McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 5 7 Drugs affecting gastrointestinal secretions

hyperacidity and hiatus hernia. See Table 57.1 for usual indications for each antacid. Pharmacokinetics Sodium bicarbonate, the oldest drug in this group, is readily available in many preparations, including baking soda powder, tablets, solutions and as an injectable for treating systemic acidosis. This drug is widely distrib- uted when absorbed orally, reaching peak levels in 1 to 3 hours, crossing the placenta and entering breast milk. It is excreted in urine and can cause serious electrolyte imbalance in people with renal impairment. Calcium carbonate is actually precipitated chalk and is available in tablet and powder forms. The main draw- backs to this agent are constipation and acid rebound. It has an onset of action in about 3 to 5 minutes. It can be absorbed systemically and cause calcium imbal- ance. When absorbed, it is metabolised in the liver and excreted in urine and faeces, with a half-life of 1 to 3 hours. Calcium carbonate is known to cross the placenta and enter breast milk. Magnesium salts are very effective in buffering acid in the stomach but have been known to cause diarrhoea; they are sometimes used as laxatives. They are available as tablets, chewable tablets and capsules and in liquid forms. Although these agents are not generally absorbed systemically and are excreted in the faeces, absorbed magnesium can lead to nerve damage and even coma, if absorbed; they are excreted in the urine. Aluminium salts, available as tablets, capsules, sus- pensions and in a liquid form, do not cause acid rebound but are not very effective in neutralising acid. They are bound in faeces for excretion. They have been related to severe constipation. Aluminium binds dietary phos- phates and causes hypophosphataemia, which can then cause calcium imbalance throughout the system. Aluminium and magnesium minimise the GI effects of constipation and diarrhoea by combining these two salts but may cause a rebound hyperacidity and alkalosis. Many of these antacids are available in combination forms to take advantage of the acid-neutralising effect and block adverse effects. For example, a combination of calcium and aluminium salts ( Mylanta ) buffers acid and produces neither constipation nor diarrhoea. Contraindications and cautions The antacids are contraindicated in the presence of any known allergy to antacid products or any compo- nent of the drug to prevent hypersensitivity reactions . Caution should be used in the following instances: any condition that can be exacerbated by electrolyte or acid– base imbalance to prevent exacerbations and serious adverse effects ; any electrolyte imbalance, which could be exacerbated by the electrolyte-changing effects of these drugs ; GI obstruction, which could cause systemic

absorption of the drugs and increased adverse effects ; renal dysfunction, which could lead to electrolyte dis- turbance if any absorbed antacid is not neutralised properly ; and pregnancy and breastfeeding because of the potential for adverse effects on the fetus or neonate. Adverse effects The adverse effects associated with these drugs relate to their effects on acid–base and electrolyte balance. Administering an antacid frequently causes acid rebound , in which the stomach produces more acid in response to the alkaline environment. Neutralising the stomach contents to an alkaline level stimulates gastrin production to cause an increase in acid production and return the stomach to its normal acidic state. In many cases, the acid rebound causes an increase in symptoms, which results in an increased intake of the antacid. This leads to more acid production and an ongoing cycle. When more and more antacid is used, the risk for systemic effects rises. Alkalosis with resultant metabolic changes (nausea, vomiting, neuromuscular changes, headache, irritability, muscle twitching and even coma) may occur. The use of calcium salts may lead to hyper- calcaemia and milk-alkali syndrome (seen as alkalosis, renal calcium deposits or severe electrolyte disorders). Constipation or diarrhoea may result, depending on the antacid being used. Hypophosphataemia can occur with the use of aluminium salts. Finally, fluid retention and heart failure can occur with sodium bicarbonate because of its high sodium content. Drug–drug interactions Antacids can greatly affect the absorption of drugs from the GI tract. Most drugs are prepared for an acidic environment, and an alkaline environment can prevent them from being broken down for absorption or can actually neutralise them so that they cannot be absorbed. People taking antacids should be advised to separate them from any other medications by 1 to 2 hours. If the pH of urine is affected by large doses of antacids, levels of drugs, such as quinidine, may increase, and levels of salicylates may decrease.

Care considerations for people receiving antacids

Assessment: History and examination

■ ■ Assess for possible contraindications or cautions : any history of allergy to antacids to prevent hypersensitivity reactions ; renal dysfunction, which might interfere with the drug’s excretion ; electrolyte disturbances, which could be exacerbated by the effects of the drug ; and current