AV Blocks: First, Second, and Third Degree
Key criteria for distinguishing first-degree, Mobitz I, Mobitz II, and complete heart block on an EKG strip.
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Questions Covered in This Set
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What two data points separate all four AV block types?
PR interval behavior (constant, lengthening, or random) and the P:QRS ratio (does every P conduct?).
First-degree AV block criteria
PR interval > 0.20 s (more than 5 small boxes), constant beat to beat, and every P wave is followed by a QRS. It's a delay, not a true block.
Hallmark of Mobitz I (Wenckebach)
Progressive PR lengthening until a P wave appears with no QRS, then the cycle restarts with a short PR (e.g., 0.20 → 0.28 → 0.36 → dropped).
Two secondary clues of Mobitz I
R–R intervals progressively shorten before the dropped beat (grouped beating), and the pause is less than two full R–R intervals.
Hallmark of Mobitz II
PR interval is identical on all conducted beats, then a P wave suddenly arrives with no QRS; the pause equals exactly two P–P intervals.
Where is the lesion in Mobitz I vs Mobitz II?
Mobitz I: high, in the AV node itself — QRS usually narrow, often transient/stable. Mobitz II: below the AV node (His or bundle branches) — QRS often wide, dangerous.
Why is Mobitz II dangerous, and how is it treated?
It doesn't respond well to atropine and can abruptly degenerate into complete heart block or asystole; it usually requires a pacemaker.
Findings in third-degree (complete) heart block
Regular P waves at a normal sinus rate, regular but slow escape QRS (30–50/min), and randomly varying PR intervals — AV dissociation.
What is 2:1 AV block and how do you approach it?
Every other P is dropped, so you never see two consecutive conducted beats to judge PR lengthening. Wide QRS + normal PR favors Mobitz II; narrow QRS + long PR favors Mobitz I. When in doubt, treat as the dangerous one and get a longer strip.
Why march out the P waves first?
The sinus node keeps firing on schedule, so the P–P interval stays regular in every AV block; marking P waves reveals dropped beats and dissociation.
