McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 4 8 Drugs affecting blood coagulation

■■ TABLE 48.2 Review of clotting studies Test Measure

Therapeutic range

Uses

Activated partial thromboplastin time (aPTT) Partial thromboplastin time (PTT) International Normalised Ratio (INR)

Activity of intrinsic pathway of coagulation Standardised measure of prothrombin levels Time required for clotting to occur; extrinsic pathway activity

1.5–2.5 times baseline Dose adjustment for heparin, low-molecular-weight heparins, bivalirudin

2–3.5

Warfarin dose adjustment, fondaparinux dose adjustment

Prothrombin time (PT)

1.3–1.5

Warfarin dose adjustment

Safe medication administration

Safe medication administration

Injectable vitamin K is used to reverse the effects of warfarin. Vitamin K promotes the liver synthesis of several clotting factors. When these pathways have been inhibited by warfarin, clotting time is increased. If an increased level of vitamin K is provided, more of these factors are produced, and the clotting time can be brought back within a normal range. Because of the way in which vitamin K exerts its effects on clotting, there is a delay of at least 24 hours from the time the drug is given until some change can be seen. This occurs because there is no direct effect on the warfarin, but rather an increased stimulation of the liver, which must then produce the clotting factors. The usual dose for the treatment of anticoagulant-induced prothrombin deficiency is 2.5 to 10 mg intramuscularly (IM) or SC or, rarely, 25 mg IM or SC. Oral doses can be used if injection is not feasible. A prothrombin time (PT) response within 6 to 8 hours after parenteral doses or 12 to 48 hours after oral doses will determine the need for a repeat dose. If a response is not seen and the person is bleeding excessively, fresh-frozen plasma or an infusion of whole blood may be needed. Prototype summary: Heparin Indications: Prevention and treatment of venous thrombosis and pulmonary emboli; treatment of atrial fibrillation with embolisation; diagnosis and treatment of DIC; prevention of clotting in blood samples and heparin locksets. Actions: Inhibits thrombus and clot production by blocking the conversion of prothrombin to thrombin and fibrinogen to fibrin. Pharmacokinetics: Route Onset Peak Duration IV Immediate Minutes 2–6 hours SC 20–60 mins 2–4 hours 8–12 hours T 1/2 : 30 to 180 minutes; metabolised in the cells and excreted in urine. Adverse effects: Loss of hair, bruising, chills, fever, osteoporosis, suppression of renal function (with long-term use).

cephalosporins. Decreased anticoagulation can occur if heparin is combined with glyceryl trinitrate. Warfarin has documented drug–drug interactions with a vast number of other drugs (Table 48.3). It is a wise practice never to add or take away a drug from the regimen of the person receiving warfarin without careful monitoring and adjustment of the warfarin dose to prevent serious adverse effects. Because of the many factors that can affect the therapeutic levels of warfarin, it is often very difficult to reach a stable level and maintain that level. In cases of a heparin overdose, the antidote is protamine sulfate (generic). This basic protein drug forms stable salts with heparin as soon as the two drugs come in contact, immediately reversing heparin’s anticoagulant effects. Paradoxically, if protamine is given to a person who has not received heparin, it has anticoagulant effects. The dose is determined by the amount of heparin that was given and the time that elapsed since then. A dose of 1 mg IV protamine neutralises 90 USP of heparin derived from lung tissue or 110 USP of heparin derived from intestinal mucosa. The drug must be administered very slowly—not to exceed 50 mg IV in any 10-minute period. Care must be taken to calculate the amount of heparin that has been given to the person. Potentially fatal anaphylactic reactions have been reported with the use of protamine sulfate and so life support equipment should be readily available when it is used.

Care considerations for people receiving anticoagulants

Assessment: History and examination

■ ■ Assess for any known allergies to these drugs. Also screen for conditions that could be exacerbated by increased bleeding tendencies , including haemorrhagic disorders, recent trauma, spinal puncture, GI ulcers, recent surgery, intrauterine device placement, tuberculosis, presence of