A 60-year-old woman with hypertension presented with chest pain. Her initial ECG showed sinus rhythm with ST segment changes. A repeat ECG showed an atrial rhythm with P waves inverted in some leads and upright in others. This finding suggested an ectopic left atrial rhythm. A subsequent ECG reverted back to sinus rhythm similar to the initial test. The document discusses using P wave morphology, particularly in leads V1 and aVL, to differentiate between right and left atrial rhythms. A positive P wave in V1 indicates a left atrial focus while an upright or biphasic P wave in aVL suggests a right atrial origin.