McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

The evidence

BOX 44.2

Preventing digoxin toxicity in children and the elderly Children and older adults are at increased risk for digoxin toxicity. Individuals in both of these groups have body masses that are smaller than the average adult body mass, and they may have immature or ageing kidneys. Digoxin is excreted unchanged in the kidneys, so any change in kidney function can result in increased serum digoxin levels and subsequent digoxin toxicity. Extreme care should be taken when administering digoxin to people in either of these age groups. Paediatric findings Many institutions require that paediatric digoxin doses be checked by a second healthcare professional before administration.This practice provides an extra check to help prevent the toxicity of this potentially dangerous drug.The child should then be assessed before the drug is given, including careful cardiac auscultation and apical pulse measurement to monitor heart rate and rhythm to detect any possible toxic effects.

Geriatric findings Older adults may not receive the same kind of attention as a policy, but they should be monitored for any factor that might affect digoxin levels when the drug is administered. Such factors may include: • Renal function (Is the blood urea nitrogen concentration elevated?) • Low body mass (Is the person underweight, undernourished, taking laxatives?) • Current pulse, including quality and rhythm • Hydration (Is the skin loose? Are the mucous membranes dry?The presence of these conditions could signal potential electrolyte disturbances.) Many geriatric people eventually need a decrease in dose, from 0.25 mg once a day to 0.125 mg once a day or 0.25 mg every other day.The healthcare professional administering the drug is often in the best position to detect any changes in the person’s condition that might indicate a need for further evaluation. Prototype summary: Digoxin Indications: Treatment of HF, atrial fibrillation. Actions: Increases intracellular calcium and allows more calcium to enter the myocardial cell during depolarisation; this causes a positive inotropic effect (increased force of contraction), increased renal perfusion with a diuretic effect and decrease in renin release, a negative chronotropic effect (slower heart rate), and slowed conduction through the atrioventricular (AV) node. Pharmacokinetics: Route Onset Peak Duration Oral 30–120 mins 2–6 hours 6–8 days IV 5–30 mins 1–5 hours 4–5 days T 1/2 : 30 to 40 hours; largely excreted unchanged in the urine. Adverse effects: Headache, weakness, drowsiness, visual disturbances, arrhythmias, GI upset.

not be taken at the same time but should be adminis­ tered 2 to 4 hours apart. Box 44.4 highlights important information about the interactions between digoxin and common herbal remedies.

■■ BOX 44.3  Digoxin antidote: Digoxin immune Fab

Digoxin immune Fab ( Digibind ) [not available in New Zealand] is an antigen-binding fragment (Fab) derived from specific anti-digoxin antibodies. These antibodies bind molecules of digoxin, making them unavailable at their site of action. The digoxin antibody–antigen complexes accumulate in the blood and are excreted through the kidney. Digoxin immune Fab is used for the treatment of life-threatening digoxin intoxication (serum levels 0.10 ng/mL with serum potassium 0.5 mEq/L in a setting of digoxin intoxication) and potential life- threatening digoxin overdose. The amount of digoxin immune Fab that is infused intravenously is determined by the amount of digoxin ingested or by the serum digoxin level if the ingested amount is unknown. The person’s cardiac status should be monitored continually while the drug is given and for several hours after the infusion is finished. Because there is a risk of hypersensitivity reaction to the infused protein, life-support equipment should be on standby. Serum digoxin levels will be very high and unreliable for about 3 days after the digoxin immune Fab infusion because of the high levels of digoxin in the blood. The person should not be redigitalised for several days to 1 week after digoxin immune Fab has been used, because of the potential of remaining fragments in the blood.

St John’s wort and psyllium have been shown to decrease the effectiveness of digoxin; this combination should be avoided. Increased digoxin toxicity has been reported with ginseng, hawthorn and licorice. People should be advised to avoid these combinations. Herbal and alternative therapies BOX 44.4