Various cardiac glycosides are found in a number of plant species. The term digitalis is often used to refer to the whole group of cardiac glycosides. In 1785 William Withering published his article concerning the use of foxglove in oedema. Subsequently the power of other cardiac glycosides was observed. During his exploration of the Zambesi in the 1860s, Livingstone observed the use of a poison so powerful that an arrow smeared with it, could kill a cape buffalo or a hippopotamus. The botanist John Kirk tried to find out where the poison came from, but no one would tell him. In the end he found that the poison originated from the seeds of a climbing plant, Strophanthus kombe (Apocynaceae). These were ground and applied to the arrow tip with an adhesive prepared from a spurge-like plant. From these seeds Fraser, a pharmacologist from Edinburgh, isolated strophantine, the active cardiac glycoside. The mechanism of action is similar to that of digoxin. Ouabain is obtained from Strophanthus hispidus and related species. Digitalis for pharmacological use is obtained from the leaves of Digitalis lanata and digitoxin from Digitalis purpurea (Scrophulariaceae). The content of cardiac glycoside varies from 0.1 to 1%, depending on the site of growth, the growth season and the local microclimate in which the plant grew.

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Every glycoside is a combination of an aglycon with one to four molecules of sugar. The aglycons are released from the glycoside by hydrolysis. They are chemically related to bile salts and steroid hormones. The pharmacological activity is found in the aglycon part. The sugar groups change the water and fat solubility of the molecule. The aglycons of digoxin and digitoxin are digoxigenin and digitoxigenin respectively. Digoxin has a positive inotropic effect and slows the ventricular frequency during atrial fibrillation. Digoxin is excreted chiefly via the kidneys (the normal half-life is 36 hours). The half-life of digitoxin is 7 days.

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In Sri Lanka at present the majority of suicides are due to eating oleander seeds (Thevetia peruviana, Thevetia nerifolia, Nerium oleander). Common oleander and yellow oleander are attractive plants with beautiful flowers. They are widely distributed and are often planted around gardens. The plants contain high concentrations of nerioside and oleandroside, similar to digoxin and digitoxin. These are examples of substances in which the distinction between medicament and poison is very narrow. Just a few seeds can lead to death. Cases have also been reported in which people became ill after the use of oleander twigs as the central pin in barbecue meat. The clinical picture is that of an acute digitalis intoxication: nausea, vomiting, anorexia, diarrhoea, confusion, disturbed vision and cardiac rhythm problems. On an ECG, conduction disturbances are observed (AV-block, ventricular arrhythmia). The toxic effects are higher if hypokalaemia or hypomagnesaemia are present. The treatment consists of prevention of further absorption of the poison (gastric lavage, introduction of activated charcoal) and correction of electrolyte deficiencies. Therapy with cardiac pacing and anti-digoxin Fab fragments is very expensive. These antibodies are stimulated in sheep by immunising the animals with a digoxin conjugate. They are stored as lyophilisate (DigiFab®).

Another type of cardiotoxic toxin is cocaine. Cocaine use accounts for a substantial percentage of all deaths secondary to illicit drug use (e.g. 10% in Australia in 1998). Cocaine is extracted from the leaves of the plant Erythroxylon coca, and is available as cocaine hydrochloride (a water-soluble powder or granule which can be taken orally, intravenously or intranasally) and as "freebase" or "crack" cocaine (heat stable, melting at high temperatures) which can be smoked. Acute myocardial infarction due to coronary spasms and cardiomyopathy are the most commonly reported cardiac consequences of cocaine misuse, usually occurring in men who are young, fit and otherwise healthy. Cocaine effect should be seriously considered in any young patient with minimal risk factors for cardiac disease presenting with angor pectoris, acute myocardial infarction, dilated cardiomyopathy, myocarditis or cardiac arrhythmias.