Digitalis glycosides cardiac effects

Cardiac steroids, or cardenoUdes, are steroid glycosides. Their effect has been known since the time of the ancient Egyptians. In more recent times, the foxglove (Digitalis purpurea) and its effect were described in 1785 by William Withering, who knew of its use in folk medicine. 

Chemicals can also interfere with the removal of the signal such as the level of a cation inside the cell. The signal is terminated by export of the cation through channels or transporters. Inhibition of these processes will prolong the signal and therefore the effect. Digitalis glycosides, which inhibit the Na+K+ ATPase so that Na+ levels rise in cardiac cells, is an example (see chap. 7). 

Adverse effects Thiazide diuretics induce hypokalemia and hyperuricemia in 70% of patients, and hyperglycemia in 10% of patients. Serum potassium levels should be monitored closely in patients who are predisposed to cardiac arrhythmias (particularly individuals with left ventricular hypertrophy, ischemic heart disease, or chronic congestive heart failure) and who are concurrently being treated with both thiazide diuretics and digitalis glycosides (see p. 160). Diuretics should be avoided in the treatment of hypertensive diabetics or patients with hyperlipidemia. 

The risk of digitalis toxicity during the therapeutic use of cardiac glycosides is similar in children to that in adults, ranging in 12 separate published series from 12 to 50% (median 21%) (141). The most common non-cardiac effects are vomiting and feeding problems, and the most common cardiac effects are conduction defects, particularly atrioventricular block and ectopic rhythms, although (as in adults) any dysrhythmia can occur. 

The digitalis glycosides interfere with the Na, K -ATPase pump with a resultant intracellular loss of potassium and intracellular increases in sodium and calcium. The net effects of this are increased myocardial contractility and decreased cardiac conduction. 

Alteration of the movement of ions, such as potassium, in heart tissue may be a toxic response to some foreign compounds. For example, the digitalis glycosides cause changes in tissue potassium and this may lead to serious cardiac effects. Diuretics may also cause low plasma potassium with potentially dangerous effects on heart function. 

To better understand certain aspects of the mechanism of digitalis drugs, it would be useful to outline briefly their cardiovascular properties. The increased force of myocardial contraction produced by these glycosides is by far their most dramatic pharmacodynamic property. This positive inotropic (increased contractile force) action translates into increased cardiac output and effects on cardiac size and blood volume through diuresis (i.e., the relief of the edema that accompanies CHF). The rate of tension development is apparently affected, not the length of time during which contraction is maintained by the muscle fiber. Digitalis exerts its effect even in the presence of p-blockers or reserpine. 

What is the effect of digitalis glycosides on the cardiac action potential ... 

Glycosides, known as digitalis glycosides, inhibit the sodium-potassium pump and increase intracellular calcium. This causes the cardiac muscle fibers to contract better. Glycosides have three effects on the heart ... 

V. Drug or laboratory interactions. The positive inotropic effects of inamrinone are additive with other inotropic agents, including digitalis glycosides. These drugs can be used together, but the patient should be monitored for cardiac dysrhythmias. 

Phenytoin has a stabilising effect on the myocardial cells so that the toxic threshold of digoxin at which arrhythmias occur is raised. However, the bradycardic effects of the digitalis glycoside are not opposed and the lethal dose is unaltered, so that the cardiac arrest reported would appear to be the result of excessive bradycardia. It seems possible that the fall in plasma digitoxin levels may be due to a phenytoin-induced increase in the metabolism of the digitoxin by the liver. ... 

The effects of digitalis glycosides might be increased by rises in blood calcium levels, and the use of intravenous calcium may result in the development of potentially life-threatening digitalis-induced cardiac arrhythmias. Teriparatide appears not to affect the calcium-mediated pharmacodynamics of digoxin. 

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