Third degree AV block (more frequently referred to as complete AV block) occurs when none of the atrial impulses are conducted to the ventricles. In this situation, the ventricles are driven by an escape pacemaker located within the AV node, the His- Purkinje system or the ventricular myocardium itself. Electrocardiographically, the P waves and the QRS complexes are dissociated and the RR interval is greater than the PP interval, i.e. the ventricular rate is less than the atrial rate.
Complete AV block most often results from a variety of diseases that affect the conduction system or from drugs that block impulse conduction in the AV node. However, it may also be congenital and occur in the absence of any other cardiac disease. The tracing shown here is from an 18 year old female with no evidence if intrinsic cardiac disease. The atrial rate is slightly irregular with PP intervals that range from 0.72 to 1.04 seconds. The P waves appear to be of sinus origin, but none are conducted to the ventricles. There is a regular narrow QRS complex escape rhythm with an RR interval of 1.50 seconds (rate = 40). The narrow QRS complexes indicate that the site of the escape pacemaker is above the bifurcation of the His bundle and this indicates that the site of block is most likely within the AV node.