Final answer:
A second-degree Mobitz type II AV block is diagnosed by examining an ECG for regular P waves with an occasional non-conducted P wave leading to a missing QRS complex, whereas the preceding PR intervals are constant. This condition implies a block at the level of or below the AV node, typically involving the bundle of His.
Step-by-step explanation:
A second-degree Mobitz type II AV block is diagnosed by analyzing an electrocardiogram (ECG). This condition is characterized by a series of conducted P waves (atrial activity) followed by a sudden non-conducted P wave without progressive PR interval lengthening beforehand, leading to a missing QRS complex (ventricular activity). Unlike second-degree Mobitz type I AV block (also known as Wenckebach), the PR intervals in Mobitz type II do not lengthen prior to a dropped beat. The described ECG pattern is associated with the infra-Hisian or AV nodal blocks, which can occur at the level of or below the AV node, often involving the bundle of His.
The ECG presentation of a Mobitz type II block would include regular P waves with an occasional non-followed QRS complex, while the preceding PR intervals are constant. When present, this ECG finding warrants clinical attention, as it can potentially lead to more severe forms of heart block. Clinically, infra-Hisian blocks are more likely to progress to a third-degree (complete) AV block compared to AV nodal blocks.