Curriculum · Cardiovascular and Hypertension
Complete (third-degree) heart block
What it is
Third-degree heart block, also called complete heart block, is complete AV conduction failure. There is no P:QRS relationship at all, and the atria and ventricles contract in a disordered way, which drops cardiac output.
It is the end of a line of three AV conduction blocks. First-degree block is only a prolonged AV delay, greater than 0.2 sec, that is a prolonged PR interval with the P:QRS ratio still 1:1, and its management is nil. Second-degree block loses beats but keeps some link between P waves and QRS complexes, and it comes in two forms: Mobitz type I, Wenckebach, is a gradual lengthening of the PR interval until a QRS is lost, and its management is nil, with atropine if symptomatic; Mobitz type II is an intermittent loss of AV conduction with a fixed PR interval, the block may last for 2 or more beats, and its management is a pacemaker. In third-degree block that link is gone.
How it is diagnosed
The ECG shows no relationship between P waves and QRS complexes. Each side of the block keeps its own timing: the P-P intervals are equal to one another and the intervals from QRS to QRS are equal to one another, while nothing ties the two together.
At the bedside the JVP carries the same news. The a-wave is atrial contraction, before the carotid pulse, and a canon a-wave points to complete heart block or to ventricular tachycardia.