## 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]
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