An article in the European Heart Journal, investigators report that patients taking non-cardiac drugs that prolong the QT interval experience a threefold increase in the risk of sudden death. The drugs they studied were the antibiotics erythromycin and clarithromycin; the antipsychotic drugs chlorpromazine, haloperidol, and pimozide; and cisapride and domperidone, drugs used for gastrointestinal disorders. All of these drugs are prescribed fairly often.
The investigators, working in the Netherlands, reviewed 775 instances in which individuals died suddenly, as well as 6000 matched individuals who served as controls. They found that the use of these QT-interval prolonging drugs was associated with three times the risk of sudden death. They further estimated that the use of these drugs is associated with 15,000 sudden deaths each year, worldwide.
The fact that these particular drugs, and similar drugs that prolong the QT interval, increase the risk of sudden death is not new. The new information provided in this report is the quantification, for the first time, of the magnitude of that increased risk. A threefold increase is certainly significant. Still, the baseline risk of sudden death is about one sudden death per thousand individuals each year; so the risk for patients taking one of these drugs is three per thousand per year - significantly higher, but still quite low.
What the authors do not discuss is the fact that the risk of fatal arrhythmias from drugs that increase the QT interval varies from person to person. That is, some individuals are quite susceptible to anything that prolongs the QT interval, while others are not. Genetic markers are being recognized that identify many of these susceptible individuals, and in the foreseeable future we will be able to do genetic screening to identify those who ought to avoid the QT prolonging drugs. Certainly, for now, anyone who has had arrhythmias associated with the long-QT syndrome, as well as closely related family members, should avoid drugs that prolong the QT interval.
The Arizona Center for Education and Research on Therapeutics maintains a list of drugs that often prolong the QT interval, which can be found here.