The rhythm is the most important and possibly difficult thing to assess. The first three beats appear to be sinus at a rate of 50 beats/minute. The rhythm then abruptly changes rate to 33 beats/minute. This patient was symptomatic when she had long periods of a rate of 33/minute. One might assume sinus node dysfunction, but look closer…

This rhythm strip demonstrates something deforming the terminal portion of the T wave that precedes the beats of the slower rate (arrows). The deflect that deforms these T waves is a very early atrial beat, a premature atrial contraction (PAC), that is so early that the conduction system is not yet ready to accept a premature beat – thus what we see here is bradycardia simulated by blocked PACs occurring in a bigeminal pattern.

Remember,
The most common cause of a pause
in sinus rhythm is a blocked PAC!