Which AV block is Second Degree Heart Block Type 1 (Wenckebach), with causes including anoxia, edema after open‑heart surgery, digoxin toxicity, hyperkalemia, and anterior MI?

Which AV block is Second Degree Heart Block Type 1 (Wenckebach), with causes including anoxia, edema after open‑heart surgery, digoxin toxicity, hyperkalemia, and anterior MI?

• A. First Degree Heart Block
• B. Second Degree Heart Block Type I (Wenckebach)
• C. Second Degree Heart Block Type II
• D. Third Degree Heart Block

Answer: (B)

Wenckebach is an AV nodal block pattern where conduction through the AV node slows progressively with each beat, until a QRS is dropped, then the cycle repeats. The ECG hallmark is a gradually lengthening PR interval followed by a nonconducted beat, after which the PR interval resets and the pattern starts again. This reflects a reversible, nodal-level delay rather than a block below the AV node.

The causes listed fit this AV nodal mechanism because anything that increases vagal tone or directly depresses AV nodal conduction can produce Wenckebach: hypoxia or edema around the heart after surgery can enlarge vagal influence or compress the node; digoxin toxicity directly slows AV nodal conduction; electrolyte disturbances such as hyperkalemia can also slow nodal tissue. Anterior myocardial infarction, however, more often injures tissue below the AV node (the His-Purkinje system), leading to Mobitz type II or higher-grade blocks rather than Wenckebach, which helps distinguish it from other AV blocks.

For contrast, first-degree block shows simply a longer PR interval without dropped beats; second-degree type II shows dropped beats with a relatively constant PR interval rather than progressively lengthening; third-degree block shows complete AV dissociation with no fixed relationship between P waves and QRS complexes.

Thus, the description corresponds to Second Degree Heart Block Type I (Wenckebach).