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.
Mark Alan
29 Jan, 2026
OH MY GOD. I JUST REALIZED MY GRANDMA WAS ON METHADONE + ONDANSETRON + CITALOPRAM. 🤯 SHE ALMOST DIED LAST YEAR AND NO ONE TOLD US WHY. THIS IS A MASSIVE COVER-UP. WHY ISN'T THE FDA DOING MORE? 🚨
Amber Daugs
29 Jan, 2026
Of course this happens. People these days take every pill they see advertised. No discipline. No responsibility. If you're going to pop a mood pill, at least get your electrolytes checked. It's not 1995 anymore. We have labs, we have guidelines, we have CredibleMeds. But no, people want the easy fix and then blame the system when their heart goes haywire.
And don't even get me started on smartwatches. 'Oh, my Apple Watch said I had an irregular rhythm!' Yeah, and your phone says you're 10% more productive. Neither is clinically valid.
Ambrose Curtis
30 Jan, 2026
Biggest thing nobody talks about? Magnesium. I used to think it was just for constipation. Then I worked ER for a year and saw 7 cases of TdP-all resolved within minutes after 2g IV mag. Even if their labs were normal. It's like giving oxygen to someone who's blue-you don't wait for a pulse ox to confirm they're hypoxic. You act.
Also, citalopram at 40mg? That's like driving a truck with no brakes. 20mg for older folks? Yeah. I've seen 78-year-olds on 40mg of citalo and their QTc was 560. No wonder they coded. Stop being lazy and check the damn ECG.
And for real-don't mix two known-risk drugs. Ever. I had a guy on azithromycin and haloperidol. He was fine until the doc added ondansetron. Three days later, he's flatlining. It wasn't bad luck. It was bad prescribing.
Also, women. Always assume they're at higher risk. Their hearts are wired differently. Testosterone is literally a natural QT stabilizer. So if you're prescribing to a woman over 65? Double-check everything. Don't just assume she's 'just a little old lady.' She's a walking risk profile.
And yes, you can check your own QT on a smartwatch. But you can also check your blood sugar with a piece of paper and a lemon. Doesn't mean it's accurate. Don't be that guy.
Robert Cardoso
30 Jan, 2026
Let's not romanticize this. TdP is statistically negligible compared to the millions of prescriptions issued annually. The real issue is the medicalization of risk. Every time a drug gets flagged, it creates a cascade of unnecessary monitoring, increased costs, and provider burnout. We're turning pharmacology into a minefield of paranoia.
Yes, QT prolongation can be dangerous. But so can untreated depression, uncontrolled pain, or untreated fungal infections. The risk-benefit calculus is not linear. We're not robots. We're clinicians. We assess context. We don't follow checklists like automatons.
And the FDA’s 1.2 million dollar testing requirement? That's not patient safety-it's corporate protectionism. Small pharma can't afford it. Innovation is stifled. The drugs we *need* don't get made.
And machine learning models? They're trained on biased datasets. If your model only includes white, elderly, urban patients, it won't predict risk in rural young adults. You're not solving the problem-you're automating ignorance.
Katie Mccreary
31 Jan, 2026
My mom died from this. No warning. No ECG. Just a 'quick script' for citalopram and ondansetron after her chemo. They didn't even check her potassium. She was 68. Healthy. No heart disease. Just a woman who trusted her doctors.
Now I make sure every relative on meds gets a baseline ECG. No excuses.
SRI GUNTORO
1 Feb, 2026
In India, we don't have access to ECGs for most patients. We prescribe citalopram because it's cheap and works. You say 'check QTc'? How? We don't have machines in villages. You're preaching to the choir in a country where people die because they can't afford insulin.
This is rich people's medicine advice.
Kevin Kennett
3 Feb, 2026
Thank you for this. Seriously. I'm a PA and I read this before my shift today. I had a patient on methadone and ketoconazole. QTc was 490. I stopped the ketoconazole, checked K+ (was 3.3), gave 2g IV mag, and repeated the ECG in 48 hours. Down to 455.
This isn't theory. This is what we do. And we need more people like you spreading this info.
Also, if you're on a QT-prolonging drug and you feel dizzy or lightheaded? Don't wait. Go to the ER. Don't 'wait and see.' TdP doesn't ask if you're busy.
Howard Esakov
5 Feb, 2026
Look, I appreciate the effort, but this is just another example of medicine becoming a bureaucratic nightmare. You're telling me I can't prescribe a perfectly effective drug because someone, somewhere, might have low magnesium? What's next? Do we ban all drugs because someone might have an allergic reaction? This isn't prevention-it's fear-based medicine.
And the FDA’s $1.2 million testing requirement? That's not science. That's rent-seeking by big pharma to kill competition. I'm not buying it.
Rhiannon Bosse
5 Feb, 2026
Okay but what if this is all a lie? What if the pharmaceutical companies are *using* QT prolongation as a scare tactic to push expensive monitoring and ECGs so they can sell more devices? Think about it. Who profits from ECG machines? Who profits from magnesium IV bags? Who profits from 'CredibleMeds' subscriptions?
And why is it always women? Coincidence? Or is it just easier to blame hormones than to admit we don't understand how female physiology works?
Also-has anyone checked if smartwatches are secretly collecting QT data to sell to insurers? 👀
John Rose
7 Feb, 2026
@7271 - I'm so sorry for your loss. That's the quiet tragedy no one talks about: a perfectly healthy person gone because a prescription was written without context.
@7272 - You're absolutely right. This advice is useless if the infrastructure isn't there. We need global advocacy-not just checklists. Maybe we start by training community health workers to recognize syncope and send ECGs via mobile apps. Not perfect, but better than nothing.
@7280 - The irony is that the same people who think this is a conspiracy also don't get their blood pressure checked. Pick a delusion and stick with it.
But seriously-thank you for sharing. This is why we write these posts. Not to scare. To save.