What Is QT Prolongation?
The QT interval on an ECG represents the total time for ventricular depolarization and repolarization. When this interval is abnormally long, it creates a window of electrical vulnerability during which a premature beat can trigger Torsades de Pointes (TdP) — a potentially fatal polymorphic ventricular tachycardia. The corrected QT interval (QTc) adjusts for heart rate and is the standard clinical measure.
QTc Reference Values
| Category | Males | Females | Clinical Action |
|---|---|---|---|
| Normal | < 440 ms | < 460 ms | Continue monitoring per protocol |
| Borderline | 440 – 460 ms | 460 – 470 ms | Increase monitoring frequency; review risk factors |
| Prolonged | > 460 ms | > 470 ms | Notify provider; consider dose reduction / alternative |
| Critical | > 500 ms (any sex) | Hold offending agent; urgent provider notification; continuous monitoring | |
Bazett's formula: QTc = QT ÷ √RR (in seconds). Also used: Fridericia, Framingham corrections.
High-Risk Medications
| Drug Class | Examples | Risk Level |
|---|---|---|
| Antiarrhythmics (Class IA / III) | Amiodarone, Sotalol, Quinidine, Dofetilide | High |
| Antipsychotics | Haloperidol, Quetiapine, Ziprasidone, Thioridazine | High |
| Antibiotics | Azithromycin, Ciprofloxacin, Levofloxacin, Moxifloxacin | Moderate–High |
| Antiemetics | Ondansetron, Droperidol, Metoclopramide | Moderate |
| Opioids | Methadone | High |
| Antidepressants (TCAs) | Amitriptyline, Imipramine | Moderate |
Risk compounds with drug combinations and electrolyte abnormalities. Check crediblemeds.org for full risk classifications.
Electrolyte Causes
K⁺ < 3.5 mEq/L
Reduced intracellular potassium prolongs Phase 3 repolarization. Common in patients on diuretics or with poor nutrition.
Mg²⁺ < 1.7 mg/dL
Magnesium stabilizes cardiac membranes; deficiency amplifies QT-prolonging effects of other drugs. IV Mg²⁺ is the treatment for TdP.
Ca²⁺ < 8.5 mg/dL
Reduced calcium prolongs the plateau phase (Phase 2) of the action potential, extending the QT interval.
Torsades de Pointes (TdP)
A polymorphic ventricular tachycardia characterized by QRS complexes that appear to "twist" around the isoelectric baseline on ECG. TdP can degenerate into ventricular fibrillation and sudden cardiac death. First-line treatment: IV Magnesium Sulfate 1–2 g over 5–15 minutes. If hemodynamically unstable: immediate defibrillation. Correct underlying electrolyte abnormalities and discontinue offending agents.
Assessment Workflow
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Baseline 12-Lead ECG
Obtain before initiating any QT-prolonging medication. Measure and document QTc.
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Electrolyte Panel
Check K⁺, Mg²⁺, Ca²⁺ and correct deficiencies before and during therapy.
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Medication Reconciliation
Identify all QT-prolonging agents in the medication list. Alert provider to combinations.
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Continuous Cardiac Monitoring
Place on telemetry. Monitor for QTc trends, premature ventricular contractions, or TdP pattern.
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Hold & Notify at QTc > 500 ms (or Δ > 60 ms from baseline)
Hold offending medication. Notify provider immediately. Ensure crash cart / defibrillator is accessible.
QT Prolongation — Concept Overview
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root((QT Prolongation))
Causes
Medications
Antiarrhythmics
Amiodarone
Sotalol
Quinidine
Dofetilide
Antipsychotics
Haloperidol
Quetiapine
Ziprasidone
Thioridazine
Antibiotics
Azithromycin
Fluoroquinolones
Antiemetics
Ondansetron
Droperidol
Opioids
Methadone
Antidepressants
TCAs
Electrolytes
Hypokalemia
Hypomagnesemia
Hypocalcemia
Congenital
Long QT Syndrome
Measurement
QTc Formula
Bazett
Fridericia
Thresholds
Normal under 440ms male
Normal under 460ms female
Critical over 500ms
Complication
Torsades de Pointes
Polymorphic VT
Ventricular Fibrillation
Sudden Cardiac Death
Management
Hold offending agent
Correct electrolytes
IV Magnesium Sulfate
Potassium replacement
Monitor QTc trend
Defibrillation if unstable "QT prolongation is often asymptomatic until it isn't — the first symptom can be a fatal arrhythmia. Prevention and monitoring are the nurse's primary tools."
"Never give IV ondansetron as a rapid bolus — slow infusion (15 min) significantly reduces the QT impact. Dose reduction applies in elderly and patients with hepatic impairment."
"Combination of two QT-prolonging drugs + hypokalemia is a recipe for TdP. Each risk factor multiplies the others — always think in combinations."