McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

■■ BOX 49.3  Chelating agents

Heavy metals, including iron, lead, arsenic, mercury, copper and gold, can cause toxicity in the body by their ability to tie up chemicals in living tissues that need to be free in order for the cell to function normally. When these vital substances (thiols, sulfurs, carboxyls and phosphoryls) are bound to the metal, certain cellular enzyme systems become deactivated, resulting in failure of cellular function and eventual cell death. Drugs that have been developed to

counteract metal toxicity are called chelating agents (from the Greek word for “claw”). Chelating agents grasp and hold a toxic metal so that it can be carried out of the body before it has time to harm the tissues. The chelating agent binds the molecules of the metal, preventing it from damaging the cells within the body. The complex that is formed by the chelating agent and the metal is non-toxic and is excreted by the kidneys.

Chelating agent

Toxic metal

Notes

desferrioxamine mesylate (Desferal) penicillamine (D-Penamine)

iron

Given IM, SC or IV; rash and vision changes are common Administered orally on an empty stomach, and at least 2 hours prior to a meal or 1 hour after other drugs, food or milk Given IM or IV; monitor renal and hepatic function, because serious and even fatal toxicity can occur

copper, gold, mercury, lead, zinc

sodium calcium edetate (Calcium DisodiumVersenate)

lead

Clinically important drug–food interactions Iron is not absorbed if taken with antacids, eggs, milk, coffee or tea. These substances should not be admin- istered concurrently. Acidic liquids may enhance the absorption of iron and should not be given concurrently.

chloramphenicol; people receiving this combination should be monitored closely for any sign of iron toxicity. The effects of levodopa may decrease if it is taken with iron preparations; people receiving both of these drugs should take them at least 2 hours apart.

CRITICAL THINKING SCENARIO Iron preparations and toxicity

THE SITUATION L.L., a 28-year-old woman, suffered a miscarriage 6 weeks ago. She lost a great deal of blood during the miscarriage and underwent a dilation and curettage to control the bleeding. On her 6-week routine follow-up visit, she was found to have recovered physically from the event but was still depressed over her loss. Her haematocrit was 31%, and she admitted feeling tired and weak. She was offered emotional support and given a supply of ferrous sulfate tablets, with instructions to take one tablet three times a day. At home, L.L. transferred the pills to a decorative bottle that had once held vitamins and left it on her table as a reminder to take the tablets. The next day, she discovered her 2-year-old daughter eating the tablets and punished her for getting into them. About 1 hour later, the toddler complained of a really bad “tummy ache” and started vomiting. She then became lethargic, and L.L. called the paediatrician, who told them to go immediately to the emergency department and bring the remaining tablets with them. The toddler was found to have a weak, rapid pulse (156 beats/minute), rapid, shallow respirations (32 per minute), and a low blood pressure (60/42 mmHg). When

a diagnosis of acute iron toxicity was made, L.L. became distraught. She said she had no idea that iron could be dangerous because it can be bought over-the-counter (OTC) in so many preparations. She had not read the written information given to her because it was “just iron”. CRITICAL THINKING What interventions should be done at this point? What sort of crisis intervention would be most appropriate for L.L.? Think about the combined depression from the miscarriage, fear and anxiety related to this crisis, and L.L.’s iron-depleted state. What kind of reserve does she have for dealing with this crisis? Which measures would be appropriate for helping the mother cope with this crisis and for treating the toddler? DISCUSSION The first priority is to support and detoxify the child suffering from iron toxicity. In cases of acute iron poisoning, eggs and milk are given to bind the iron and prevent absorption. Gastric lavage, using a 1% sodium bicarbonate