What is the workup for a patient with a prolonged QT interval on an electrocardiogram (EKG)?

Workup for Prolonged QT Interval on EKG

Immediately obtain a detailed medication history, baseline electrolytes (potassium, magnesium, calcium), and repeat a manual QT measurement to confirm the automated reading, as automated ECG measurements are frequently inaccurate. 1, 2

Initial Assessment and Confirmation

Verify the QT Prolongation

• Manually measure the QT interval using the tangent method in the same lead consistently, as automated measurements cannot be relied upon for accurate diagnosis 3, 2
• Calculate the corrected QT interval (QTc) using alternative rate adjustment formulas (Fridericia, Hodges, or Sagie/Framingham) rather than the classical Bazett correction, particularly for patients with abnormal heart rates 4
• QTc >500 ms represents an unequivocal risk threshold requiring immediate intervention 4, 5
• QTc >450 ms (men) or >470 ms (women) is considered prolonged and warrants further workup 4

Essential Laboratory Workup

Electrolyte Panel

• Check serum potassium (target >4.0 mEq/L or >4 mmol/L), magnesium (target >1.8 mg/dL or >2 mmol/L), and calcium 4, 5
• Severe hypokalemia or hypomagnesemia, especially with other risk factors, requires continuous monitoring until corrected 4
• Electrolytes should be repleted before any repeat ECG assessment 5

Additional Laboratory Tests

• Baseline liver function tests (LFTs) 4
• Renal function (U&Es) 4
• Thyroid function tests (as hypothyroidism can prolong QT) 1

Comprehensive Medication Review

Identify QT-Prolonging Drugs

• Systematically review all current medications for QT-prolonging potential, including antiarrhythmics (quinidine, procainamide, disopyramide, sotalol, dofetilide, ibutilide), antibiotics (macrolides, fluoroquinolones), antipsychotics, antidepressants, and antiemetics 4, 6
• Check for drug interactions that may impair metabolism of QT-prolonging agents 6, 7
• Consider discontinuing or substituting any offending medications 4, 5
• Reference updated drug lists at www.qtdrugs.org or crediblemeds.org 6

Risk Stratification

Assess Risk Factors for Torsades de Pointes

• Older age, female sex, heart disease (left ventricular hypertrophy, ischemia, low ejection fraction) 4
• Bradyarrhythmias, slow heart rate, or long pauses 4
• Family history of syncope, sudden death, or long QT syndrome 4
• Baseline QT prolongation suggesting genetic predisposition 4
• History of cardiac arrhythmias, particularly polymorphic ventricular premature beats or nonsustained polymorphic VT 4

Clinical History

Key Historical Elements

• History of syncope, palpitations, dizziness, or seizure-like episodes (which may represent misdiagnosed torsades de pointes) 1, 3
• Recent vomiting, diarrhea, or starvation diet (causing electrolyte disturbances) 4, 6
• Family history of sudden cardiac death, drowning, or unexplained syncope (suggesting congenital long QT syndrome) 4, 3
• Previous cardiac disease or structural heart abnormalities 4

Cardiac Evaluation

ECG Monitoring Strategy

• Continuous ECG monitoring is indicated if QTc >500 ms, if patient has symptoms, or if multiple risk factors are present 4
• For drug-induced prolongation, monitor at least every 8 hours and before/after dose changes 4
• Repeat ECG after 1-2 weeks when medications are adjusted or restarted, then again at 1 month to ensure stability 5
• Monitor for QT-related arrhythmias including T wave alternans, enhanced U waves, and polymorphic ventricular ectopy 4

Additional Cardiac Testing

• Exercise treadmill testing can be useful in suspected congenital long QT syndrome, particularly if resting QTc is borderline; QTc prolongation ≥445 ms at 4-minute recovery or >460 ms at 7 minutes suggests long QT syndrome 4
• Echocardiogram to assess for structural heart disease, left ventricular hypertrophy, or reduced ejection fraction 4

Genetic Considerations

When to Consider Genetic Testing

• Genetic testing for long QT syndrome should be performed if there is marked QT prolongation, family history of sudden cardiac death, or recurrent syncope with QT prolongation 4
• Yield of genetic testing in phenotype-positive patients is 50-86%, with higher yield in those with marked prolongation 4
• A negative genetic test does not exclude long QT syndrome; clinical diagnosis remains paramount 4

Immediate Management Considerations

Critical Actions

• If QTc >500 ms, discontinue all QT-prolonging medications immediately and continue monitoring until QTc decreases 4, 5
• A QTc increase >60 ms from baseline warrants immediate attention and medication review 5
• Correct electrolyte abnormalities aggressively before any further intervention 4, 5
• For patients on arsenic trioxide (ATO) or similar high-risk agents, monitor QT/QTc at least twice weekly 4

Common Pitfalls to Avoid

• Do not rely solely on automated QT measurements; always verify manually 1, 2
• Do not use Bazett correction formula in patients with abnormal heart rates, as it overcorrects at high rates and undercorrects at low rates 4
• Do not overlook concomitant use of multiple QT-prolonging drugs, even if individual agents are considered "low risk" 4, 6
• Do not assume normal electrolytes without checking; hypokalemia and hypomagnesemia are frequently present and easily correctable 4, 5