Pre-Medication

Pre-medication is the administration of medication before anaesthesia. Pre-medication is used to prepare the patient for anaesthesia and to help provide optimal conditions for surgery. This includes:¹

• Reduction of anxiety and pain
• Promotion of amnesia
• Reduction of secretions
• Reduction of volume and pH of gastric contents (avoid Mendelson's syndrome)
• Reduction of post-operative nausea and vomiting
• Enhancing the hypnotic effects of general anaesthesia
• Reduction of vagal reflexes to intubation
• Specific indications, e.g. prevention of infective endocarditis

Pre-medication is traditionally given intramuscularly but the oral route is preferred for children and those with bleeding disorders. Pre-medication is usually given 1-3 hours preoperatively. Topical anaesthetic creams (e.g. EMLA) are often prescribed for children before cannulation.

The practice of pre-medication has changed substantially in recent years. The use of strongly sedative drugs, e.g. morphine and hyoscine, to aid smooth induction and reduce salivation has been abandoned with the advent of modern intravenous and inhalational anaesthetic agents, which have far fewer side effects and a faster onset of action.² Other factors that have reduced the use of a sedative pre-medication include:²

• Increasing use of day-case surgery
• Same-day admissions – patients often do not find a bed until just before surgery
• Changes to the surgical list, making the timing of drug delivery difficult

The choice of drug(s) used for pre-medication depends on the procedure, patient and anaesthetic technique. Some patients prefer not to have pre-medication and potential benefits may be outweighed by potential problems (except for specific indications), especially with day-case surgery. A Cochrane review found no evidence of a difference in time to discharge from hospital following adult day surgery in patients who received anxiolytic premedication.³

• Careful discussion of the patient's concerns is essential, including at the pre-operative assessment.
• Benzodiazepines are ideal agents to reduce anxiety. They provide anterograde amnesia and light sedation. If given orally 1-2 hours before surgery they have only a small effect on cardiorespiratory function but large doses can interfere with the speed and quality of recovery. In day care cases short acting benzodiazepines, e.g. temazepam, are often preferred.
• Anxiolysis and sedation may also be achieved by morphine, pethidine³ and fentanyl citrate.⁴
• In children, oral antihistamines, e.g. trimeprazine, may be used for sedation. If given alone, oral trimeprazine may cause postoperative restlessness in the presence of pain.

• Especially useful in the young or those having repeated general anaesthetics. May allow a lighter depth of anaesthesia by reducing risk of awareness during surgery.
• The most effective agents are lorazepam and midazolam.

Opioids, paracetamol and NSAIDs reduce the required dose of anaesthetic agent and improve patient comfort in the immediate postoperative period.

• Caution must be taken when considering the use of cyclo-oxygenase-2 inhibitors, because of their association with increased risk of myocardial infarction and stroke.²
• Opioids are the drugs of choice in the presence of acute pain. In the absence of pain, some people may experience intense dysphoria.
• Opioids also cause variable sedation and cardiorespiratory depression. All opioids cause nausea and vomiting and this may outweigh any beneficial effects. Opioids may also precipitate bronchospasm or anaphylaxis.

Response to surgery often includes vagally mediated bradycardia. Antisialogogues (e.g. glycopyrrolate intramuscularly or intravenously) are rarely needed but may be indicated for awake fibre-optic intubation or before ketamine anaesthesia.²

• Hyoscine has strong sedative, amnesic and antisalivation properties. It is a moderately effective antiemetic and potentiates opioids. Intramuscular atropine or hyoscine is therefore often prescribed together with an opioid.
• However, antisialogogues cause unpleasant dry mouth.
• Hyoscine is the most potent agent available with the added advantage of amnesia and sedation. However it can cause significant peri-operative confusion in elderly patients.

• Are used to either reduce the emetic effects of anaesthetic agents (antihistamines, butyrophenones, hyoscine) or enhance gastric emptying (metoclopramide).
• Those with a risk of regurgitation of gastric contents or undergoing procedures with a high incidence of nausea and vomiting e.g. laparoscopy, should receive agents to reduce gastric acidity.
• Can use H₂-receptor antagonist or proton pump inhibitors several hours preoperatively and oral sodium citrate 15-30 minutes before induction.