Cardiac glycosides are similar in their chemical structure to bile acids, cortisone, vitamins, hormones, cholesterol. These substances are natural metabolites that support the normal functioning of the heart.
Cardiac glycosides are composed of glycon (sugar) and aglycon. It is aglycone that acts as a determining factor in the pharmacological activity of the drug. Reacting with other active substances provides an unsaturated five-membered ring in their structure.
Cardiac glycosides are distributed unevenly across tissues and organs. The target of their action is the myocardium, which accumulates only 10% of the drug, another 50% is fixed in the striated muscles, where they do not cause any effect.
According to the duration of action, cardiac glycosides are divided into:
- long-term - digitoxin,
- medium - digoxin,
- short - strophanthin, korglikon.
Cardiac glycosides: mechanism of action
In a normal state, a cardiomyocyte contains 30 times more potassium and 20 times less sodium. At rest, the outer surface of the cell membrane is positively charged, and the inner - on the contrary, negatively. Upon excitation, membrane depolarization is caused: sodium and calcium enter the cell. During this process, potassium cannot pass through the cell membrane. As soon as they exit the cell, the process of repolarization begins. This moment occurs immediately before muscle contraction. Thus, the amount of calcium ingested provides cell contraction. Potassium ions remaining in the cell are antagonists of this process.
Cardiac glycosides accelerate the onset of repolarization and provide maximum potassium output from cardiomyocytes. After reduction, it is necessary to restore the balance of ions inside and outside the cell (potassium return and sodium yield). This process depends on the energy supply of the cell, is provided by the enzyme ATPase. Its active center is the sulfhydryl group. Cardiac glycosides block the active center, reducing activity. All this leads to the fact that sodium ions remain in the cell, and this accelerates the onset of the next depolarization. If the content of potassium ions decreases significantly, the synthesis of creatinine phosphate is disrupted, which leads to a decrease in the force of contraction of the heart muscle.
Cardiac glycosides: contraindications
Absolute contraindications to their use are: atrioventricular block II degree, sinus bradycardia, allergic reactions to these drugs, intoxication with them.
Relative contraindications include the following: atrial bradyarrhythmia, grade I AV block, sinus node weakness syndrome, coronary heart disease, sinus bradycardia, hypertonic heart, hypercalcemia, hypokalemia, a history of renal or pulmonary insufficiency .
It is inappropriate to use cardiac glycosides in the following conditions: amyloidosis of the heart, diastolic overload of the heart with aortic insufficiency, thyrotoxicosis, acute myocardial infarction, cardiomyopathy, constrictive pericarditis, heart failure while maintaining high cardiac output, anemia, and so on.
The contraindications themselves are directly due to the lack of systolic dysfunction, which explains the inefficiency of using cardiac glycosides. And, for example, with the development of acute myocardial infarction, their use can provoke the development of arrhythmias, which gives an unfavorable prognosis for the future life of the patient.
Before using cardiac glycosides, be sure to consult a specialist. This will help prevent the development of serious complications.