Approach Considerations
The documentation and characterization of second-degree atrioventricular (AV) block by means of ECG is usually an essential investigation. This is ideally performed with a 12-lead ECG, although ambulatory ECG monitoring may be needed to identify and characterize infrequent episodes.
Electrophysiologic study to assess the level of block and subsidiary pacemaker rate may be beneficial in some patients. In patients with suspected carditis or cardiomyopathy, an endomyocardial biopsy may be performed at the same time.
Histologic findings
When endomyocardial biopsy is performed to assess heart block in conjunction with myocardial dysfunction, histochemically stained sections should be assessed for myofibrillar destruction or for lymphocytic, fatty, or fibrotic infiltration. Electron microscopy should be performed to assess any mitochondrial or other ultrastructural alterations that may be present.
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A common pattern of second-degree atrioventricular (AV) block consists of gradual prolongation of the PR interval leading up to a nonconducted P wave; this pattern is known as Wenckebach AV block, or Mobitz I AV block. This rhythm strip is an example of classic Mobitz I, or Wenckebach, AV block, in which the PR interval prolongs by sequentially smaller increments, with consequent shortening of the RR intervals until the blocked beat occurs. However, classic Wenckebach block is present in only a minority of cases. Wenckebach block is most easily diagnosed by comparing the PR interval following the blocked beat with the PR interval preceding the blocked beat; if the PR interval shortens following the blocked beat, the block is most likely of the Wenckebach type.
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If there is no progressive prolongation of the PR interval and the PR interval fails to shorten following a blocked beat, non-Wenckebach AV block (or Mobitz II AV block) is said to be present. This block is usually located more distally in the His bundle or the His bundle branches, or both, and the escape rates are usually slower and less stable.
