Identifying which type of atrioventricular (AV) block you’re looking at comes down to three things: the PR interval, whether any beats are dropped, and the relationship between P waves and QRS complexes. There are four distinct patterns, and each one tells a different story about how electrical signals travel from the upper chambers of the heart to the lower chambers.
First-Degree AV Block
This is the mildest form. Every P wave is followed by a QRS complex, so no beats are actually “blocked.” The only abnormality is that the PR interval is longer than 0.20 seconds (one large box on standard ECG paper). The normal PR interval falls between 0.12 and 0.20 seconds, so anything beyond that upper limit qualifies. When the PR interval stretches past 0.30 seconds, it’s considered a “marked” first-degree block.
The key features to spot on a rhythm strip:
- Every P wave conducts. There are no dropped beats.
- The PR interval is constant but longer than 0.20 seconds.
- The rhythm is regular. P waves and QRS complexes march along in a steady 1:1 pattern.
Because every impulse still reaches the ventricles, first-degree block is really more of a conduction delay than a true block. Most people have no symptoms, and it often shows up incidentally on a routine ECG.
Second-Degree AV Block, Type I (Wenckebach)
This is the pattern students often remember with the rhyme: “longer, longer, longer, drop, now you have a Wenckebach.” The PR interval gets progressively longer with each beat until one P wave fails to conduct entirely, producing a “dropped” QRS complex. After that pause, the cycle resets and starts over with a shorter PR interval that gradually lengthens again.
The key features:
- PR interval progressively lengthens over several beats.
- A single P wave is eventually not followed by a QRS complex (the dropped beat).
- The cycle then repeats. The first conducted beat after the dropped beat has the shortest PR interval in the group.
- The atrial rate stays consistent (P waves are regular), but the ventricular rhythm is irregular because of the periodic dropped beat.
Wenckebach typically reflects a problem at the AV node itself, which is relatively high in the conduction system. It can be caused by medications that slow conduction, increased vagal tone, or inferior heart attacks that affect the blood supply to the AV node. It is generally considered the more benign of the two second-degree blocks.
Second-Degree AV Block, Type II (Mobitz II)
Unlike Wenckebach, the PR interval here stays the same from beat to beat. Then, without warning, a P wave simply fails to conduct, and a QRS complex is missing. The hallmark is that the PR intervals before and after the dropped beat are identical. An unchanged PR interval after the block is the defining requirement of Mobitz type II.
The key features:
- PR intervals are constant on all conducted beats.
- Dropped beats occur unpredictably, without any preceding PR lengthening.
- At least two consecutive conducted P waves must be visible to confirm this diagnosis. Without them, you can’t tell whether the PR interval was truly constant or was lengthening subtly.
This distinction matters because Mobitz type II reflects damage lower in the conduction system, in the bundle of His or the bundle branches. That location makes it more dangerous: it can progress suddenly to complete heart block. A pacemaker is indicated regardless of whether the person has symptoms.
The 2:1 Block Problem
When every other P wave is blocked (two P waves for every one QRS complex), you only see one conducted PR interval at a time. There is no way to tell whether the PR interval was lengthening because there’s no second consecutive conducted beat to compare. This pattern is simply called a 2:1 AV block, and additional testing is needed to classify it further.
Third-Degree (Complete) AV Block
In complete heart block, the electrical connection between the atria and ventricles is entirely severed. The atria fire at their own rate, and the ventricles beat independently at a slower escape rhythm. No atrial impulse ever reaches the ventricles.
The key features:
- More P waves than QRS complexes are present, and no consistent relationship exists between them.
- The P-P intervals are regular (the atria keep a steady rhythm).
- The R-R intervals are also regular (the ventricles keep their own steady rhythm), but at a slower rate.
- P waves “march through” the QRS complexes. You’ll see P waves appearing at different spots relative to the QRS, sometimes buried in or overlapping with them, because the two rhythms are completely independent.
The atrial rate is typically in the normal range of 60 to 100 beats per minute, while the ventricular escape rate is slower, often 30 to 50 depending on where the backup pacemaker sits. The lower the escape pacemaker, the slower and wider the QRS complexes tend to be. Complete heart block always warrants a pacemaker.
How to Identify the Block on Your Strip
Start by answering three questions in order. First, is every P wave followed by a QRS? If yes and the PR interval is simply prolonged beyond 0.20 seconds, it’s first-degree. If some P waves lack a QRS, move to the second question: do the P waves and QRS complexes have any predictable relationship at all? If they are completely independent, with regular P-P intervals and regular but slower R-R intervals that bear no fixed connection to each other, it’s third-degree.
If there is a relationship (some P waves conduct and some don’t), you’re looking at second-degree block. Now ask the third question: does the PR interval change before the dropped beat? If the PR gets progressively longer before the drop, it’s Type I (Wenckebach). If the PR stays exactly the same on every conducted beat and the drop comes without warning, it’s Type II (Mobitz II). If the conduction ratio is 2:1 with no consecutive conducted beats to compare, you can’t definitively distinguish between the two subtypes.
Why the Type Matters
The clinical urgency varies dramatically across these categories. First-degree block rarely needs treatment. Wenckebach is often benign and may resolve on its own, especially if it’s caused by a medication or by high vagal tone during sleep. It only requires a pacemaker when symptoms like dizziness or fainting are present.
Mobitz type II is a different story. Because the block sits low in the conduction system, it can abruptly progress to complete heart block without warning. A pacemaker is recommended even in the absence of symptoms, particularly when the QRS complex is wide (suggesting extensive disease in the bundle branches). Complete heart block also calls for a pacemaker whether or not the person has symptoms, because the heart is entirely dependent on an escape rhythm that may be unreliable.