Sudden Stops: Second-degree AV Block (Mobitz II)

Second-degree atrioventricular (AV) block, Mobitz type II, is characterized by the intermittent failure of atrial impulses to conduct to the ventricles, occurring without the progressive PR interval prolongation seen in Mobitz I.¹⁸² This type of block typically occurs at the level of the His-Purkinje system.¹⁸²

Analogy: “The communication between the upper and lower sections of the orchestra is suddenly and unexpectedly interrupted, causing a missed beat.”

ECG Characteristics

The ECG in Mobitz II second-degree AV block shows a consistent PR interval in the conducted beats, which may be either normal or prolonged.¹⁸² The hallmark of this block is the sudden, intermittent dropping of QRS complexes (P waves are not followed by a QRS complex) without any preceding lengthening of the PR interval.¹⁸² P waves from the sinus node continue to occur at a regular rate, but not all of them are conducted to the ventricles. This can manifest as a fixed ratio of conducted to non-conducted beats, such as 2:1 (two P waves for every QRS complex) or 3:1.¹⁸²

Other Symptoms

Compared to Mobitz I, Mobitz II second-degree AV block is more likely to cause symptoms such as lightheadedness, dizziness, and syncope (fainting).¹⁸² It is considered more serious than Mobitz I due to a higher risk of progressing to third-degree (complete) heart block.¹⁸² Mobitz II is often associated with underlying structural heart disease or conduction system disease, such as a left bundle branch block or bifascicular block.¹⁸⁵

Analogy

“Like an unexpected silence in the music, the ventricles suddenly fail to respond to the atrial signal.”