• reduction of excessive salivation, gastric contents and undesirable autonomic reflexes. Preoperative fasting has traditionally been of great importance. There are several issues . Anaesthesia involves the abolition of airway reflexes and an increased risk of vomiting and regurgitation, with subsequent lung aspiration. This potential is greater as the volume and acidity of stomach contents increase. The balance of risks is then established between the danger of aspiration and the danger of delaying surgery. For this reason, non-essential surgery with a full stomach is normally contraindicated.
Many departments have established protocols to deal with the issue of preoperative fasting. Most anaesthetists would accept a period of 4-6 h for solids and 2-4 for clear liquids in healthy adults and an even shorter period in small children.
Many factors determine the volume and acidity of the stomach contents. The most obvious is the time since the last meal. Other factors, like trauma, fear, pain, opioids and other drugs, and gastrointestinal disorders reduce stomach emptying and increase gastric contents up to 24 h after a meal. An excessively long starvation, conversely, has numerous metabolic consequences, including dehydration and protein catabolism, and potentiates nausea and vomiting. Some patients prefer not to have a premedication.
The commonest preparations are described below.
These are excellent anxiolytics; benzodiazepines provide antegrade amnesia and light sedation. They are agonists at the gamma-aminobutyric acid (GABA) receptor. When administered orally 1-2 h before surgery they have only a small effect on cardiorespiratory function. i.v., particularly in frail patients, they cause profound cardiorespiratory depression. Large doses can interfere with the speed and quality of recovery. The differences between benzodiazepines are largely due to pharmacokinetic differences (Table 2.8).
Metoclopramide (10 mg orally 1-2 h preoperatively) increases gastric motility and oesophageal sphincter pressure; these effects combine to reduce the risk of reflux. This also gives the drug a synergistic effect with the benzodiazepines, increasing their intestinal absorption. In addition, metoclopramide has antiemetic properties, an effect mediated by its antagonism at the central dopaminergic and 5-hydroxy-tryptamine Class 3 (5HT-3) receptors. It can produce dystonia and other extrapyramidal effects, more so when given intramuscularly (i.m.) or i.v.
Atropine (0.6 mg i.m.), the classical anticholinergic, is a tertiary amine; it is therefore more lipid soluble and penetrates the blood-brain barrier better, which confers more central activity. It has strong vagolytic action which protects against bradycardia, together with a moderate antisialagogue and weak sedative effect. Hyoscine (0.6 mg i.m.), also a tertiary amine, has strong sedative, amnesic and antisalivation properties. It is a moderately effective antiemetic and potentiates opioids. It is, thus, customary to prescribe i.m. atropine or hyoscine as a premedication, together with an opioid. Glycopyronium (0.4 mg i.m. or i.v.) is not usually administered in the premedication. It is a quaternary amine with little penetration of the blood-brain barrier and limited central action. It is a strong antisialagogue and protects against bradycardia.
These are probably agents of choice as premedicants in the presence of acute pain (Table 2.9). In the absence of pain, however, some individuals may experience intense dysphoria. They also cause variable sedation (but not anxiolysis) and cardiorespiratory depression. All opioids cause nausea and vomiting which may overshadow their favourable effects. Most cause histamine release and can precipitate bronchospasm or anaphylaxis.
Eutectic mixture of local anaesthetic (EMLA) cream is a mixture of 2.5% lignocaine and 2.5% prilocaine in an emulsified and viscous white cream with an approximate density of 1 (1g = 1 ml). It is used as a topical anaesthetic at the anticipated site of venepuncture. It should be administered between 1 and 3h before anaesthesia. EMLA is contraindi-cated in children under 1 year old. Ametop is a topical 4% amethocaine gel which is applied 1 h before venepuncture. Hypersensitivity can occur.
Table 2.9. Opioids: differential characteristics.
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