Answer: A-V dissociation due to sinus bradycardia and junctional escape rhythm, with occasional sinus capture beats.
Explanation: At first glance you might be tempted to call this 3rd degree A-V block, but do not be mislead into temptation! The ACLS course teaches that the escape rhythm in complete heart block must be regular. In the strip above, several beats are early (beats 4 and 8 in the top strip and 4 and 9 in the lower strip). How can we explain these? The inherent rhythm is a sinus bradycardia (rate of 40) with clearly defined P waves. In the lower strip, someone was nice enough to circle them for our benefit (although they forgot the one peeking out of the front of the 4th QRS). In addition, there is a junctional escape rhythm (rate of 50). A-V dissociation is, therefore, present.
How do we explain the early beats? Each of these is preceded by a P wave (buried in the preceding T wave) indicating that they must be capture beats. To have complete heart block, one must have not only A-V dissociation, but sufficient numbers of P waves having ample opportunity to conduct, yet no conduction. In the above example, whenever a P wave occurs with a sufficiently long interval after the preceding QRS, it does conduct. The slightly prolonged and slightly variable PR interval is probably explained by the principle of RP-PR reciprocity (more on that another time).
Sol Nevins, M.D.
Director of EMS Morristown Memorial Hospital