Medicine-induced QT prolongation and torsades de pointes

With every heartbeat the muscle of the lower chambers contracts, then resets electrically before the next beat. An ECG records this as a trace, and the QT interval is the time from the start of the contraction signal to the end of the reset. If the reset takes too long, the heart is briefly vulnerable to a disorganised extra beat that can start a chaotic rhythm.

That rhythm is called torsades de pointes, a French phrase for twisting of the points, from the characteristic pattern on the trace. It may stop by itself, causing a faint or a seizure-like episode, or it may continue and cause cardiac arrest.

Heart cells reset by letting potassium flow out through specialised channels. Many medicines partially block those channels as an unintended side effect, slowing the reset. The NHS lists some antibiotics, antihistamines, antidepressants and heart medicines as causes of acquired QT prolongation, in contrast with the inherited form, which is genetic. Other examples recognised by clinicians include some antipsychotics, some anti-sickness medicines and certain antimalarials.

The effect is usually modest and dose-related. A medicine on its own may lengthen the interval only slightly. Danger arises when several factors line up, such as two QT-prolonging medicines together, a high dose, a medicine that slows the clearance of another, or a low potassium level after vomiting, diarrhoea or diuretics.

The MHRA's advice on citalopram and escitalopram, which illustrates the principles, describes greater risk in people over 65, those with low potassium or magnesium, people with pre-existing heart conditions, women, and people who take other QT-prolonging medicines. It stresses that the effect is dose-dependent. Slow heart rate, an inherited long QT tendency, a family history of sudden unexplained death and severe liver disease are other recognised contributors.

The NHS adds that acquired prolongation can also arise from other cardiac conditions, brain bleeds or strokes and severe deficiencies of potassium, magnesium or calcium, so the medicine is not always the only factor.

• Ask before combining any new prescription with an antibiotic, antihistamine or anti-sickness medicine if you already take an antidepressant, antipsychotic or heart rhythm medicine.

• Report vomiting or diarrhoea lasting more than a day if you take a diuretic, since electrolytes can fall.

• Mention any family history of fainting or sudden death in young relatives.

The NHS lists palpitations, fainting, seizures and potentially life-threatening cardiac arrest as manifestations, noting that exercise, emotional stress and sudden loud noises can trigger episodes in the inherited form. A person with acquired QT prolongation may have no symptoms at all until a rhythm disturbance occurs, which is why prescribers check at risk times.

An electrocardiogram detects the prolonged QT interval. Further tests may include 24-hour heart rhythm monitoring and stress ECGs, and blood tests for potassium, magnesium and calcium. The MHRA advice for citalopram recommends baseline ECG in people with cardiac disease, monitoring of electrolytes particularly in older patients on diuretics, and treatment review at defined ECG thresholds. Your clinic will tell you what applies to the specific medicine.

Treatment starts by dealing with the cause. The NHS says that if prolongation is medication-induced, stopping the offending drug can resolve it. In practice the prescriber considers whether the medicine can be stopped, lowered or substituted, corrects low potassium or magnesium and removes interacting medicines. Do not stop heart or psychiatric medicines yourself, because abrupt changes carry their own risks.

Ongoing prevention includes avoiding strenuous exercise during an unstable period if advised, avoiding dehydration and avoiding high-caffeine energy drinks, and consulting a healthcare professional before taking any new medicine or supplement, as the NHS recommends. In persistent cases beta blockers may be used to stabilise the rhythm, and an emergency plan may be agreed.

Call 999 for fainting that is unexplained, a seizure, collapse or someone who is unresponsive and not breathing normally. The NHS says to contact emergency services for unconsciousness, seizures or cardiac arrest. For recurrent palpitations or fainting spells, seek an urgent GP assessment. The NHS citalopram page lists a fast, irregular heartbeat with light-headedness and fainting as a serious side effect and says to call 111 if you think you are having serious side effects.