Introduction to CV Pharmacology - Orlando Health

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CV Pharmacology Positive Inotropic Therapy Digoxin Our discussion of digitalis preparations will be limited to digoxin (Lanoxin, Digitek®) which is the most commonly prescribed form of digitalis. Digoxin is a positive inotropic agent as it increases the strength of myocardial contraction. Digoxin also decreases automaticity and prolongs electrical conduction through the AV node. Indications Digoxin is used as maintenance therapy for symptom control in patients with heart failure to increase myocardial contractility, stroke volume, and cardiac output. It also is used to control the ventricular rate with atrial fibrillation or atrial flutter by slowing AV nodal conduction. Note: For initial ventricular rate control in atrial fibrillation, IV calcium channel blockers and beta-blockers are used before digoxin due to the length of time digoxin takes to produce a slowing of the ventricular rate. Precautions and Interactions Digoxin has a very narrow therapeutic range and toxicity can develop rapidly. Symptoms of digoxin toxicity include nausea, vomiting, diarrhea, visual disturbances, and changes in mental status and level of consciousness. Toxicity also causes a wide variety of dysrhythmias, including heart blocks, junctional rhythms, and ventricular dysrhythmias. Toxicity is precipitated by hypokalemia, hyperkalemia, and hypomagnesemia. Many patients taking digoxin for heart failure also take diuretics that can deplete potassium and lead to toxicity. Be sure to provide patient education concerning the need to take any prescribed potassium supplements if the patient is on diuretic therapy. Digoxin is potentiated in such drugs as amiodarone, calcium channel blockers, and quinidine. In patients taking these drugs, digoxin should be used with caution and the dose might be reduced by 50%. Overdose of digoxin can be treated by the drug Digibind ® . Clinical Application Digoxin should be avoided with those patients with sick sinus syndrome or significant atrioventricular nodal heart blocks such as complete heart block and second degree AV block. Dobutamine Dobutamine (Dobutrex) is a synthetic catecholamine. It is a potent β 1 agonist to increase myocardial contractility and cardiac output. It is also a mild alpha agonist; however, the alpha effects are counteracted by the more potent β 2 properties, resulting in a mild vasodilation effect. This vasodilation will decrease preload and afterload with a beneficial effect on myocardial oxygen supply and demand. At doses of 2–20 mcg/kg/min, it has only a minimal effect on the heart rate. By increasing the cardiac output in the presence of mild vasodilation, it will increase coronary, renal and mesenteric perfusion. 2010 Orlando Health, Education & Development 39
CV Pharmacology Indications Dobutamine is used to increase myocardial contractility and cardiac output in patients with left ventricular systolic dysfunction. By increasing cardiac output, dobutamine may result in an increase in arterial blood pressure. It is used concomitantly with volume loading in the presence of right ventricular myocardial infarction. It is commonly used with moderate dose dopamine (5- 10 mcg/kg/minute) to increase blood pressure and enhance organ perfusion rather than using higher doses of dopamine and risking the adverse effects of dopamine. Precautions and Interactions Dobutamine may cause headache, nausea, tremors, and hypokalemia. Remember that hypokalemia may increase ventricular irritability and precipitate digoxin toxicity, so patients receiving dobutamine should be monitored for hypokalemia. While dobutamine may induce tachydysrhythmias and myocardial ischemia, it does so to a much lower extent than other catecholamines. Phosphodiesterase (PDE) Inhibitors PDE’s inhibit the breakdown of cyclic AMP (cAMP) in cardiac and peripheral vascular smooth muscle, resulting in potent inotropic effects while dilating both arteries and veins. The net effect is similar to dobutamine. By increasing contractility and causing Examples of Drugs: vasodilation, the PDE inhibitors reduce preload and afterload and thus decrease myocardial oxygen demand. milrinone (Primacor) Note: Since is rarely used, our discussion will focus on inamrinone (Inocor) milrinone. Inamrinone is seldom used due its significant problems amrinone with thrombocytopenia. Indications PDE inhibitors are used for patients with heart failure or systolic dysfunction cardiomyopathy. These agents are more effective in heart failure and cardiomyopathy than they are in the presence of ischemic heart disease. Precautions and Interactions Due to the potent vasodilation properties, hypotension can occur. Likewise, an increase in myocardial irritability may occur inducing both supraventricular and ventricular dysrhythmias. Although, Milrinone may cause a mild thrombocytopenia, the incidence is far lower than with inamrinone. Milrinone may also cause mild to moderate headaches. © 2010 Orlando Health, Education & Development 40
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