ECG Case 043

Main Abnormalities

Sinus rhythm with evidence of 2nd degree AV block

• The QRS complexes cluster in groups separated by non-conducted P waves.
• There is a 3:2 relationship between the P waves and QRS complexes.
• With fixed ratio blocks like this, it can sometimes be difficult to distinguish between Mobitz I and Mobitz II conduction, as there are not always enough successive PR intervals to judge whether progressive PR prolongation is occurring.
• The PR interval seems relatively constant between the first and second beats of each group, suggesting Mobitz II conduction.
• However, the clustering of QRS complexes into repeating groups with P:QRS ratios of 3:2, 4:3, 5:4, etc. is a characteristic feature of Mobitz I.
• There is an atypical RBBB — typical RSR’ pattern in V1 with slurred S wave in lead I, but atypical morphology elsewhere.

Pragmatically, I wouldn’t get too focussed on trying to work out the rhythm from a single tracing. I would just record multiple ECGs aiming to capture any longer P:QRS cycles to assess whether successive PR prolongation is present (Mobitz I) or absent (Mobitz II).

Progressive PR prolongation with Mobitz I

Constant PR interval with Mobitz II

Another clue to aid differentiation is QRS width:

• Mobitz I is typically due to block at the level of the AV node — hence QRS complexes are usually (but not always) narrow.
• Mobitz II typically occurs on a background of widespread conducting system disease (LBBB or bifascicular block). The block is typically at the level of the fascicles — hence QRS complexes are usually (but not always) broad.

There is some prognostic value in differentiating between Mobitz I and II — Patients with Mobitz II have a much higher rate of complete heart block and are more likely to require a permanent pacemaker.

However, in the acute situation it is more important to look at the patient rather than to fixate on the ECG.

Treatment decisions such as whether to commence temporary pacing will be largely determined by the patient’s clinical stability.