QT Prolongation Risk Calculator
This tool helps assess the risk of Torsades de Pointes (TdP) based on key clinical factors. TdP is a life-threatening arrhythmia triggered by prolonged QT interval.
Every year, hundreds of people in the U.S. and Australia suffer a sudden, unexpected heart rhythm that can kill them in minutes. It’s not a heart attack. It’s not a stroke. It’s Torsades de Pointes - a chaotic, twisting ventricular tachycardia triggered by common medications many patients take without knowing the danger. And here’s the scary part: half of these cases happen with no warning at all.
What Exactly Is Torsades de Pointes?
Torsades de Pointes (TdP) isn’t just another irregular heartbeat. It’s a specific, life-threatening arrhythmia where the QRS complexes on an ECG appear to twist around the baseline - like a ribbon spinning out of control. This isn’t random. It only happens when the heart’s electrical reset time - measured as the QT interval - is stretched too long. The heart muscle doesn’t fully recover between beats, and that’s when dangerous electrical sparks, called early afterdepolarizations, fire off. Those sparks trigger the twisting rhythm. The QT interval is measured from the start of the Q wave to the end of the T wave on an ECG. It’s corrected for heart rate using Bazett’s formula (QTc). A QTc over 450 ms in men or 460 ms in women is considered prolonged. But when it hits 500 ms or more, the risk of TdP jumps two to three times. Even a 60 ms increase from a patient’s normal baseline can be a red flag.Which Medications Cause QT Prolongation?
Over 200 medications are linked to QT prolongation. Some are obvious - like antiarrhythmics used to treat heart rhythm problems. But many are everyday prescriptions.- Antibiotics: Erythromycin, clarithromycin, moxifloxacin
- Antifungals: Ketoconazole, voriconazole
- Antipsychotics: Haloperidol, thioridazine, ziprasidone
- Antidepressants: Citalopram, escitalopram (dose matters - over 40 mg/day increases risk)
- Antiemetics: Ondansetron (especially IV doses over 16 mg)
- Opioid replacement: Methadone (risk spikes above 100 mg/day)
Who’s Most at Risk?
It’s not just about the drug. It’s about the person taking it. Most TdP cases happen because of a perfect storm of risk factors.- Women: 70% of cases occur in women, even though men and women are equally likely to have QT prolongation.
- Age 65+: Two-thirds of TdP patients are over 65.
- Low potassium (hypokalemia): Present in 43% of cases. Potassium below 3.5 mmol/L triples the risk.
- Low magnesium (hypomagnesemia): Found in 31% of cases. Magnesium below 1.6 mg/dL increases risk by 2.7 times.
- Slow heart rate: 57% of patients have a heart rate under 60 bpm.
- Multiple QT-prolonging drugs: 28% of cases involve two or more drugs that stretch the QT interval.
- Heart disease or kidney/liver problems: 41% have pre-existing heart disease. Impaired kidneys or liver can cause drugs like citalopram to build up to dangerous levels.
How to Prevent It Before It Starts
TdP is almost always preventable. Here’s what works:- Check the patient’s baseline ECG. Measure QTc before starting any high-risk drug. Do this even if they seem healthy.
- Review every medication. Use CredibleMeds.org to check if any drug on the patient’s list is known to prolong QT. Don’t assume your pharmacist flagged it.
- Fix electrolytes. If potassium is below 4.0 mmol/L or magnesium below 2.0 mg/dL, correct it before prescribing. Giving magnesium prophylactically to high-risk patients cuts TdP risk significantly.
- Avoid combinations. Never give two drugs from the Known Risk list together. Even one Known and one Possible can be dangerous.
- Follow dosing limits. Citalopram max is 40 mg/day - but only 20 mg/day for patients over 65. Ondansetron IV should never exceed 16 mg. Methadone requires ECG monitoring at initiation and again if the dose goes above 100 mg/day.
What to Do If TdP Happens
If a patient collapses or their monitor shows the twisting pattern, act fast:- Give magnesium sulfate: 1-2 grams IV over 5-15 minutes. It works in 82% of cases, even if magnesium levels are normal.
- Pace the heart: Temporary pacing to keep the heart rate above 90 bpm stops TdP by shortening the QT interval. It’s successful in 76% of cases.
- Correct electrolytes: Check potassium and magnesium again. Replenish both.
- Stop the offending drug. Immediately discontinue the medication that caused it.
- Use isoproterenol if needed: If pacing isn’t available, isoproterenol can increase heart rate and suppress the arrhythmia.
The Bigger Picture: Regulation and New Tools
The FDA now requires all new drugs to be tested for QT prolongation before approval. This adds $1.2 million and 6-8 months to development - but it’s saved lives. Since 1990, 12 drugs like terfenadine and cisapride have been pulled off the market because of TdP risk. New tools are emerging. Mayo Clinic’s machine learning model predicts individual TdP risk with 89% accuracy by analyzing 17 factors - age, sex, kidney function, meds, electrolytes, baseline QTc. It’s not in every clinic yet, but it’s coming. The 2023 CredibleMeds update added 12 new drugs to the Known Risk list, including lesinurad and fedratinib. Domperidone was moved from Known to Possible after new data showed lower risk than thought.Bottom Line: Don’t Fear the Drug - Manage the Risk
Some doctors avoid prescribing any drug that prolongs QT. That’s not the answer. Many of these drugs - like methadone for pain or citalopram for depression - are life-changing. The goal isn’t to avoid them. It’s to use them safely. Ask yourself: Is this patient female? Over 65? On more than one QT drug? Have low potassium or magnesium? Any heart or kidney disease? If yes, check the ECG. Correct the electrolytes. Limit the dose. Monitor. TdP is rare. But when it happens, it’s often sudden, fatal, and preventable. The difference between life and death isn’t always a fancy machine. It’s a simple checklist - and the discipline to use it every time.Can Torsades de Pointes happen in healthy people without heart disease?
Yes. While heart disease increases risk, most TdP cases occur in people with no prior cardiac history. The trigger is usually a combination of a QT-prolonging drug and a modifiable factor like low potassium, low magnesium, or slow heart rate. Healthy women over 65 taking multiple medications are especially vulnerable.
Is a prolonged QT interval always dangerous?
Not always. A QTc of 470 ms in a young, healthy person on a single low-risk drug carries minimal risk. Danger increases sharply when QTc exceeds 500 ms, or when there’s a rise of more than 60 ms from baseline - especially with other risk factors like hypokalemia or multiple QT drugs. The key is context, not just the number.
Can I check my own QT interval on a smartwatch?
Some smartwatches can detect irregular rhythms and may show a prolonged QT, but they’re not reliable for clinical decision-making. They lack the precision of a 12-lead ECG, especially for measuring the end of the T wave, which is critical. Never rely on a smartwatch to rule out TdP risk. Always get a proper ECG if you’re on a high-risk medication.
Why do women have a higher risk of TdP than men?
Women naturally have longer QT intervals than men, even at the same heart rate. Hormonal differences, especially lower testosterone levels, affect potassium channel function in the heart. This makes women more sensitive to QT-prolonging drugs. The risk is highest during the luteal phase of the menstrual cycle and after menopause.
What’s the safest antidepressant if I’m concerned about QT prolongation?
Sertraline and bupropion have minimal effect on the QT interval and are preferred in patients at risk. Escitalopram and citalopram carry higher risk, especially at doses above 20 mg/day in older adults. Always check the CredibleMeds database before prescribing or switching antidepressants.
Does magnesium need to be given even if levels are normal?
Yes. Magnesium sulfate works as an antiarrhythmic in TdP, regardless of serum levels. It stabilizes the heart’s electrical activity by blocking calcium channels and reducing early afterdepolarizations. Giving 1-2 grams IV is standard in acute TdP, even if the patient’s magnesium is normal.
How often should I repeat the ECG after starting a QT-prolonging drug?
For high-risk drugs like methadone or sotalol, repeat the ECG within 1-2 weeks after starting or increasing the dose. For moderate-risk drugs, check at 4-6 weeks. If the QTc increases by more than 60 ms from baseline or exceeds 500 ms, stop the drug and reassess. Don’t wait for symptoms.
John Rose
27 Jan, 2026
This is one of those posts that should be mandatory reading for every med student and prescriber. I've seen too many patients get flagged for QT prolongation without anyone connecting the dots between their new antibiotic, low potassium, and that one antidepressant they've been on for years. The 5-step checklist at the end? Print it. Laminate it. Tape it to your EHR screen.
Knowledge like this doesn't just save lives-it prevents families from losing someone in a grocery store parking lot because a doctor didn't check a lab result.