Regarding the normal ECG intervals and segments in adults, all of the following statements are correct EXCEPT:

Question

Accepted Answer

A. The QRS complex duration should not exceed 80 milliseconds in normal sinus rhythm. ## Normal ECG Intervals and Segments ### Correct Statements (Options 0, 1, 2) **Key Point:** The PR interval (120–200 ms) includes atrial depolarization (P wave) and AV nodal conduction delay, making it a marker of AV node function. **High-Yield:** QT interval correction is essential because QT varies with heart rate. Bazett's formula is the most commonly used: $QTc = \frac{QT}{\sqrt{RR}}$ where RR is in seconds. **Clinical Pearl:** The ST segment should be isoelectric (at baseline) in normal ECGs. Deviation >1 mm in limb leads or >2 mm in precordial leads is abnormal and suggests ischemia, injury, or infarction. ### Incorrect Statement (Option 3) — THE ANSWER **Key Point:** The QRS complex duration in normal sinus rhythm should **NOT exceed 120 milliseconds (0.12 seconds)**, not 80 milliseconds. A QRS >120 ms indicates **bundle branch block** or ventricular origin of the rhythm. **Mnemonic:** **QRS ≤ 120 ms** = Narrow complex; **QRS > 120 ms** = Wide complex (BBB, VT, or aberrant conduction). ### Summary Table | Interval/Segment | Normal Duration | Clinical Significance | | --- | --- | --- | | PR interval | 120–200 ms | AV conduction; prolongation = AV block | | QRS complex | ≤120 ms | Ventricular depolarization; >120 ms = BBB or VT | | QT interval (corrected) | <450 ms (♂), <460 ms (♀) | Repolarization; prolongation = risk of torsades | | ST segment | Isoelectric | Deviation suggests ischemia or injury | [cite:Harrison 21e Ch 295]

Answer

B. The QT interval varies inversely with heart rate and should be corrected using Bazett's formula for accurate interpretation

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C. The ST segment is typically isoelectric and represents the period between ventricular depolarization and repolarization

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D. The PR interval normally measures 120–200 milliseconds and represents atrial depolarization and AV nodal delay

Regarding the normal ECG intervals and segments in adults, all of the following statements are correct EXCEPT:

Explanation

Normal ECG Intervals and Segments

Correct Statements (Options 0, 1, 2)

Incorrect Statement (Option 3) — THE ANSWER

Summary Table

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Interval/Segment	Normal Duration	Clinical Significance
PR interval	120–200 ms	AV conduction; prolongation = AV block
QRS complex	≤120 ms	Ventricular depolarization; >120 ms = BBB or VT
QT interval (corrected)	<450 ms (♂), <460 ms (♀)	Repolarization; prolongation = risk of torsades
ST segment	Isoelectric	Deviation suggests ischemia or injury