She had not consumed other caffeine-containing substances. At autopsy, the patient was found to have sclerosis and myxoid change of the mitral valve leaflets. The substance is named after the Guarani Amazonian tribe, who originally used the seeds to brew a drink. In Brazil, guarana is commonly used as an astringent, a flavouring, and as a stimulant, and it is increasingly being used in similar products internationally. Such products are widely available at pharmacies and natural food outlets, and are marketed as natural sources of energy.
Caffeine is a natural alkaloid methylxanthine.
Caffeine is metabolised by the P450 hepatic enzyme system. The pharmacodynamic profile of caffeine is similar to that of theophylline, another methylxanthine, in that it inhibits the adenosine receptor and acts as a phosphodiesterase inhibitor. At high serum levels, enzyme saturation occurs, and elimination follows zero-order kinetics, with constant elimination regardless of serum level. 12 Caffeine increases intracellular calcium concentrations, causes noradrenaline release and sensitises dopamine receptors. It relaxes smooth muscle, stimulates skeletal muscle, has a weak diuretic action, and its metabolic effects include hypoglycaemia.