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Many interpreters of electrocardiograms (ECGs) will agree with the statements made in the article by Yamaji et al. (1). The authors used pattern recognition to explain their findings. I wish to use the same ECGs that they published, but offer the use of basic principles, including vector concepts, to explain the abnormalities shown in the ECGs (2).
Einthoven’s law and equilateral triangle, though not absolutely perfect, have stood the test of time when used for clinical purposes. Accordingly, an ECG deflection in lead I, plus the deflection in lead III, equals the deflection in lead II. It is also accepted that the deflection in lead aVR, plus the deflection in lead aVL, plus the deflection in aVF equals zero. All six of the extremity lead electrodes are electrically equal distance from the heart; therefore, each lead is equally capable of recording the electrical forces made by the entire heart. The only reason the deflections are different in different extremity leads is that the electrodes of the various leads have a different view of the electrical forces that produce the deflections. Accordingly, no secret information belongs to a specific extremity lead. A metaphor might be as follows: photographs of a person made in the frontal plane from six different, but from distant vantage points would all be different, but there is only one person.
As Robert Grant pointed out many years ago—it is the determination of the direction and size of spatially oriented electrical forces and their relationship to the cardiac anatomy and other electrical forces that gives insight into the meaning of the deflections (3). Grant, working at Emory University, developed the system now used to determine the spatial direction of the electrical forces that conspire to create the deflections in the ECG (3). He taught that it was possible to simply inspect the 12-lead surface ECG and achieve this goal with a degree of accuracy that was suitable for clinical purposes. He would initially establish the frontal plane direction of electrical forces by inspecting the six extremity leads and, following that, he would determine the anterior-posterior direction of the electrical forces by inspecting the six precordial leads (3). I have tried to contribute to the use of the Grant system by correlating the ECG information found by using his method with coronary arteriography, echocardiography and cardiac catheterization (2).
The illustrations shown in Figure 1were created by applying Grant’s method to the three tracings shown in Figure 1of the article by Yamaji et al. (1). The legends of each of the three parts of the Figure describe the thought process that accompanies the interpretation.
When generalized endocardial injury of this degree persists, it almost always represents endocardial infarction (Fig. 1A). Note that the deflections of the S-T segments in all 12 leads were used to determine the direction of the S-T segment which, in turn, indicates the approximate location of the damage. This abnormality has to be located in the endocardium because the S-T vector is not directed toward epicardial injury (there is no epicardium for the S-T segment vector to point toward).
Such a tracing is usually caused by severe triple-vessel obstructive coronary atherosclerosis or may occasionally be caused by atherosclerotic obstruction of the left main coronary artery. There is also an uncommon clinical condition that can lead to generalized endocardial infarction. It is the combination of left ventricular hypertrophy from hypertension or aortic valve stenosis, heart failure with an elevated left ventricular diastolic pressure, 40% to 50% diameter occlusion of several coronary arteries and hypotension due to acute blood loss or some other cause. Endocardial infarction of the left ventricle can occur in such patients without a change in the atherosclerotic process.
The mean QRS vector and mean spatial S-T vector are superimposed on the display system which includes the six extremity lead axes, the left and right ventricles and the coronary arteries. Note in Figure 1B how the S-T segment vector, which represents predominant epicardial injury, intersects the left anterior descending coronary artery. This produces an anterior myocardial infarction. Note in Figure 1C that the S-T segment vector is directed inferiorly and very slightly anterior indicating an inferior infarction. The S-T segment vector is not directed sufficiently anteriorly to diagnose additional right ventricular infarction, although the lower rim of the right ventricle may be slightly involved. The probability of right ventricular infarction increases as the mean S-T vector becomes directed more and more to the right and anteriorly, but one must remember that other conditions can be responsible for an S-T vector that is directed toward the northwest part of the hexaxial reference system.
The direction of the mean S-T vector is a powerful indicator of the location of the injury associated with myocardial infarction because it is usually an entirely new electrical force. However, abnormal Q waves or T waves produced by infarction are the vector sum of old forces plus new forces. In addition, the mean S-T vector can, at times, identify the approximate location of the obstruction in a designated coronary artery. It is important to know the exceptions to this general rule because there are areas of the heart served by several coronary arteries and in such patients one must offer a differential diagnosis as to the site of the obstruction.
- American College of Cardiology Foundation
- Yamaji H.,
- et al.
- Hurst J.W.
- Grant R.P.