A 68-year-old woman is referred to the ECG laboratory by her family physician. No history is provided:

• Sinus rhythm with bigeminal sequence
• Likely sinus rhythm with premature atrial complexes

Comment: Given the p-wave morphology of the early atrial beat is slightly different than the p-wave morphology of the later beat, we can conclude that the slower atrial rate is likely the sinus rate, and the early beat is a premature atrial complex occurring in a bigeminal pattern.