People still make Borat references, right?
Anyway, I’ve got a quick one for you today. This patient was being evaluated for possible pericarditis. What’s the rhythm?
Let’s first discuss what this rhythm is not:
- Sinus arrest or sinus pause (as it was read by one physician)
- Type II AV Block (as diagnosed by another physician and the computer)
So why were these providers mistaken?
- The first was too hasty in his reading. He saw only irregular pauses in the sinus rhythm and wrongfully assumed they were due to inactivity of the SA-node. Looking closely, however, you can appreciate that there is a “blip” in the T-wave preceding each break in the rhythm. This blip is a buried P-wave. Although it’s not followed by a QRS-complex, it resets the SA-node before the next scheduled sinus beat and results in a compensatory pause while the sinus node repolarizes and prepares to resume normal pacemaking.
- The second physician managed to avoid that pitfall and picked-up the hidden waves, but then made another common error. If you march out the P-waves in these strips, you’ll notice that the buried ones arrive early in the cardiac cycle and are thus premature atrial complexes (PAC’s). In fact, they arrive so early that the AV-node is still in its absolute refractory period and cannot conduct to depolarize the ventricles. This results in a P-wave that is not technically “blocked,” but instead is what we term “non-conducted.”
In this case semantics matter. It is the job of the AV-node to keep the ventricles from being overwhelmed if the atria fire too rapidly (i.e. atrial fibrillation), so refusing to conduct these excessively early beats demonstrates that the AV-node is in-fact behaving normally. Saying these P-waves were “blocked” would give the impression that there was pathology involved and carry a much different prognosis.
It’s easy, right? This rhythm is very simply: Normal Sinus Rhythm with Multiple Non-conducted PAC’s.
During your career the number of ECG’s you’ll see with non-conducted PAC’s will far exceed the cases of Type II AV Block. Whenever you encounter a pause in a rhythm strip, remember these two things:
- The most common cause of a pause is a non-conducted PAC,
- The most common cause of blocked P-waves is not.