McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

Therapeutic actions and indications As noted previously, the anticoagulants interfere with the normal cascade of events involved in the clotting process. Warfarin causes a decrease in the production of vitamin K–dependent clotting factors in the liver. The eventual effect is a depletion of these clotting factors and a prolongation of clotting times. It is used to maintain a state of anticoagulation in situations in which the person is susceptible to potentially dangerous clot formation (see Table 48.1 for usual indications for warfarin). See the Critical thinking scenario for additional care for people taking warfarin. Heparin, bivalirudin, apixaban, dabigatran and rivaroxaban block the formation of thrombin from pro­ thrombin. The usual indications for heparin include acute treatment and prevention of venous thrombosis and pulmonary embolism; treatment of atrial fibrilla­ tion with embolisation; prevention of clotting in blood samples, and in dialysis and venous tubing; and diagnosis and treatment of disseminated intravascular coagulation (DIC) (Box 48.3). Because heparin must be injected, it is often not the drug of choice for outpatients, who would be responsible for injecting the drug several times during the day. Individuals may be started on heparin in the acute situation and then switched to the oral drug warfarin. Apixaban, dabigatran and rivaroxaban are commonly used to prevent thrombosis following total hip or knee replacement and in individuals with atrial fibrillation. Antithrombin interferes with the formation of thrombin from prothrombin; it is a naturally occur­ ring anticoagulant, as mentioned earlier, and a natural safety feature in the clotting system. Fondaparinux is the newest of the anticoagulants, approved in 2002. It inhibits factor Xa and blocks the clotting cascade to prevent clot formation. It is supplied in prefilled syringes, making it convenient for people who self-administer the drug at home.

■■ BOX 48.3  Understanding disseminated intravascular coagulation

Disseminated intravascular coagulation (DIC) is a syndrome in which bleeding and thrombosis are found together. It can occur as a complication of many problems, including severe infection with septic shock, traumatic childbirth or missed abortion, and massive injuries. In these disorders, local tissue damage causes the release of coagulation-stimulating substances into the circulation. These substances then stimulate the coagulation process, causing fibrin clot formation in small vessels in the lungs, kidneys, brain and other organs. This continuing reaction consumes excessive amounts of fibrinogen, other clotting factors and platelets. The end result is increased bleeding. In essence, the person clots too much, resulting in the possibility of bleeding to death. The first step in treating this disorder is to control the problem that initially precipitated it. For example, treating the infection, performing dilation and curettage to clear the uterus, or stabilising injuries can help stop this continuing process. Whole-blood infusions or the infusion of fibrinogen may be used to buy some time until the person is stable and can form clotting factors again. There are associated problems with giving whole blood (e.g. development of hepatitis or AIDS), and there is a risk that fibrinogen may set off further intravascular clotting. Paradoxically, the treatment of choice for DIC is the anticoagulant heparin. Heparin prevents the clotting phase from being completed, thus inhibiting the breakdown of fibrinogen. It may also help avoid haemorrhage by preventing the body from depleting its entire store of coagulation factors. Because heparin is usually administered to prevent blood clotting, and the adverse effects that are monitored with heparin therapy include signs of bleeding, it can be a real challenge for the healthcare providers to feel comfortable administering heparin to a person who is bleeding to death. Understanding of the disease process can help alleviate any doubts about the treatment.

CRITICAL THINKING SCENARIO Oral anticoagulant therapy

THE SITUATION G.R. is a 68-year-old woman with a history of severe mitral valve disease. For the last several years, she has been able to manage her condition with digoxin, a diuretic and a potassium supplement. However, on a recent visit to her doctor she disclosed that she had been experiencing

periods of breathlessness, palpitations and dizziness. Tests showed that she was having frequent periods of atrial fibrillation (AF), with a heart rate of up to 140 beats/ minute. Because of the danger of emboli as a result of her valve disease and the bouts of AF, warfarin therapy was begun.