McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 4 4 Cardiotonic agents

Inotropic drugs work here; cardioglycosides increase Ca + levels and myocardial contractility Phosphodiesterase increases cAMP in cell and may prolong sympathetic effects

Myocardial function or failure

Cardiac output

Nitrate vasodilators work here to dilate vessels and reverse compensation Adrenergic blockers work here to block sympathetic compensation

Blood flow to kidneys

Baroreceptor stimulation

ACE inhibitors work here to block angiotensin II effects

Activation of renin-angiotensisn system

Sympathetic activation

BP

P

BP

+ Inotropic effect Cardiac output Respirations

Cardiac output

Aldosterone synthesis and release

Human B-type natriuretic works to sensitivity peptide to stress response and dilate blood vessels Diuretics work here to urine productions and Na levels

ADH release

H 2 O retention K + retention Na + retention

Blood volume

FIGURE 44.4  Sites of action of drugs used to treat heart failure (HF).

Cardiac workload

including heart block, atrial arrhythmias and ventricu­ lar tachycardia. This can be a life-threatening situation. A digoxin antidote, digoxin immune Fab , has been developed to rapidly treat digoxin toxicity (Box 44.3). See the Critical thinking scenario for additional infor- mation about inadequate digoxin absorption . Clinically important drug–drug interactions There is a risk of increased therapeutic effects and toxic effects of digoxin if it is taken with verapamil, amiodar­ one, quinidine, quinine, erythromycin, tetracycline or cyclosporin. If digoxin is combined with any of these drugs, it may be necessary to decrease the digoxin dose to prevent toxicity. If one of these drugs has been part of a medical regimen with digoxin and is discontinued, the digoxin dose may need to be increased. The risk of cardiac arrhythmias could increase if these drugs are taken with potassium-losing diuretics. If this combin­ ation is used, the person’s potassium levels should be checked regularly and appropriate replacement done. Digoxin may be less effective if it is combined with thyroid hormones, metoclopramide or penicillamine, and increased digoxin dose may be needed. Absorp­ tion of oral digoxin may be decreased if it is taken with cholestyramine, charcoal, colestipol, antacids, bleomy­ cin, cyclophosphamide or methotrexate. If it is used in combination with any of these agents, the drugs should

excreted through the kidneys and toxic levels could develop ; and electrolyte abnormalities (e.g. increased calcium, decreased potassium, decreased magnesium), which could alter the action potential and change the effects of the drug. Digoxin should be used cautiously in women who are pregnant or breastfeeding because of the potential for adverse effects on the fetus or neonate. It is not known whether digoxin causes fetal toxicity; it should be given during pregnancy only if the benefit to the mother clearly outweighs the risk to the fetus. Digoxin does enter breast milk, but it has not been shown to cause problems for the neonate. Caution should be used, however, during breastfeeding. Children and older people are also at higher risk (Box 44.2). Adverse effects The adverse effects most frequently seen with the cardiac glycosides include headache, weakness, drowsiness and vision changes (a yellow halo around objects is often reported). Gastrointestinal upset and anorexia also commonly occur. Arrhythmias may develop because the glycosides affect the action potential and conduc­ tion system of the heart. Digoxin toxicity is a serious syndrome that can occur when digoxin levels are too high. The person may present with anorexia, nausea, vomiting, malaise, depression, irregular heart rhythms