Patients with hypertension or long QT syndrome should limit their intake of energy drinks, say the authors of a small study that has found they affect heart rhythm and blood pressure.

But caffeine is unlikely to be to blame for risky heart rhythms, with the responsible ingredient yet to be identified, researchers report in the Journal of the American Heart Association.

With 34 recorded deaths in the US from energy drinks, the researchers randomly assigned 34 healthy participants, median age of 22, to drink 946mL (32oz) of one of two energy drinks (304-320mg total caffeine) or a placebo drink.

They found the QT interval (corrected for heart rate) of participants who had downed one of the energy drinks had increased by 6 milliseconds after four hours.