Anomalous left main coronary artery (ALMCA) originating from the pulmonary artery is an important cause of morbidity from heart failure and mortality in infants. Discriminating ALMCA from myocarditis or other forms of dilated cardiomyopathy (DC) in infants is critical for proper early management of this treatable disease. This study was performed to characterize electrocardiographic (ECG) patterns in infants with ALMCA, and to identify features that would allow differentiation of these infants from those with myocarditis/DC. Presenting electrocardiograms from 28 patients with ALMCA < 2 years of age were analyzed for 103 variables, and compared with electrocardiograms from 28 aged-matched infants with myocarditis/DC using the t test, Fisher's exact test, and discriminant analysis using stepwise logistic regression techniques. ECG findings characteristic of infants with ALMCA were confirmed, including deep (> or = 3 mm) and wide (> or = 30 ms) Q waves and a QR pattern in at least 1 of the following leads: I, aVL, V5 to V7. Also, the complete absence of Q waves from leads II, III, and aVF in all infants with ALMCA was noted. These and other ECG patterns were more common in infants with ALMCA than in those with myocarditis/DC (p < 0.05), but were also noted in some patients with myocarditis/DC. Stepwise logistic regression analysis identified 3 ECG variables that best discriminated ALMCA from myocarditis/DC, including Q wave width (w) in lead I, and Q-wave depth (d) and ST-segment amplitude (s) in lead aVL.(ABSTRACT TRUNCATED AT 250 WORDS)