A 74 y/o female presents after a syncopal episode. She c/o light-headedness and appears diaphoretic. V/S: Pulse 40, BP 90/50.
Q1. What rhythm does the EKG demonstrate? How do you classify this bradycardia?
A1. 2nd degree AV block with 2:1 conduction
AV block occurs in 3 types. In 1st degree AV block, there is a delay in conduction through the AV node, but all sinus impulses conduct through (i.e. there are no dropped beats). In 2nd degree AV block, some sinus impulses conduct through the AV node but some are blocked. In 3rd degree AV block, no sinus impulses conduct through the AV node.
2nd degree AV block is further categorized into 2nd degree type I (AKA Mobitz I or Wenckebach block) and 2nd degree type II (AKA Mobitz II). In 2nd degree type I AV block, there is progressive lengthening of the PR interval until an impulse is not conducted (“dropped beat”). In 2nd degree type II AV block, the PR interval does not widen. The PR interval remains the same and then an impulse is not conducted (“dropped beat”).
To differentiate 2nd degree type I from type II, you must see two consecutive beats where the sinus impulse conducts through. This allows you to determine if the PR interval is progressing (type I) or not (type II). In our EKG, every second sinus beat is dropped. So, you do not see two consecutive sinus beats conducting through. This prevents you from differentiating 2nd degree type I from 2nd degree type II. This EKG pattern is referred to a 2:1 AV block. (Every second beat is blocked by the AV node). In general, in 2:1 AV block, the presence of a prolonged PR interval makes type I block more likely, whereas the presence of wide QRS complexes makes type II block more likely. However, this is not entirely reliable.
When faced with a 2:1 AV block, running a long rhythm strip may allow you to see two consecutive conducted beats. You can then measure consecutive PR intervals to see if they are prolonging (type I) or the same (type II). In EKG 5-11b (attached), the first 3 beats represent 2:1 AV block. However, beats 4 and 5 are consecutive beats which conduct through the AV node. Here we see a lengthening PR interval from beat 4 (0.12 seconds) to beat 5 (0.24 seconds), followed by a dropped beat. This allows us to confirm this is 2nd degree type I AV block. (EKG 5-11b also demonstrates biphasic T waves in V1-V3 and T wave inversions V4-V6, I, aVL.)
[Yealy, Kosowsky. Dysrhythmias. In Robert’s Emergency Medicine, 8th Ed. Ch. 79]