In general cardioactive glycosides improve the efficiency of the heart muscle without increasing its need for oxygen. This enables the heart to pump adequate amounts of blood around the body and ensures that no fluid builds up in the lungs or extremities.
These powerful and fascinating constituents are found in various medicinal plants, specifically cardiac glycosides such as digitoxin and convallotoxin have a strong, direct action on the heart, supporting the rate of contraction when it is failing. They are also significantly diuretic, helping to stimulate urine production, thus increasing the removal of fluid from the tissues and circulatory system.
Cardiac glycosides are generally used to treat cardiac insufficiency, especially in atrial fibrillation & supra ventricular rhythm abnormalities, improving the ability of the myocardium (heart muscle) to perform it’s work. When taken internally they increase force of systole and the contraction of the myocardium, having a positive inotropic effect (increased force of contraction of the heart), a negative chronotropic effect (slowing the heart rate & reducing conduction velocity at the AV junction), increasing venous return, decreasing vasoconstriction and resistance to ventricular ejection, and increasing cardiac output overall without raising myocardial oxygen requirements.
At the cellular level, cardiac glycosides act on the cell membrane to inhibit sodium/potassium ATPase, resulting in an increase in intracellular calcium concentrations, thereby allowing for the greater force of heart muscle contraction. In addition there is a reduced activation of the renin-angiotensin-aldosterone system and a lower circulating level of catecholamines, thereby lowering blood pressure in the vasculature overall.
The use of herbs rich in cardiac glycosides requires extreme caution, as they have a narrow margin of safety and at higher doses can cause nausea, vomiting, diarrhea, yellowing of vision, confusion, headache, arrhythmia, ventricular extra systole, bradycardia, and AV block. The solubility and removal rates of these glycosides tends to be low, and high levels may easily accrue in the body becoming potentially quite dangerous. Their use requires frequent monitoring of diuresis, blood pressure, heart rate and serum potassium levels.
Drugs that reduce potassium levels in the blood (eg. diuretics, laxatives, corticosteroids, some antibiotics) increase the potency and potential toxicity of cardiac glycosides. Contraindications with concurrent drug use include: phenobarbitol, quinine, sulfasalazine, verapamil, amiodarone.
Note: In a strictly technical sense, the pharmacological term cardiotonic is synonymous with “positive inotropic”, and is used not only to describe agents that increase heart contractility, but also to indicate an increase in heart beat frequency and general cardiac performance. Specifically cardioactive plants are those that owe their effects on the heart to cardiac glycosides, thus possessing the strengths and drawbacks of these constituents.