Cardiac Safety · Chapter 1
QTc Prolongation: The 5-Minute Framework
How to think about QT risk in psychiatric prescribing — the numbers that matter, the patients who are vulnerable, and what to do when the QTc climbs.
Bottom Line Up Front
The 30-second version
- QT risk is cumulative, not about any single drug. The same medication is trivial in a healthy young patient and dangerous in a vulnerable one.
- Two numbers trigger action: a QTc >500 ms, or a rise of >60 ms above the patient's own baseline.
- Your fastest lever is potassium and magnesium. Correct them first, review every QT-prolonging drug on the list, then decide about the psychotropic.
What QTc Actually Is (60-Second Version)
The QT interval is the time it takes the ventricles to depolarize and then recover (repolarize). Because that duration shortens as heart rate rises, we correct it for rate — the QTc. Bazett's formula is the most common; Fridericia is more reliable at very fast or slow rates. Whatever you use, measure the same way each time so serial values are comparable.
When repolarization is delayed too far, the heart becomes vulnerable to torsades de pointes (TdP) — a polymorphic ventricular tachycardia that can cause syncope and, occasionally, degenerate into ventricular fibrillation and sudden death. The key clinical reality: most drug-induced QT prolongation is small and harmless. Danger emerges when the prolongation is marked, or when a modest effect lands on a patient whose "repolarization reserve" is already depleted by other factors.
The Numbers That Matter
| QTc | Interpretation | Response |
|---|---|---|
| < 450 ms (men) < 460 ms (women) | Normal | Proceed |
| 450–500 ms (men) 460–500 ms (women) | Prolonged / borderline | Optimize & recheck |
| > 500 ms | High TdP risk (roughly 2–3× baseline) | Act |
| Rise > 60 ms from baseline | High risk regardless of absolute value | Act |
Risk rises continuously, not at a cliff — in congenital long-QT cohorts, each additional 10 ms of QTc adds roughly a 5–7% exponential increase in TdP risk, meaning risk compounds multiplicatively rather than in a straight line. That figure comes from congenital LQTS data and may not translate directly to drug-induced prolongation in the general population, but the direction is the lesson: more QTc means disproportionately more risk. There is no value at which TdP is certain, and none below which it is impossible — the thresholds above are decision points, not guarantees.
The Core Principle: Risk Is Additive
This is the single idea worth carrying out of this chapter. A QT-prolonging drug rarely causes TdP on its own in a low-risk patient. TdP appears when multiple hits stack: the drug, plus low potassium, plus a second QT drug, plus bradycardia, plus impaired clearance, plus an older female patient. Each factor is a layer; TdP lives at the top of the pile.
So the question at the point of care is not "Is this drug safe?" — it's "What is this patient's total QT burden, and what can I subtract?" The same sertraline-or-citalopram decision that is unremarkable in a 30-year-old becomes a real conversation in an 80-year-old who is hypokalemic and already on methadone.
Patient Risk Factors (the Multipliers)
Scan this list before you prescribe a QT-prolonging agent. The more boxes a patient ticks, the lower your threshold for an ECG, electrolyte correction, or a lower-risk alternative.
- QTc already > 500 ms or rise > 60 ms
- Female sex
- Age > 65
- Bradycardia
- Heart failure with reduced EF
- Recent acute myocardial infarction
- Hypokalemia
- Hypomagnesemia
- Hypocalcemia
- Hepatic or renal impairment (higher drug levels)
- Congenital long-QT syndrome
- Prior drug-induced TdP
- Two or more QT-prolonging drugs
- CYP interactions raising drug levels
- Rapid IV administration
Which Psychotropics Carry the Most Risk (Orientation)
This chapter gives you the map, not the full drug-by-drug tables. At a high level:
| Tier | Examples (orientation only) |
|---|---|
| Highest concern CredibleMeds “known risk” | Thioridazine, pimozide (largely abandoned for this reason); IV haloperidol (route- and rate-dependent); TCAs (especially in overdose); citalopram (known TdP risk, dose-dependent — present at therapeutic doses, not just high doses). Methadone is the major non-“psych” offender clinicians forget. |
| Intermediate | Ziprasidone (greater QTc effect than most other atypicals, but classed as possible/conditional rather than known risk); iloperidone; escitalopram; other antipsychotics — effect varies by dose, route, and patient. |
| Generally lower | Most other SSRIs (e.g., sertraline); aripiprazole; lurasidone. "Lower" is not "zero" once risk factors stack. |
The canonical worked example — citalopram. The FDA caps citalopram at 40 mg/day, and at 20 mg/day for patients over 60, those with hepatic impairment, CYP2C19 poor metabolizers, or those on a CYP2C19 inhibitor (e.g., cimetidine, omeprazole) — all of which raise blood levels and therefore QT effect. It should be avoided in congenital long-QT syndrome, bradycardia, untreated hypokalemia or hypomagnesemia, recent MI, uncompensated heart failure, or alongside other QT-prolonging drugs. Notice that the dose rule is the additive principle in action: the ceiling drops precisely when other multipliers are present.
When to Get a Baseline ECG
There is no universal mandate, and local protocols vary — but a practical rule:
| When | Indication |
|---|---|
| Baseline — before starting | Get an ECG first if the patient carries one or more major risk factors:
|
| Recheck — during treatment | Repeat the ECG when:
|
| Usually not needed | A young, healthy patient starting a low-risk SSRI generally does not need routine ECG screening. |
The Action Ladder
QTc normal (< 450 / 460 ms)
Proceed. Routine care; no special monitoring required for low-risk patients.
QTc 450/460–500 ms (borderline)
Optimize the modifiable factors before anything else: correct potassium and magnesium to the AHA targets (K⁺ > 4.0 mEq/L, Mg²⁺ > 2.0 mg/dL), review and remove other QT-prolonging drugs where possible, consider a dose reduction or a lower-risk agent, then repeat the ECG.
QTc > 500 ms or rise > 60 ms
High risk. Stop or switch the offending agent unless there is a compelling reason to continue; correct electrolytes; place on telemetry; obtain cardiology input. If the QTc does not normalize after withdrawal, investigate for congenital long-QT syndrome.
Symptomatic or documented TdP
Syncope, palpitations, or a documented run of TdP means stop now and escalate — telemetry/acute care pathway, IV magnesium per protocol, cardiology. (Acute TdP management is covered in the Monitoring chapter; this is the trigger, not the protocol.)
Clinical Pearls
Pearls
- Fix electrolytes first. Potassium and magnesium are the fastest, most reversible levers. Magnesium is protective in TdP even when the serum level reads "normal."
- IV haloperidol ≠ oral haloperidol. Haloperidol is a known-risk agent for TdP, and intravenous administration carries a higher reported TdP incidence than oral — route and infusion rate matter, and rapid IV administration is itself a listed risk factor.
- Congenital LQTS hides in the history. Ask about unexplained syncope, "seizures," drowning, and family members with sudden or unexplained death.
- Impaired clearance silently raises the dose. Hepatic impairment and CYP inhibition (for example, omeprazole or cimetidine added to citalopram) push drug levels — and QT effect — up without changing the number on the prescription.
- A normal baseline is not a guarantee. Patients with reduced repolarization reserve can develop TdP from a starting QTc that looked entirely normal once enough factors stack.
Red Flags — Stop and Escalate
Stop the agent and get help
- QTc > 500 ms, or a rise > 60 ms above baseline
- Syncope, presyncope, or palpitations on a QT-prolonging drug
- Any documented torsades de pointes or polymorphic VT
- New, uncorrectable bradycardia or refractory hypokalemia during therapy
Patient Counseling Script
Plain-language script
"This medicine can slightly change the electrical timing of your heart. For most people that's harmless, but to be safe we'll check a quick heart tracing and your bloodwork. Please call us or seek care right away if you faint, feel dizzy, or notice your heart racing, pounding, or skipping beats. And let us know before you start any new medicine or antibiotic, because some of them add to the same effect — don't stop this medication on your own without talking to us first."
EMR / Documentation Template
References
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of Torsade de Pointes in Hospital Settings: A Scientific Statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060.
- Tisdale JE, Chung MK, Campbell KB, et al. Drug-Induced Arrhythmias: A Scientific Statement from the American Heart Association. Circulation. 2020;142(15):e214–e233.
- U.S. Food & Drug Administration. Drug Safety Communication: Abnormal heart rhythms associated with high doses of Celexa (citalopram hydrobromide). 2011 (revised 2012).
- Citalopram FDA prescribing information (current label, DailyMed).
- CredibleMeds (formerly AzCERT). QTdrugs lists. crediblemeds.org — the maintained reference for drug-specific TdP risk categories.
Last reviewed June 2026. This chapter is part of the Psychiatry Education Forum Academy and is for clinician education; it supports, and does not replace, individual clinical judgment and current local protocols.
You have the framework. Now get the drug-by-drug answers.
This chapter taught you how to weigh QT risk. The member chapters give you the tiered tables, dose ceilings, and route-specific cautions for every agent — the answers you reach for mid-clinic.
Educational use only. Refer to the sources cited above and current prescribing information for clinical decisions. Psychiatry Education Forum and authors assume no liability for use of this material.
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