Mobitz type I vs type II heart block: reading the PR interval

The deciding finding: what the PR interval does before the dropped beat

Measure the PR interval of each conducted beat in a row. If it grows a little longer each time until a P wave stands alone with no QRS after it, and the next PR is the shortest of the cycle, the rhythm is Mobitz type I. The lengthening is the signature.

If the PR intervals of the conducted beats are all the same and a P wave is suddenly not followed by a QRS, the rhythm is Mobitz type II. Type II is often seen with a wide QRS, reflecting disease below the AV node in the His bundle or bundle branches.

A practical approach is to march out the P waves with calipers or paper first, confirm they are regular, then compare each PR interval to the one before it. Working in that order stops the eye from being distracted by the irregular QRS rhythm.

What both blocks share, and what cannot tell them apart

Both rhythms show more P waves than QRS complexes, so the ventricular rhythm is irregular and the pulse may feel like it skips. Either can be symptom free or cause dizziness, fatigue, fainting or chest discomfort if the ventricular rate falls far enough.

A pulse check, a heart rate or the patient's symptoms cannot separate the two; only the rhythm strip can. A pattern where every second P wave is blocked is a well known trap, because with only one conducted beat between drops there is no run of PR intervals to compare. Treat that pattern cautiously and report it.

Why type II carries more risk

Type I usually reflects slowed conduction within the AV node, tends to progress slowly, and if complete block develops a junctional escape rhythm usually takes over. Treatment is often unnecessary unless the patient has symptomatic bradycardia.

Type II is considered pathological. It can progress abruptly to complete heart block, and the escape rhythm that follows tends to be slow and unreliable, with a risk of asystole. For that reason a pacemaker is indicated when no transient or reversible cause is found.

For the exam, the reasoning to show is that location predicts danger. A block inside the AV node usually leaves a reasonable backup pacemaker below it, while a block lower down leaves only slow and unstable backup tissue, which is why the same dropped beat means more in type II.

Nursing priorities for each rhythm

For Mobitz type I, assess the patient rather than the monitor alone: blood pressure, mental status, dizziness and perfusion. Report the new rhythm, review medications that slow AV conduction with the prescriber, and keep monitoring. A stable patient without symptoms can usually be observed.

For Mobitz type II, notify the provider promptly even if the patient feels well, keep continuous cardiac monitoring, and have transcutaneous pacing equipment ready under local protocol. Atropine is standard teaching for symptomatic bradycardia, but it is often less helpful when the block lies below the AV node, so pacing is the expected bridge to a permanent device.

Hypothetical exam-style scenario

This is an invented practice item, not a real NCLEX question. A client on telemetry has a constant PR interval with intermittent P waves that are not followed by a QRS. The client reports light-headedness and the blood pressure has dropped from baseline. What should the nurse do first?

The best answer is to notify the provider urgently and prepare transcutaneous pacing. Documenting and rechecking in an hour treats a high-risk rhythm as benign. Encouraging the client to cough or bear down suits some tachycardias, not a conduction block. Relying on atropine alone overlooks that it may not work when the block is infranodal.

Sources and further reading

MSD Manual Professional: Atrioventricular Block. PR patterns in Mobitz I and II, site of block, QRS width, progression risk, escape rhythms, pacemaker indications.

MedlinePlus: Arrhythmias. Symptoms of slow or irregular rhythms, ECG as the first test, pacemakers among treatment options.