In second-degree AV block Mobitz type II, which of the following ECG findings is pathognomonic?

Explanation

## Second-Degree AV Block: Mobitz Type II ### Pathophysiology Mobitz type II block occurs due to **infranodal conduction disease** (block below the AV node, typically in the His bundle or bundle branches). The conduction delay is fixed, not progressive. ### ECG Hallmark **Key Point:** The defining feature of Mobitz II is a **sudden, unexpected dropped QRS complex without any preceding PR prolongation**. The PR interval remains constant in the conducted beats. ### Comparison with Mobitz Type I | Feature | Mobitz Type I | Mobitz Type II | |---------|---------------|----------------| | **Site of block** | AV node (supranodal) | His bundle / bundle branches (infranodal) | | **PR interval pattern** | Progressive prolongation | Fixed PR interval | | **Dropped beat** | After longest PR | Sudden, unpredictable | | **Prognosis** | Usually benign | High risk of progression to complete block | | **Treatment** | Observation (often) | Pacemaker indicated | ### Clinical Significance **High-Yield:** Mobitz II is a **dangerous rhythm** because it can suddenly progress to complete heart block (CHB). It is an indication for **permanent pacemaker implantation** even if asymptomatic. **Clinical Pearl:** Mobitz II often occurs in acute inferior MI (when the block is at AV node level, it is Mobitz I; when at infranodal level in inferior MI, it is Mobitz II) or chronic degenerative conduction disease. **Mnemonic:** **"Mobitz II = Sudden drop, no warning"** — the block occurs without progressive PR lengthening, unlike Mobitz I which gives you a "heads-up" with increasing PR intervals. [cite:Harrison 21e Ch 226]