## ECG Segments, Intervals, and Waves — Detailed Analysis ### U Wave — The Incorrect Statement **Key Point:** The U wave represents repolarization of the **Purkinje system and ventricular muscle**, NOT atrial repolarization. Atrial repolarization occurs during ventricular depolarization (QRS complex) and is typically buried within and obscured by the QRS complex. **High-Yield:** The U wave is a small deflection that follows the T wave and is best visualized in leads **V2–V4** (correct location in the option). However, its origin is Purkinje and ventricular muscle repolarization, not atrial repolarization. The U wave is often absent or small and becomes prominent in **hypokalemia**, **bradycardia**, and **prolonged QT interval**. ### Correct ECG Components | Component | Duration/Measurement | Represents | Normal Features | |-----------|----------------------|-----------|------------------| | **ST segment** | Isoelectric or near-isoelectric | Plateau phase of ventricular action potential (entire ventricle depolarized) | Should be at baseline | | **QT interval** | Measured QRS start to T wave end | Total ventricular depolarization + repolarization | QTc < 0.44 s (males); < 0.46 s (females) | | **TP segment** | Baseline between T and P waves | Isoelectric reference line | Used for ST segment measurement | | **U wave** | Small deflection after T wave | Purkinje + ventricular muscle repolarization | Best in V2–V4; absent in many normal individuals | **Clinical Pearl:** Atrial repolarization (Ta wave) occurs during the QRS complex and is normally invisible because it is buried within the much larger QRS deflection. This is why we do not see a separate atrial repolarization wave on the normal ECG. **Mnemonic:** **PQRSTU** — P (atrial depol), QRS (ventricular depol), T (ventricular repol), U (Purkinje repol). Remember: **U = Purkinje, NOT atria**.
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