Abstract
Antacids neutralise gastric acidity. They are used to relieve the symptoms of dyspepsia and gastro-oesophageal reflux. They promote ulcer healing but less effectively than other therapies.
Antacids
Antacids neutralise gastric acidity. They are used to relieve the symptoms of dyspepsia and gastro-oesophageal reflux. They promote ulcer healing but less effectively than other therapies.
Aluminium- and Magnesium-Containing Antacids
Neither is absorbed from the gut significantly and due to their relatively low water solubility they are long-acting providing that they remain in the stomach. Aluminium-containing antacids have a slower action and produce constipation, while magnesium-containing antacids produce diarrhoea. Aluminium ions form complexes with some drugs (e.g. tetracycline) and reduce their absorption.
Sodium Bicarbonate and Sodium Citrate
These antacids are water-soluble and their onset of action is faster than the aluminium- and magnesium-containing antacids. They are absorbed into the systemic circulation and may cause a metabolic alkalosis if taken in excess. Sodium bicarbonate releases carbon dioxide as it reacts with gastric acid, resulting in belching. Thirty millilitres of 0.3 m sodium citrate is often used with ranitidine to reduce gastric acidity before caesarean section. It should be given less than 10 minutes before the start of surgery due to its limited duration of action.
Drugs Influencing Gastric Secretion
Physiology
Gastrin and acetylcholine (ACh) stimulate parietal cells (via gastrin and muscarinic receptors) to secrete H+ into the gastric lumen. ACh is released from parasympathetic postganglionic fibres while gastrin is released from G-cells in the antral mucosa. However, the main stimulus for parietal cell acid secretion is via histamine receptor activation. Gastrin and ACh also stimulate the adjacent paracrine cells to produce and release histamine, which acts on the parietal cell, increasing cAMP and therefore acid secretion.
H2 Receptor AntagonistsCimetidine
Cimetidine is the only H2 receptor antagonist with an imidazole structure.
Uses
It is used in peptic ulcer disease, reflux oesophagitis, Zollinger–Ellison syndrome and pre-operatively in those at risk of aspiration. It has not been shown to be of benefit in active haematemesis.
Mechanism of Action
Cimetidine is a competitive and specific antagonist of H2 receptors at parietal cells.
Effects
Gut – the gastric pH is raised and the volume of secretions reduced, while there is no change in gastric emptying time or lower oesophageal sphincter tone.
Cardiovascular – bradycardia and hypotension follow rapid intravenous administration.
Central nervous system – confusion, hallucinations and seizures are usually only seen when impaired renal function leads to high plasma levels.
Respiratory system – low-grade aspiration of gastric content that has been stripped of its acidic, antibacterial environment will result in increased nosocomial pulmonary infections in critically ill ventilated patients.
Endocrine – gynaecomastia, impotence and a fall in sperm count are seen in men due to the anti-androgenic effects of cimetidine.
Metabolic – it inhibits hepatic cytochrome P450 and will slow the metabolism of the following drugs: lidocaine, propranolol, diazepam, phenytoin, tricyclic antidepressants, warfarin and aminophylline.
Ranitidine
Ranitidine is more potent than cimetidine.
Uses
Ranitidine has similar uses to cimetidine. However, because it does not inhibit hepatic cytochrome P450, it is often preferred to cimetidine. It is used in combination with antibiotics to eradicate H. pylori. It is also used widely in labour with apparently no deleterious effects on the fetus or progress of labour.
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