Grad Student With Palpitations

ANSWER
The correct interpretation of this ECG includes sinus rhythm with marked sinus arrhythmia and a blocked premature atrial contraction (PAC).

Sinus rhythm is defined as a heart rate between 60 and 100 beats/min, with a P wave for every QRS complex, a QRS complex for every P wave, and a consistent PR interval between 120 and 200 ms.

A sinus arrhythmia is defined as a variation of the P-P interval 
≥ 120 ms in the presence of normal P waves and a normal PR interval. The most common cause of a sinus arrhythmia is respiratory variation. 

A blocked PAC is seen following the fifth QRS complex. Careful inspection of the terminal portion of the T wave reveals a P wave without a corresponding QRS complex. There is no QRS complex because the PAC occurs while the AV node is refractory. The subsequent pause occurs because the PAC blocks the sinus node and resets the sinus rate.

 The palpitations were correlated to PACs observed on a continuous rhythm strip, and the patient was reassured.