This 79-year-old woman with heart failure with a reduced ejection fraction is seen in the cardiac clinic for follow-up. She is stable and euvolemic, and denies any presyncope: 

• Sinus rhythm with possible sino-atrial block (Mobitz I)
• Left bundle branch block

The P waves are positive in I and aVF, in keeping with their origin from the sinus node. The PR interval is slightly prolonged to 230 msec and constant. There is a bigeminal rhythm. The longer PP interval is not a multiple of the short one. These findings suggest the possibility of 3:2 type I sino-atrial block. A possible alternative diagnosis of this arrhythmia is:” premature atrial complexes with a pattern of bigeminy, originating near the sinus note”. Because this possibility cannot be excluded, the diagnosis of 3:2 sino-atrial block is always open to question, unless other conduction ratios are also documented.

In type I sino-atrial block, calculation of the sinus rhythm is obtained by assuming that the sinus node discharges regularly and the sino-atrial conduction time increases until a sinus discharge is blocked. The interval between the P waves at the end of two successive long pauses corresponds to 3 sinus node discharges (two conducted, one blocked). Dividing it by 3, we obtain the interval between sinus discharges.