Regarding the normal electrocardiogram (ECG) and its components, all of the following statements are correct EXCEPT:

Explanation

## Normal ECG Intervals and Waves ### PR Interval — The Correct Abnormality **Key Point:** The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The normal range is **0.12–0.20 seconds (3–5 small squares)**, NOT 0.20–0.24 seconds. **High-Yield:** A PR interval > 0.20 seconds (> 5 small squares) is classified as **first-degree atrioventricular (AV) block**. The option states 0.20–0.24 seconds as normal, which is incorrect — 0.20 seconds is already at the upper limit of normal, and anything beyond it is pathological. ### Normal ECG Components — Correct Statements | Component | Duration | Represents | Normal Features | |-----------|----------|-----------|------------------| | **P wave** | < 0.12 s (< 3 squares) | Atrial depolarization | Upright in most leads; biphasic in V1 | | **QRS complex** | 0.08–0.12 s (2–3 squares) | Ventricular depolarization | Narrow in supraventricular origin | | **T wave** | Variable | Ventricular repolarization | Biphasic in V1; upright in II, III, aVF | | **PR interval** | **0.12–0.20 s (3–5 squares)** | AV conduction time | Constant in sinus rhythm | **Clinical Pearl:** The PR interval reflects the time for the impulse to travel through the atrium, AV node (the slowest component), and the His bundle to the start of ventricular activation. Prolongation beyond 0.20 seconds indicates AV nodal delay or block. **Mnemonic:** **PQRST** — P (atrial depolarization), QRS (ventricular depolarization), T (ventricular repolarization). Remember: **PR < 0.20 s is normal**; ≥ 0.20 s is first-degree AV block.