## QT Interval: Determinants and Clinical Significance **Key Point:** The QT interval represents the total duration of ventricular depolarization and repolarization. Its length is influenced by multiple physiologic and pharmacologic factors, but atrial wall thickness is NOT one of them. ### Factors Affecting QT Interval | Factor | Effect on QT | Mechanism | |--------|--------------|----------| | **Heart rate (↑ RR)** | ↓ QT (shortens) | Faster rate → faster repolarization | | **Serum K^+^ (↓)** | ↑ QT (prolongs) | Reduced K^+^ gradient slows repolarization | | **Serum Ca^2+^ (↓)** | ↑ QT (prolongs) | Reduced inward Ca^2+^ current | | **Serum Mg^2+^ (↓)** | ↑ QT (prolongs) | Cofactor for K^+^ channels | | **Class IA, III antiarrhythmics** | ↑ QT (prolongs) | Block K^+^ channels, delay repolarization | | **Antipsychotics (haloperidol, risperidone)** | ↑ QT (prolongs) | HERG channel blockade | | **Hypothermia** | ↑ QT (prolongs) | Slowed cellular metabolism | | **Female sex** | ↑ QT (prolongs) | Hormonal effects on ion channels | | **Atrial wall thickness** | **NO effect** | Does not influence ventricular repolarization | **High-Yield:** QT interval is **rate-dependent**. The corrected QT (QTc) is calculated using Bazett's formula: $QTc = \frac{QT}{\sqrt{RR}}$ (in seconds). Normal QTc = 350–450 ms in men, 350–460 ms in women. QTc >500 ms indicates prolongation and increased risk of torsades de pointes. **Clinical Pearl:** Hypokalemia, hypomagnesemia, and hypocalcemia are the "lethal triad" causing QT prolongation and sudden cardiac death. Multiple QT-prolonging drugs increase torsades risk synergistically. **Mnemonic:** **"QT PROLONGED"** — **Q**uinidine, **T**ricyclics, **P**henothiazines, **R**isperidone, **O**ld age (female), **L**ow K^+^/Mg^2+^, **O**ther drugs (macrolides, fluoroquinolones), **N**ew antiarrhythmics, **G**ene mutations, **E**lectrolyte abnormalities, **D**isease (hypothermia, myocarditis). 
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