The orthogonal electrocardiogram as risk indicator for the prediction of myocardial infarction and/or cardiac death

Analysis of electrocardiograms for subcutaneous monitors

We are developing a subcutaneous cardiac arrest monitor and alarm with electrodes that have spacing of a few centimeters. We hypothesize that closely spaced bipolar electrodes that provide QRS amplitudes of a millivolt or more in sinus rhythm (SR) will not provide similar amplitudes during ventricular tachyarrhythmia (VT/VF), and that an orthogonal set of electrode pairs in diagonal is necessary to produce signals of sufficient amplitude in order to distinguish these rhythms.

METHODS

Forty patients were studied during ICD implantation in the clinical electrophysiology laboratory. A square array of 9 electrodes with 4- to 10-cm spacing between adjacent electrodes was placed on the patient's anterior chest over the left heart. The center electrode in the left most column is located in the 5th intercostal space at the left sternal border. Eight-channel recordings were made in SR and during induced VT/VF from the 8 peripheral electrodes referenced to the central member of the array. From these recordings all 20 bipolar ECGs of adjacent (including diagonally adjacent) electrodes were constructed algebraically. QRS peak-peak amplitudes in SR and VT/VF were measured in each bipolar lead and in the spatial vector formed by summing the squares of each adjacent and orthogonal pair of these leads.

RESULTS

Electrode pairs that yielded optimal ECG amplitudes in SR were not always the pairs that yielded optimal amplitudes in VT/VF. But in every patient two orthogonal pairs could be found whose QRS vector amplitude is sufficient in both SR and VT/VF to separate the rhythms.

CONCLUSIONS

A patient-defined set of bipolar electrode pairs is suitable for automatic separation of SR and VT/VF by rate in subcutaneous ambulatory monitoring.

Risk assessment of future myocardial infarction from automated serial ECG analysis

From 1980 to 1990 a cohort of 7,542 employees took part in the MARISK study. This study was designed in order to explore the utility of computerized electrocardiographic (ECG) measurements for predicting future myocardial infarction. After an initial period of annual examinations, follow-up information was collected by mailed questionnaires and obtained from 89.3% of all study participants with a median follow-up time of more than 7 years. Eighty-three participants suffered from myocardial infarction during the course of the study. Multivariate logistic regression analysis revealed that automated ECG measurements contribute significant additional information to risk estimates obtained from classical risk factors. Serial ECG analysis yields stronger risk indicators than single measurements. Results are represented via receiver operating characteristic (ROC) curves and regression coefficients, which can be used in independent applications. The results of this study indicate that it is useful to include automated serial ECG analysis in cardiovascular screening programs. It still has to be evaluated whether successful preventive measures may be based on such screening programs.

Electrocardiographic effects of experimental myocardial infarction in pigs

Myocardial infarctions were produced in young domestic farm pigs by ligating coronary arteries. Arteries ligated in four groups were (A) left anterior descending, (B) branches of left anterior descending, (C) left circumflex, and (D) right coronary artery. After 7 or 8 days, the hearts were removed and dissected and the infarcted areas measured. Vectorcardiograms were taken before ligation and just before termination. High recording speeds and sensitivities were used so that details of the QRS complexes could be seen. Difference vectors were computed as a function of time during QRS. In groups A and B, statistically significant differences in QRS components before and after ligation were found. Also, there were statistically significant correlations between vectorcardiographic deflections and the ratio of infarct weight to heart weight. The changes were related to known excitation patterns of the pig heart.