Scenario: This is the resting 12-lead ECG waveform of a 75-year-old white man presenting to an acute care clinic with complaints of shortness of breath, intermittent chest pain, and dizziness.

Interpretation: Third degree atrioventricular (AV) block with accelerated idioventricular rhythm (AIVR).

Evaluation of the V1 rhythm strip at the bottom of the 12-lead ECG waveform is necessary for diagnosing this cardiac rhythm. In this example, there is no relationship between the P waves and the QRS complexes indicating that there is complete failure of the atrial impulses to conduct to the ventricles. The atria and ventricles are acting independently—each firing at an intrinsic rate with no relationship to one another. Third-degree (complete) heart block is caused by a conduction block most commonly in bundle branch-Purkinje system (61%) at the level of the AV node (21%) or the His...

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