Pharmacology for Anaesthesia and Intensive Care

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Drugs acting on the gut

Antacids


Drugs influencing gastric secretion


Drugs influencing gastric motility


Mucosal protectors


Prostaglandin analogues

Antacids

Antacids neutralize gastric acidity. They are used to relieve the symptoms of dyspep-

sia 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 sys-

temic circulation and may cause a metabolic alkalosis if taken in excess. Sodium

bicarbonate releases carbon dioxide as it reacts with gastric acid, resulting in belch-

ing. Thirty milliliters of 0.3msodium 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 ACh stimulate parietal cells (via gastrin and muscarinic receptors) to

secrete H+into the gastric lumen. ACh is released from parasympathetic postgan-

glionic fibres while gastrin is released from G-cells in the antral mucosa. How-

ever, the main stimulus for parietal cell acid secretion is via histamine receptor