We all know that the PR interval is prolonged in the presence of a 1st degree AV block, that the PR interval is short when ventricular pre-excitation is present and that it is generally normal otherwise.
If 1st degree AV block is present, all the normally conducted PR intervals will be prolonged. When an accessory pathway is present - creating the opportunity for ventricular pre-excitation - the PR interval is shortened whenever a delta wave is present. On rare occasions, slow AV conduction combined with an accessory pathway in the far left lateral border of the heart can result in a "normal" PR interval with a delta wave (but again, this is not common!).
But there are times when the PR interval changes and there is no 1st degree AV block and no evidence (or history) of ventricular pre-excitation. What is happening?
We generally want to know if the cause of the change in the duration of the PR interval is within the AV node itself (usually relatively benign) or below the AV node (usually pathological and often dangerous).
If a new 1st degree AV block suddenly appears on the ECG of a patient who already has either a LBBB, a RBBB with LAFB, or a RBBB with LPFB - beware! This 1st degree AV block is likely occurring below the AV node. What this means is that this patient is being kept alive by only one of three fascicles and the one that is still conducting is now having problems. If the patient had a documented 1st degree AV block for years and then developed a bifascicular block, that probably isn't quite as serious since the 1st AV block was probably in the AV node to begin with. Block within the AV node generally does not carry the same bad prognosis as a block below the AV node.
But a number of conditions and situations can mess with the PR interval duration: vagal tonicity and a premature junctional ectopic impulse can affect the PR interval. Electrolyte imbalances can also affect the PR interval. How can we be sure that the change or variation in the PR interval is due to a process in the AV node?
Conduction in the AV node follows the rule of RP/PR reciprocity. If the RP interval changes, the PR interval that follows will change in the opposite direction by the same amount. In other words, if the RP interval decreases by 40 msec, the following PR interval will increase by 40 msec. The only place this is going to happen is in the AV node!
So how do we recognize this? Are a bunch of measurements and calculations necessary? Well, you could do a bunch of measurements and calculations and arrive at the answer, but (as you should have guessed by now) the idea behind this post is that there is a much simpler and easier way...
If the PR intervals are changing, look closely at the P waves. If the P-P intervals are remaining basically the same (and remember: in sinus rhythm they can vary by a few msec), that is RP/PR reciprocity in action and proof that the variation in the PR interval is due to something going on within the AV node and not below it! This is the basis of Wenckebach conduction (Mobitz I block). And just because the RP/PR ratio is operative does not mean that there must be a dropped or non-conducted QRS. Many Mobitz I blocks resolve before an impulse fails to conduct. In such cases, it is probably better to call the rhythm "Wenckebach conduction." If the PR intervals are changing but there is no RP/PR reciprocity, one must consider AV dissociation or some other process.