect of electrode position variations on the diagnostic interpretation of an ECG, ECG and VCG interpretations were combined. The reduction was assessed by generating ECGs with displaced electrodes for a group of subjects using Body Surface Potential Maps (BSPMs). VCGs were reconstructed from the ECGs. The group consisted of normals, cases with myocardial infarction (MI), and with left ventricular hypertrophy (LVH). The effects of four types of electrode position changes were assessed for the diagnostic categories MI and LVH. The combined interpretation proved to be less sensitive to large changes than either the ECG or the VCG interpretation alone. The number of small changes increased for the combined interpretation. The combined interpretation showed higher agreement with a human expert than the ECG interpretation alone.

The time course of ECG-parameters has no quantitative significance in the thrombolytic era

Spatial ST-vector magnitudes, mod ST/sub 40-80/ mod , were measured every 3 min for at least 3 h after admission, and at wider intervals for 3 days. Likewise, the magnitudes of the difference vector mod Delta QRS mod between the vectorcardiogram (VCG) on admission and a subsequent VCG were determined. The VCG was synthesized from eight electrocardiogram (ECG) leads through linear transformation. The study included 18 patients, only three of whom did not undergo interventions. mod ST/sub 40-80/ mod was always maximal on admission and declined to a stable level in about 4.5. Over the same time span mod Delta QRS mod showed an inconspicuous rise. Correlation of these parameters with infarct size determined by 76 h cumulative serum a-hydroxy-butyrate dehydrogenase activity was poor. Evidently, important ECG charges have already taken place before admission, but even allowing for this, little quantitative relation between VCG parameters and infarct size was apparent.<<ETX>>

Dissociation of heart rate and ECG morphology

The authors studied thirteen healthy subjects in which identical heart rates (/spl Delta/HR<1%) could be obtained under different autonomous conditions: by increasing the angle of the legs with the horizontal plane, with tilt angles ranging from 0 till 60/spl deg/ and back, or by performing a handgrip maneuver. During all measurements the thorax was kept at a 700 angle. Heart rate increased from 65.2a/spl plusmn/9.0 (control) to 72.1/spl plusmn/8. 7 (tilt) and 72.1/spl plusmn/8.8 (handgrip) bpm. A number of vector cardiographic parameters differed significantly (P<0.05) between tilt and handgrip, e.g. QRS azimuth (-33.5/spl plusmn/15.0 vs. -22.4/spl plusmn/22.5/spl deg/), QRS duration (103/spl plusmn/10 vs. 107/spl plusmn/13 ms), maximal T vector (646/spl plusmn/200 vs. 703/spl plusmn/184 /spl mu/V), T azimuth (45.3/spl plusmn/14.5 vs. 38.6/spl plusmn/13.6/spl deg/) and the heart rate corrected QT interval (418/spl plusmn/15 vs. 435/spl plusmn/21 ms). This study demonstrates that tilt and handgrip dissociate heart rate and ventricular depolarization and repolarization.

Body surface potential mapping of QRST changes during and after percutaneous transluminal coronary angioplasty

The present study reports preliminary results on 23 patients undergoing percutaneous transluminal coronary angioplasty (PTCA); 120 lead body surface potential maps (BSPM) were recorded before, during and after balloon inflation. Twelve patients underwent PTCA for LAD coronary disease, 6 for RCA and 4 for LCx; one patient had combined LAD and RCA disease. Instantaneous voltage measurements during the QRST complex were obtained by sampling time-normalized QRS and STT waveforms. In each patient, maps recorded during the various phases of the procedure were compared with the patients own baseline map. Also, for each patient, successive maps were compared to group-mean normal maps computed from 159 normal control subjects. In 6 out of 23 patients, maps were also recorded 24 hours after PTCA. In most cases, QRST changes occurred in areas corresponding approximately to the vascular beds of the inflated arteries. In the LAD group, mid-to-late QRS voltage increase accompanied ST elevation in the left anterior and superior torso. In the RCA group, QRS changes were observed in the lower anterior and posterior chest with corresponding ST elevation. Similarly, inflation of the LCx produced QRST changes mostly in the back. In all but 3 subjects, QRST changes were present after the PTCA procedure was completed and in 6 patients with map recordings 24 hours post-PTCA, QRST changes could still be observed.<<ETX>>

Methods for Beat-to-Beat and Statistical Classification

Our interest in ECG monitoring is centered around the crucial periods of life: at it’s beginning and when it is threatened in its most vital function. Monitoring provides the physician with a means on which to base his decisions. Both periods of monitoring have their own problems and difficulties, and consequently different solutions.