40-year-old woman with palpitations.
Answer & Explanation
This patient has a slightly irregular rhythm which corresponds to her sensation of “a skipped beat”. The rhythm is essentially sinus at 75/minute with occasional pauses in the regular rhythm. The pauses are caused by an early atrial depolarization (a PAC) which does not conduct because it occurs so early that the conducting system is still refractory to further beats (a blocked PAC). This is evident in the rhythm strip as a deformed T wave of the complex preceding the pause (P wave within the usual T wave).
“The most common cause of a pause during sinus rhythm is a blocked PAC”.