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

Question

Accepted Answer

A. The PR interval represents the time from atrial depolarization to the start of ventricular depolarization and normally measures 0.20–0.24 seconds (5–6 small squares). ## 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.

Answer

B. The QRS complex represents ventricular depolarization and normally has a duration of 0.08–0.12 seconds (2–3 small squares)

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C. The P wave represents atrial depolarization and normally has a duration of less than 0.12 seconds (3 small squares)

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D. The T wave represents ventricular repolarization and is typically biphasic in lead V1 but upright in leads II, III, and aVF

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

Normal ECG Components — Correct Statements

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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