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By Ken Grauer, MD
Clinical Scenario: The ECG shown in the Figure was obtained from a 60-year-old woman with chest pain. As was the case for the last month’s ECG Review (Intern Med Alert 1999;21:168), the QRS complex is predominantly negative in lead I and positive in lead aVR. Is this another case of lead misplacement? Is there any additional information that can be surmised from this tracing?
Interpretation: The ECG in the Figure is unusual in several ways. The rhythm is regular at a rate of about 80 beats/minute, but the nature of atrial activity is uncertain. That is, although an upright P wave is clearly seen and obviously conducting (constant PR interval) in lead III—no definite P wave is seen in lead II.
The most common form of lead misplacement results from interchange of the left and right arm electrodes. This technical mishap produces a picture of global negativity (of P wave, QRS complex, and T wave) in lead I—and an upright complex in lead aVR. However, limb lead misplacement should not affect QRS morphology in the precordial leads, since chest lead placement is not changed in any way. The Figure in this case shows a precordial lead pattern consistent with reverse R wave progression (small r wave in lead V1 that gets even smaller as one approaches lead V6). This strongly suggests that this patient has true dextrocardia, a finding confirmed on physical examination and repeat ECG. The unusual finding about this dextrocardia tracing is that instead of global negativity in lead I, the T wave is peaked and upright—a subtle finding suggesting ischemia as the cause of her chest pain.