There’s only one letter between them, which I find really confusing:
AVNRT=AV nodal re-entry tachycardia
AVRT=AV re-entry tachycardia
What’s also confusing is that symptoms of AVNRT and AVRT are pretty similar (and sound like a lot of tachyarrhythmias): lightheadedness, palpitations or a “flutter” in the chest, cough, syncope. You may hear both of them referred to as short or long “RP tachycardia” because of EKG appearance.
So what are the differences?
Normally, there are two pathways to the AV node: a slow pathway and a fast pathway. In normal beats, the fast pathway gets activated, and the slow pathway signal gets extinguished before it causes trouble.
But sometimes, a premature or aberrant beat—like with exercise, caffeine, anxiety, etc—can cause the slow pathway to get activated first. Because the slow pathway has a shorter refractory period, the circuit becomes an endless loop. This, friends, is AVNRT.
AVNRT is much more common. On the EKG, you may see a retrograde p-wave, the p-wave may be buried in the QRS complex, and there may be a “pseudo-R wave or pseudo s-wave.”
On the other hand, AVRT is caused by accessory pathways that are outside of the node, that are only present in some individuals. Accessory bypass tracts can conduct anterograde conduct signals antegrade, from the atrium to the ventricle, or retrograde, from the ventricle to the atrium.
AVRT can be either orthodromic (antegrade) or antidromic (retrograde). Both of these can present with rates in the 200-300s, but orthodromic AVRT has narrow QRS complexes while antidromic AVRT, which is much rarer, has wide QRS complexes that can be mistaken for VT. The p-wave is generally greater than 50% of the RR interval. (To distinguish antidromic AVRT from true VT, use Brugada criteria.) These patients should always have cardiology or electrophysiology follow up.
Luckily, typical cases of AVNRT and AVRT or any other SVT should be treated the same:
- start with vagal maneuvers (Valsalva or carotid massage)
- if that doesn’t work and the patient is stable, and not having active ischemia, CHF exacerbation, or bronchospasm, can give adenosine
- f you were 100% sure that a patient had antidromic AVRT as a cause of their wide complex tachy, adenosine would be perfectly reasonable…but you also don’t want to be responsible for stopping someone’s heart so would always confirm this with an expert!