The research was published in HeartRhythm, the official journal of the Heart Rhythm Society and the Cardiac Electrophysiology Society.
The QT interval is the time it takes the heart muscle to recharge between beats. Over 200 commonly used medications prolong the QT interval including not only antiarrhythmic drugs, but medications with no cardiac indications such as some antibiotics, antihistamines, and antipsychotic drugs.
Investigators have now confirmed the QT-prolonging effects of grapefruit juice and call for a stronger warning to patients taking such drugs.
"With so many drugs, of such varied composition blocking the IKr channel, it is reasonable to assume that food compounds may also have IKr-channel-blocker properties, raising the possibility that 'proarrhythmic food' exists," said Dr Sami Viskin, MD, Department of Cardiology, Tel Aviv Sourasky Medical Center and Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel, who led the study.
The drugs work by blocking a specific "IKr" potassium channel on the cardiac muscle cell membrane and prolonging the repolarization in the heart ventricles.
"Previous studies showed that flavonoid compounds contained in grapefruit juice have IKr channel- blocking properties. We therefore tested the possibility that grapefruit juice has QT-prolonging properties," said said Dr Sami Viskin.
Thirty healthy volunteers and ten patients with congenital long QT syndrome took part in the study, which confirmed that grapefruit juice prolongs the QT interval.
Dr Viskin and his colleagues conclude that stronger warnings should be given against drinking grapefruit juice in large quantities for patients taking cardiac or non-cardiac medications that prolong the QT interval.
The study does not imply that daily consumption of grapefruit juice is risky for the general population, and grapefruit juice is low in calories, high in nutrients, promotes appetite controls and has been shown to aid weight loss.
"The net increase in QT interval caused by grapefruit among healthy volunteers was small, but in the range that, if grapefruit juice were a new drug in development, the results of the present study would probably lead the FDA to call for additional studies before issuing a final recommendation based on its expected benefits and risks," commented Dr Viskin.
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