Case in Electrocardiography
Clearly, the patient was not displaying symptoms of acute coronary syndrome, nor had she had symptoms in the past. The tracing in Figure 1 shows poor R-wave progression across the precordium, with an abrupt transition from R wave amplitude < S wave amplitude (leads V1-V3) to R wave amplitude > S wave amplitude (leads V4-V6). This triggered the computer and the health care personnel in preadmission testing to interpret this loss of anterior forces in the right precordial leads as a sign of possible anterior myocardial infarction.
In the ED, an ECG was performed with the same result, then repeated with close attention to precordial electrode placement; the latter tracing is found in Figure 2. Note the patient has “grown an R wave” in lead V3, and not coincidentally, the computer no longer interpreted the ECG as being suggestive of age-indeterminate anterior myocardial infarction. The tracing also has evidence of a right-sided conduction delay (or incomplete right bundle branch block) in leads V1 and V2, manifested by an rSR' QRS complex in those leads.
Precordial electrode placement is a tricky business. No two people have the same torso. Electrode placement should not result from simply eyeballing the chest and guessing where the electrodes belong. Navigating the breast tissue in women adds to the difficulty of this process. Remember that the precordial electrodes should be placed as guided by anatomic findings. (Clinical Procedures in Emergency Medicine (4th ed). Philadelphia, Elsevier, 2003;270.) (See table.)
We have found that inattention to proper electrode placement frequently leads to pseudopathology on the ECG. Taking this a step further, when comparing old and new ECGs, analyze the ST/T wave changes in the company they keep, particularly across the precordium. This means that ST/T wave changes should always be viewed with attention to the relative amplitude of the R and S waves of that complex. Variability in those amplitudes suggests that the placement of that particular electrode may have been different last time (e.g., compare the size and vector of the R and S waves in the new lead V lead compared with the old V lead). A change in the relative amplitude of the R and S waves in a given lead suggest that yesterday's precordial electrode placement may be at variance with today's.
One can logically expect the ST/T waves to vary to some degree as well. Moreover, coronary ischemia should not appear in one lead; a new isolated T wave inversion in one precordial lead without changes in the adjacent leads is likely not due to ischemia. Remember to consider precordial electrode positioning when evaluating the ECG.
One further nuance deserves discussion. The second ECG (Figure 2) shows a new rSR' in leads V1 and V2. This can occur when those two precordial leads are placed one interspace higher than the previous ECG. (ECG in Emergency Medicine and Acute Care. Philadelphia, Elsevier Mosby, 2005;16.) Perhaps the patient's earlier tracing (Figure 1) also suffered from slightly lower than optimal placement of leads V1 and V2.