ECG Case 029

Main Findings

• Irregular narrow-complex rhythm (overall rate = 72 bpm)
• Normal sinus P waves are seen (upright in lead II), indicating a sinus origin of the rhythm
• QRS complexes cluster in groups, separated by non-conducted P waves
• The PR interval progressively prolongs within each group
• The PR prolongation can be appreciated by comparing the first and last PR interval of each group

Diagnosis 

This is the typical appearance of 2nd degree AV block with Mobitz I conduction (Wenckebach phenomenon).

Clinical Significance

In comparison to patients with Mobitz II, who typically require a pacemaker for prophylaxis of complete heart block and ventricular standstill, patients with Mobitz I do not necessarily need any intervention.

This is provided that they are asymptomatic with a normal BP, and that reversible causes such as drug toxicity (beta-lockers, digoxin), hyperkalaemia and myocardial ischaemia have been excluded.

The risk of progression to haemodynamically unstable AV block in these patients is very low.

Further Reading

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.
• Wiesbauer F. Little Black Book of ECG Secrets. Medmastery PDF