Determining what the fast rhythm is

Most atrial tachycardias are due to reentry. Only some are due to delayed afterdepolarizations, which is a triggered activity that occurs when the heart is racing.

So, the thing to look for is to see if there is a P wave before the QRS.

There are two types of paroxysmal lone AF. There is the vagal type that occurs in men at night or after a meal. And there is the catecholamine type that occurs in young women under stress or after coffee.


So, it's all down to the P wave for SVT.
If before QRS then this is atrial tachycardia (which can be automatic or reentrant) or atrial flutter - differentiate by looking to see if the QRS is distorted.


If just after QRS, then this is AVNRT..or...AUTOMATIC JUNCTIONAL TACHYCARDIA

If a little after QRS but still before the T wave then this is AVRT or AJT. The P wave will be inverted.


If a lot after the QRS, such that it's almost at the next QRS, then it's atypical (fast-slow)AVNRT with retrograde conduction up the slow pathway


So, once again:
P wave in QRS= AVNRT or AJT
P wave in ST or ascending limb of T wave= AVRT or AJT
P wave after T= "AT" or "flutter" or "atypical AVNRT" or "AVRT with  retrograde conduction via a slow bypass tract"...how to tell the difference? The pearls are:
- distorted baseline means flutter
- large P waves means AVRT or atypical AVNRT ("size does matter")


However, often you don't see any P waves, so then have to rely upon the initiating beat:
- if the initiating beat or terminating beat is VPB, then it's AVRT.
- if the initiating beat is a PAC and the PR interval is long, then it's an AVNRT because you've just seen the dual AV node pathway!

The other trick is to see if there develops a rate-related bundle. If it does AND the ventricular rate slows, then this is an AVRT.

Differentiating between AVNRT and AJT:
AVNRT doesn't go slower than 150 and both appears and disappears suddenly , AJT doesn't go faster than 130 and can be seen to speed up and down.