Prolonged QTc on an ECG: What Does It Mean and What Happens Next?

A prolonged QTc on an ECG means the electrical recharging phase of the heart's lower chambers looks longer than expected for your heart rate. It is a measurement, not a diagnosis. It can come from medicines, low blood salts, other heart problems or an inherited condition, and it can sometimes raise the risk of a dangerous fast heart rhythm.

This guide is for adults who have seen "prolonged QT" or "long QTc" on an ECG report. It explains what the measurement is and what to ask. It cannot tell you whether your result is significant; that needs a clinician who knows your history, medicines and full tracing.

Call your local emergency number if someone collapses and is not breathing normally, has a seizure, or has fainted and cannot be woken within about a minute. The NHS lists these as emergencies in long QT syndrome because they can signal a dangerous heart rhythm.

Seek urgent same-day advice if any of the following happen, even if the person seems to recover fully.

• Fainting or a seizure, especially during exercise, swimming, a sudden fright, a loud noise such as an alarm, or sleep.
• Palpitations that keep coming back, last several minutes or are getting worse.
• Noisy gasping during sleep, which NHLBI lists as a possible symptom.
• A parent, child, brother or sister who has been diagnosed with long QT syndrome or died suddenly and unexpectedly, particularly at a young age.

On an ECG, the QT interval runs from the start of the Q wave to the end of the T wave. It reflects the time the lower chambers (ventricles) take to fire and then recharge. NHLBI describes a normal QT as roughly one third of each heartbeat cycle.

The QT naturally shortens when the heart beats faster and lengthens when it slows. To compare readings fairly, it is adjusted for heart rate and reported as the QTc, measured in milliseconds. Different correction formulas exist, and automated machine readings can differ from a careful manual measurement, so a clinician often re-checks the value on the tracing.

There is no single cut-off used everywhere. UK Specialist Pharmacy Service guidance gives upper limits of normal of about 450 ms for men and 460 ms for women, with values above 500 ms linked to a high risk of a dangerous rhythm called torsade de pointes. A rise of 20 ms or more from a person's own earlier ECG can also be significant.

Other guidelines and laboratories use slightly different numbers, and a single value is only one part of the picture. Clinicians also weigh symptoms, heart rate, blood salts, medicines, family history and the shape of the T wave.

Many medicines can lengthen the QT interval, and the effect usually depends on the dose, how long it is taken and whether several such medicines are combined. NHS sources name broad groups rather than a fixed list.

• Some antibiotics, including macrolides and fluoroquinolones, and some azole antifungals.
• Some antidepressants and antipsychotics.
• Some anti-sickness medicines.
• Some antihistamines and some heart medicines.
• Medicines that lower potassium or magnesium, such as some water tablets (diuretics) or long-term laxative use, which can add to the risk indirectly.

Yes. Very low potassium, magnesium or calcium can prolong the QT interval. NHLBI notes that eating disorders, thyroid disease and severe diarrhoea or vomiting can lower these levels. Doctors often check blood salts and thyroid tests when a long QTc is found.

Other listed contributors include a slow heart rate, heart failure or other structural heart disease, older age, female sex, reduced liver or kidney function, and a recent stroke or bleed in the brain. A slow rhythm may also appear on the same ECG; see our guide to sinus bradycardia for that finding.

Some people are born with gene changes that affect the heart's ion channels. The NHS describes inherited genes as the most common cause of long QT syndrome. MedlinePlus Genetics estimates the most common inherited form affects about 1 in 2,000 people, and notes that some people with the gene change never have symptoms.

NHLBI notes that symptoms of inherited long QT often appear during stress or sleep and usually start in childhood; first symptoms after age 40 are rare. Because the QT interval can vary from day to day, one normal or borderline ECG does not settle the question.

Inherited long QT may be found on an ECG done for another reason. Assessment can include repeat ECGs, a 24-hour monitor, an exercise ECG and genetic testing. If it is confirmed, close relatives are usually offered checks because they may carry the same change.

Bring a full list of prescription medicines, over-the-counter products and supplements, including recent changes. Do not stop or change a prescribed medicine yourself; ask the prescriber or a pharmacist first. The NHS advises people with long QT syndrome to check with a doctor or pharmacist before taking any new medicine or supplement, and to avoid becoming dehydrated during vomiting or diarrhoea.

• What was my QTc, which correction was used, and was it checked by hand as well as by the machine?
• Do any of my current medicines or supplements prolong the QT interval, and should anything be reviewed?
• Have my potassium, magnesium, calcium and thyroid levels been checked?
• Should the ECG be repeated, and do I need a heart monitor, exercise test or genetic testing?
• Does my family history matter here, and should relatives be checked?
• Is there anything I should avoid, and which symptoms mean I should seek urgent help?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a long QTc on an ECG were assessed, such as which blood tests, medicine reviews or monitoring were discussed, can help you prepare clearer questions for your own appointment.

Other people's cases cannot show what your ECG means or whether your medicines are safe for you. Use them to plan the conversation, not to decide treatment. Related findings such as extra beats are covered in our guide to PVCs on an ECG.