QTc Interval Quick Reference

Evidence-Based Guidelines for QT Interval Assessment and Management

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Normal QTc Ranges by Population

Important Notes on QTc Interpretation

• QTc values should be interpreted in clinical context

• Multiple factors can affect QT interval including medications, electrolyte abnormalities, and cardiac conditions

• First-time ECGs obtained after syncope must be interpreted with caution

Pediatric Population

Age Group Normal QTc (ms) Borderline QTc (ms) Prolonged QTc (ms)
0-1 years <440 440-460 >460
1-7 years <440 440-450 >450
8-17 years <440 440-460 >460

Adult Population by Sex

Population Normal QTc (ms) Borderline QTc (ms) Prolonged QTc (ms) High Risk (ms)
Adult Males <430 430-450 450-470 >470
Adult Females <450 450-470 470-480 >480

Special Populations

Population Considerations QTc Guidelines
Athletes • Often have bradycardia at rest
• May have transient AV blocks
• Fridericia formula preferred		 Males: <470 ms
Females: <480 ms
(Using Fridericia)
Geriatric (>65 years) • Increased risk of drug interactions
• Multiple comorbidities
• Altered drug metabolism		 Same as adults with increased vigilance
Consider lower thresholds for intervention

Critical QTc Values

QTc >500 ms: Associated with significantly increased risk of Torsades de Pointes

QTc increase >60 ms from baseline: Also indicates high risk regardless of absolute value

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QTc Correction Formulas

The QT interval varies with heart rate and requires correction. Multiple formulas exist, each with advantages and limitations.

Bazett Formula
QTc = QT / √(RR)

Most commonly used

✓ Simple calculation

✓ Accurate for HR 60-100 bpm

✗ Over-corrects at HR >100 bpm (tachycardia)

✗ Under-corrects at HR <60 bpm (bradycardia)

Fridericia Formula
QTc = QT / ∛(RR)

Preferred for athletes

✓ Better for bradycardia

✓ More accurate across HR ranges

✓ Recommended for HR <60 or >100

Framingham Formula
QTc = QT + 0.154(1 - RR)

Linear correction

✓ Population-based derivation

✓ Good for epidemiological studies

✗ Less commonly used clinically

Hodges Formula
QTc = QT + 1.75(HR - 60)

Linear correction

✓ Simple mental calculation

✓ Reasonable accuracy

✗ Less validated than others

Rautaharju Formula
QTc = QT - 0.185(RR - 1) + k

Sex-specific correction

✓ Accounts for sex differences

✓ k = +0.006 for women

✓ k = 0 for men

Clinical Recommendations

• Standard practice: Bazett formula for HR 60-100 bpm

• Bradycardia/Tachycardia: Consider Fridericia formula

• Athletes: Fridericia formula strongly preferred

• Research: Consider using multiple formulas for validation

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Psychotropic Medications and QTc Risk

Critical Safety Information

• Risk increases with polypharmacy, especially combining QT-prolonging agents

• Monitor closely in patients with electrolyte abnormalities, cardiac disease, or other risk factors

• Baseline ECG recommended before initiating high-risk agents

Typical Antipsychotics (First Generation)

Medication QTc Risk Level Average QTc Prolongation Clinical Notes
Thioridazine HIGH 28-37 ms FDA black box warning; avoid if possible
Haloperidol (IV) HIGH 16 ms IV form has higher risk than oral (4-9 ms)
Chlorpromazine HIGH Variable Low-potency typical with significant risk
Droperidol HIGH Variable FDA black box warning

Atypical Antipsychotics (Second Generation)

Medication QTc Risk Level Average QTc Prolongation Clinical Notes
Ziprasidone HIGH 15.9 ms Highest risk among SGAs
Iloperidone MODERATE 9-12 ms Dose-dependent effect
Quetiapine MODERATE Variable Risk increases with other QT drugs
Risperidone LOW 3.6 ms Clinically insignificant in most cases
Olanzapine LOW 1.7 ms Minimal QTc effect
Aripiprazole LOW Minimal Not associated with QTc prolongation
Lurasidone LOW Minimal Lowest risk among antipsychotics
Brexpiprazole LOW Minimal Limited data, appears safe

Antidepressants

Tricyclic Antidepressants (TCAs)

High Risk:

• Amitriptyline

• Imipramine

• Doxepin

• Clomipramine

Moderate Risk:

• Nortriptyline

• Desipramine

SSRIs

Medication QTc Risk Level Clinical Notes
Citalopram HIGH FDA max dose: 20mg in elderly, 40mg in adults
Escitalopram MODERATE Dose-dependent; caution >10mg in elderly
Fluoxetine LOW Minimal QTc effect at standard doses
Sertraline LOW Minimal QTc effect at standard doses
Paroxetine LOW Lowest risk among SSRIs
Fluvoxamine LOW Minimal QTc effect

Other Antidepressants

SNRIs (Low Risk):

• Venlafaxine

• Duloxetine

• Desvenlafaxine

Others:

• Mirtazapine (Low risk)

• Bupropion (Minimal risk)

• Trazodone (Low-moderate risk)

Psychostimulants

Generally considered low risk for QTc prolongation:

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Non-Psychotropic QT-Prolonging Medications

Antiarrhythmics

Class Medications Risk Level Notes
Class IA Quinidine, Procainamide, Disopyramide HIGH TdP risk 1-8% with quinidine
Class III Amiodarone, Sotalol, Dofetilide HIGH Sotalol: 10-40ms prolongation
Others Dronedarone HIGH Significant interaction risk

Antibiotics

Class High Risk Moderate Risk Lower Risk
Macrolides • Erythromycin
• Clarithromycin		 • Azithromycin • Ketolides (e.g., Solithromycin)
Fluoroquinolones • Moxifloxacin
• Sparfloxacin		 • Levofloxacin
• Ciprofloxacin		 • Delafloxacin
Others • Pentamidine • Bedaquiline • Beta-lactams
• Aminoglycosides

Antifungals

Azoles (Moderate-High Risk):

• Ketoconazole

• Itraconazole

• Fluconazole

• Voriconazole

Note: Also CYP3A4 inhibitors

Safer Alternatives:

• Echinocandins:

  - Caspofungin

  - Micafungin

  - Anidulafungin

Antiemetics

Medication/Class Risk Level Safer Alternatives
Ondansetron MODERATE • Scopolamine
• Promethazine
• Cyclizine
• Aprepitant
Dolasetron MODERATE
Droperidol HIGH
Domperidone HIGH

Other Notable QT-Prolonging Medications

Anticancer Agents:

• Arsenic trioxide

• Vandetanib

• Nilotinib

• Sunitinib

Opioids:

• Methadone (HIGH RISK)

• Buprenorphine (Low risk)

Others:

• Hydroxychloroquine

• Chloroquine

• Ranolazine

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Clinical Management of QTc Prolongation

Risk Assessment

Risk Factors for QTc Prolongation

Patient Factors:
  • Female sex
  • Age >65 years
  • Bradycardia
  • Structural heart disease
  • Previous QTc prolongation
  • Family history of LQTS
Clinical Factors:
  • Hypokalemia (<3.5 mEq/L)
  • Hypomagnesemia (<1.5 mg/dL)
  • Hypocalcemia
  • Hepatic impairment
  • Renal impairment
  • Drug interactions

Monitoring Guidelines

Clinical Scenario Recommended Monitoring
Before initiating QT-prolonging medication • Baseline ECG (within 1 month)
• Electrolyte panel
• Assess risk factors
• Review current medications
High-risk patients • ECG before and 8-12 hours after initiation
• Regular electrolyte monitoring
• Consider continuous telemetry if inpatient
Multiple QT-prolonging drugs • ECG with each new agent
• More frequent monitoring
• Consider alternative agents

Management Algorithm

When to Take Action

QTc Value Action Required
450-470 ms (♂) / 470-480 ms (♀) • Assess modifiable risk factors
• Consider dose reduction
• Increase monitoring frequency
470-500 ms (♂) / 480-500 ms (♀) • Correct electrolytes urgently
• Consider medication change
• Cardiology consultation
>500 ms or ↑>60 ms from baseline • STOP offending medication(s)
• Urgent cardiology consultation
• Continuous cardiac monitoring
• Correct electrolytes STAT

Psychiatric Red Flags for QTc Monitoring

High-Priority Psychiatric Scenarios

  • Polypharmacy: Patients on ≥2 psychotropic medications with QT-prolonging potential
  • Eating Disorders: Anorexia nervosa or bulimia with electrolyte disturbances
  • Substance Use: Active alcohol use disorder (risk of hypomagnesemia)
  • High-Dose Antipsychotics: Especially with typical antipsychotics or ziprasidone
  • Geriatric Psychiatry: Age >65 with multiple medical comorbidities
  • Medication Changes: Recent addition of CYP3A4 inhibitors (e.g., fluoxetine, fluvoxamine)

Torsades de Pointes Management

Emergency Management

  1. Hemodynamically unstable:
    • Immediate cardioversion
    • Magnesium sulfate 2-4g IV bolus
  2. Hemodynamically stable:
    • Magnesium sulfate 2g IV over 15 minutes
    • Correct electrolyte abnormalities
    • Stop all QT-prolonging medications
    • Consider isoproterenol or temporary pacing

Electrolyte Replacement Guidelines

Electrolyte Target Level Replacement Protocol Monitoring
Potassium 4.0-5.0 mEq/L • Mild (3.0-3.5): KCl 40 mEq PO
• Moderate (2.5-3.0): KCl 20 mEq/hr IV
• Severe (<2.5): KCl 40 mEq/hr IV with cardiac monitoring		 Recheck q2-4h during replacement
Magnesium 2.0-2.5 mg/dL • Mild (1.5-1.8): MgO 400-800mg PO
• Moderate (1.0-1.5): MgSO₄ 2g IV over 1h
• Severe (<1.0): MgSO₄ 4-6g IV over 2-4h		 Recheck after infusion complete
Calcium 8.5-10.5 mg/dL
(ionized: 4.5-5.5)		 • Calcium gluconate 1-2g IV over 10 min
• Calcium carbonate 1-2g PO TID with meals		 Monitor ionized calcium if available

Prevention Strategies

Prescribing Practices:

• Use lowest effective dose

• Avoid drug combinations

• Regular medication review

• Consider alternatives first

Patient Education:

• Report palpitations, syncope

• Avoid grapefruit juice

• Maintain hydration

• Adherence to monitoring

System Safeguards:

• Electronic alerts

• Pharmacy screening

• Standardized protocols

• Staff education