23-17: Third-Degree (Complete) AV Block

ECG Findings

• Atrial and ventricular electrical activities are entirely dissociated.

• The P-P and R-R intervals remain constant.

• P waves may be hidden in the QRS complex or may distort the shape of the T wave.

• The atrial rate is usually faster than the ventricular rate, and the ventricular rate is generally slow.

Pearls

1. Third-degree block is also called complete heart block because no impulses are conducted from the atria to the ventricles.

2. AV rate and QRS morphology depend upon the location of the escape pacemaker.

3. A nodal escape rate is typically 40 to 60 bpm, with a narrow QRS complex.

4. Ventricular escape rate is usually 20 to 40 bpm, with a widened QRS complex.

5. Complete heart block may be caused by MI, conduction system disease, or drugs such as digoxin.

6. Complete heart block may dramatically decrease cardiac output and cause hypotension. Emergent cardiac pacing is required if the patient is unstable.