In this interesting case, we have a middle-aged lady complaining of intermittent palpations associated with shortness of breath.
EKG at the time she presented to the ER can be read as follow:
Regular, narrow complex tachycardia at a ventricular rate of 127bpm; that would exclude atrial fibrillation from our differential.
Now, do we see P waves? In the presenting EKG, we see an RSR’ complex in V1 that is not seen on repeat EKG (EKG1) where the patient was in sinus rhythm; suggestive of a P wave embedded in QRS complex.
Is the atrial rate more than the ventricular rate? No, that would exclude Atrial flutter (in cases of 1:1 atrial flutter, ventricular rate would be >250bpm).
A pathognomonic feature that can be seen on the patient’s presenting EKG is a short RP interval of ~40ms (short RP is usually <70-90ms) that is diagnostic for AVNRT. AVNRT physiology depends on the presence of 2 pathways around the AV node (a slow paced and a fastpaced pathway) which is called the dual AV physiology. Looking to our patient we can see the 2 pathways inaction at different points in time; EKG 1 shows a normal PR interval of 180ms indicating a conduction via the fast pathway; while EKG 2 shows a prolonged PR interval of 338ms indicating conduction via the slow pathway. Based on the way the impulse travels from the atrium to ventricle and back, AVNRT is classified into 2 types*: Typical AVNRT or the “slow-fast” type is the commonest (>80%) form of AVNRT. It is usually initiated by an atrial premature depolarization which, by virtue of prematurity, finds the fast pathway refractory (remember, the fast pathway has a longer refractory period) and conducts to the ventricles via the slow pathway.
Atypical AVNRT or the “fast-slow” type, where the impulse travels via the fast pathway towards the ventricles and returns via the slow pathway to the atria.
Our patient case is consistent with typical AVNRT as the p wave can be seen at the end of the QRS (RSR’ in V1), on the other hand patients with atypical ANVRT usually have the p wave following the QRS. *= Less than 5% are classified as slow/slow AVNRT where the impulse follows a complex route through the AV node and the surrounding area.