Why three degrees of heart block?

Heart block, or atrioventricular (AV) block, is either complete or incomplete. The term third-degree AV block, or complete heart block, is applied when there is no relationship between the atrial and ventricular beats and the atrial rate is faster than the ventricular. Incomplete AV block is divided into first-degree, second-degree, and advanced AV block. By definition, first-degree AV block occurs when the PR interval is 0.20 seconds and each atrial beat is followed by a ventricular complex. Second-degree AV block results in intermittent failure of atrial impulses to be conducted to the ventricles and is divided into two basic types: Type I or Wenckebach (also called Mobitz I) shows progressive lengthening of the PR interval from beat to beat until an atrial complex is blocked. Because of these periodic pauses, grouped beating occurs and, when present, aids in the diagnosis of Mobitz I block. It may be seen transiently in acute inferior wall myocardial infarction. Type II AV block (advanced or Mobitz II) shows intermittent blocked P waves where the PR intervals of the conducted beats are constant. In these situations, the block is usually below the His bundle and may be accompanied by an associated bundle branch block.

Mobitz II is usually considered more serious than Mobitz I and may require artificial pacemaker treatment. In third-degree block, pacemaker cells in the AV node or in the His Purkinje system may initiate ventricular contraction. Except for some cases of congenital third-degree AV block, this condition usually requires an artificial pacemaker.

First-degree block (PR interval = 0.26 seconds).

Mobitz type I second-degree AV block (Wenckebach). Note the gradual prolongation of the PR interval (1-5), the missing QRS complex after the sixth P wave, and the return of the PR interval to its shortest duration (7).

Third-degree AV block with a ventricular escape rhythm at 32 bpm. P-wave activity is somewhat irregular.

Figures from Seelig CB: Simplified EKG Analysis. Philadelphia, Hanley & Belfus, 1992, pp 77-80, with permission.

Why three degrees of heart block? Photo Gallery

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