Right atrial infarction is more common than left atrial infarction, possibly because of the higher oxygen concentration in the left atrial blood. Infarction is more common in the atrial appendages than in the lateral or posterior walls of the atria. Right atrial infarctions have been reported in 81% to 98% of myocardial infarctions and left atrial infarctions in 2% to 19% of infarctions, while infarction of both atria have been reported in 19% to 24% of infarctions. The data will vary with whether it is autopsy evidence of atrial infarction or clinical evidence and also on how meticulously atrial infarction was looked for.
Ta wave or sometimes called as the PTa segment is the atrial repolarization seen on ECG. Changes in Ta wave analogous to ST segment elevation in ventricular infarction can be looked for. Atrial infarction may be accompanied by arrhythmias like paroxysmal atrial fibrillation, wandering atrial pacemaker, premature atrial complexes and runs of atrial tachycardia. Liu CK and colleagues  had suggested a few major and minor criteria for diagnosis of atrial infarction from the ECG. Major criteria where PTa segment elevation of more than 0.5 mm in leads V3 and V6, with reciprocal depression of PTa segments in leads V1 and V2; PTa segment elevation of more than 0.5 mm in lead I, with reciprocal depressions in leads II and III; and PTa segment depression of more than 1.5 mm in precordial leads, and 1.2 mm in leads I, II and III, associated with any atrial arrhythmia. Minor criteria were abnormal P waves, flattening of P wave and irregular or notched P wave. Sivertssen et al  proposed additional criteria like PR segment prolongation (normal PR segment duration is less than 200 ms); P wave axis changes (normal P wave axis on frontal plane is 30° to 60°); and abnormal atrial rhythms, including atrial flutter, atrial fibrillation, wandering atrial pacemaker and atrioventricular nodal rhythm. PR segment elevation in aVR and V1 may also be an evidence of atrial infarction. Atrial infarction can contribute to low output state due to loss of atrial contribution (atrial kick) to ventricular filling. Lewis lead ECG may be able to visualize atrial repolarization abnormalities better than standard leads.
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Sivertssen E, Hoel B, Bay G, Jorgensen L. Electrocardiographic atrial complex and acute atrial myocardial infarction. Am J Cardiol. 1973;31:450–6.