What the diagram shows

Why the QRS differs

• Junctional escape. The backup pacemaker is at or just below the AV node, above the bifurcation of the His bundle. The impulse then travels down the normal His-Purkinje system, so ventricular depolarization is fast and the QRS is narrow.
• Ventricular escape. The backup pacemaker is in the ventricular muscle or the distal conduction system, below the block. The impulse spreads cell to cell through myocardium, not along the fast conduction system. Depolarization is slow, so the QRS is wide. Pacemakers lower in the conduction system are also slower and less reliable.

ECG features of complete heart block

• P waves and QRS complexes are completely dissociated (AV dissociation). The atria beat at their own, faster rate.
• The PP interval and the RR interval are each regular, but they are unrelated to each other.
• The ventricular rate is slow, because it is set by the escape focus.

Textbook support

• Fuster and Hurst's The Heart, 15th Edition, Third-Degree AV Block: with block at the AV node, "the escape rhythm is usually junctional, with a narrow QRS complex (unless associated with preexisting bundle branch block)." With His-Purkinje disease, the escape rhythm is ventricular, "with a wide QRS interval and a ventricular escape rhythm at 20 bpm to 40 bpm."

Clinical points

• Narrow-QRS escape. This is usually more stable. It often follows an AV nodal cause such as inferior MI, increased vagal tone, drugs, or congenital block. It can sometimes be transient or reversible.
• Wide-QRS escape. This is unstable and can lead to asystole or syncope (Stokes-Adams attacks). It is associated with infranodal disease, such as anterior MI or degenerative conduction disease. It usually needs urgent pacing.
• Treatment. Treat reversible causes. For unstable patients, use atropine (works best for nodal block), transcutaneous or transvenous pacing, then a permanent pacemaker.

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