Third Degree Atrioventricular Block

With third degree atrioventricular block there is complete interruption of the conduction from the atria to the ventricles. The block is either at, or below, the atrioventricular node.

The QRS complexes are caused by slower, automatic escape rhythms. The nervous system is unable to influence the rate of these rhythms, the ECG will therefore show a constant RR interval.

When the conduction starts within, or close to, the atrioventricular node, a narrow QRS complex escape rhythm is seen. If a wide complex escape rhythm is visible, then the conduction is of ventricular origin and thus may be considered as a ventricular escape. The closer the origin to the atrioventricular node, the faster the rate.

Third degree atrioventricular block is also referred to as complete heart block.

Conduction: Third Degree Atrioventricular Block

In the following example, whilst some P waves are obscured, the underlying atrial activity (shaded) at a rate of 59 bpm is still apparent. The ventricular rate (arrowed) is slower at 38 bpm.

Electrocardiogram: Third Degree Atrioventricular Block
Electrocardiogram: Third Degree Atrioventricular Block