AV blocks
Heart blocks on an EKG strip: first, second and third degree
A heart block, or atrioventricular (AV) block, is a delay or failure in passing the electrical impulse from the atria to the ventricles. There are four to know. All four have normal P waves; what separates them is what happens between each P wave and the QRS that should follow it. That is why the heart blocks are told apart by one measurement: the PR interval.
Criteria from the American Heart Association guidelines and published ECG standards · Every strip checked by measurement · Sources and method · Updated November 15, 2026
The four heart blocks, measured
Each strip has calipers on its PR intervals, in seconds. Read the numbers from left to right and the difference between the blocks is in front of you.
1First-degree AV block
PR 0.28 s on every beat, at 76 per minute
Every P wave gets through, late. One long PR interval, repeated on every beat. Nothing is dropped.
2Second-degree AV block, type I (Wenckebach)
PR 0.18 → 0.26 → 0.31 → 0.34 s, then a dropped QRS
Each P wave gets through later than the one before, until one does not get through at all. Then it starts over. The red flag marks the P wave with no QRS.
3Second-degree AV block, type II (Mobitz II)
PR 0.17 s on every conducted beat, QRS 0.14 s (wide)
P waves get through on time, with the same PR interval, until one suddenly does not. The red flag marks the P wave with no QRS.
4Third-degree (complete) AV block
Atria 72 per minute, ventricles 43, QRS 0.09 s (narrow)
No P wave gets through. The atria and the ventricles each keep their own regular beat. Blue ticks mark every P wave.
Three questions that name any heart block
- 1
Is there a QRS after every P wave?
Yes, and the PR is longer than 0.20 s: first-degree block. No: some beats are being dropped, so go to question 2.
- 2
Is the R-R interval regular?
Yes, with a slow ventricular rate and P waves that have no fixed relationship to the QRS: third-degree block. No: it is a second-degree block, so go to question 3.
- 3
Does the PR interval change before the dropped beat?
It gets longer, and is shorter again after the drop: type I (Wenckebach). It is the same before and after the drop: type II (Mobitz II).
The comparison that settles most arguments: put your calipers on the PR interval just before a dropped beat, then move them to the PR interval just after it. Different means Wenckebach. Identical means Mobitz II.
A way to remember the four heart blocks
One short line for each block. Say it while you move your calipers along the PR intervals, and the strip either fits the line or it does not.
- First degreeLate every time.
- Every P wave is followed by a QRS, and the wait is the same long one on every beat: a PR interval over 0.20 s that never changes.
- Second degree, type ILater, later, later, gone.
- Each PR interval is longer than the one before it, until a P wave stands alone with no QRS. Then the count starts again.
- Second degree, type IIOn time, on time, gone.
- The PR interval does not change from beat to beat, and then a P wave is not followed by a QRS, with no warning.
- Third degreeTwo rhythms, no link.
- The P waves keep one steady pace and the QRS complexes keep another, slower one. No PR interval is repeated, because nothing connects them.
Many courses teach a rhyme for the heart blocks. Whatever words yours uses, each line comes down to the same check: what the PR interval does from one beat to the next.
Heart block comparison chart
| First degree | Second degree, type I (Wenckebach) | Second degree, type II (Mobitz II) | Third degree (complete) | |
|---|---|---|---|---|
| Do all P waves conduct? | Yes | No, some are dropped | No, some are dropped | None conduct |
| PR interval | Long (> 0.20 s), constant | Lengthens before each drop | Constant before and after each drop | Different on every beat |
| R-R intervals | Regular | Irregular, in groups | Regular except at the drop | Regular |
| P waves vs. QRS complexes | Equal | More P waves | More P waves | More P waves, unrelated |
| QRS | Narrow | Narrow | Often wide | Narrow or wide |
| Where the block is | AV node (a delay) | AV node | Below the AV node | AV node or below |
| Usual course | Benign; monitor | Often temporary; treat if symptomatic | Can progress suddenly; pacing | Urgent; pacing |
The last row is a study summary, not a protocol. Follow current American Heart Association guidelines and your facility’s policies.
Practice: which heart block is it?
Six strips, all AV blocks. There are no calipers on these: count the small squares yourself. One small square is 0.04 s.
Strip 1 of 6
Lead II · 25 mm/s · 6 s
Why the heart blocks get mixed up
- Type I and type II both drop beats. The dropped beat catches the eye, but it does not tell them apart. Only the PR intervals around it do.
- A 6-second strip can hide the evidence. If it shows only the last two beats before a drop, a Wenckebach cycle can look constant. Find the first PR after a pause and compare it with the last PR before one.
- Third-degree block has P waves in front of some QRS complexes by chance. One of them can pass for a long PR. Measure three; they will not match.
- P waves hide. A P wave can sit inside a T wave or a QRS. Set calipers to the P-P interval and walk them across the strip to find the ones you cannot see.
Questions about heart blocks
How do you tell the heart blocks apart?
Use the PR interval. If every P wave has a QRS and the PR is long but constant, it is first degree. If beats are dropped and the PR lengthens before each drop, it is second-degree type I (Wenckebach). If beats are dropped and the PR does not change, it is second-degree type II (Mobitz II). If the PR is different on every beat and the R-R is regular, it is third degree.
Which heart block is the most serious?
Third-degree (complete) heart block, because no atrial impulse reaches the ventricles and the patient depends on a slow backup rhythm. Second-degree type II is also treated as serious, because it can turn into complete block without warning.
What is an easy way to remember the heart blocks?
Give each one a short line about its PR interval. First degree: late every time. Second-degree type I (Wenckebach): later, later, later, gone. Second-degree type II (Mobitz II): on time, on time, gone. Third degree: two rhythms, no link. Each line is what you see when you walk your calipers from one PR interval to the next.
What is 2:1 AV block?
A second-degree block in which every other P wave is dropped. Because there are never two conducted beats in a row, you cannot see whether the PR interval would have lengthened, so it cannot be labeled type I or type II from the strip alone. A wide QRS makes type II more likely.
What is the difference between a heart block and a bundle branch block?
An AV heart block is a problem getting the impulse from the atria to the ventricles, and it shows in the PR interval and in dropped beats. A bundle branch block is a delay inside the ventricles after the impulse has arrived, and it shows as a wide QRS. This page covers AV blocks.