Preanaesthetic Medication

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Chapter: Essential pharmacology : General Anaesthetics

Preanaesthetic medication refers to the use of drugs before anaesthesia to make it more pleasant and safe.


PREANAESTHETIC MEDICATION

 

Preanaesthetic medication refers to the use of drugs before anaesthesia to make it more pleasant and safe. The aims are:

 

1.   Relief of anxiety and apprehension preoperatively and to facilitate smooth induction.

2.   Amnesia for pre and postoperative events.

3.   Supplement analgesic action of anaesthetics and potentiate them so that less anaesthetic is needed.

4.    Decrease secretions and vagal stimulation caused by anaesthetics.

5.    Antiemetic effect extending to the postoperative period.

6.    Decrease acidity and volume of gastric juice so that it is less damaging if aspirated.

 

Different drugs achieve different purposes. One or more drugs may be used in a patient depending on the needs.

 

1. Sedative-antianxiety Drugs

 

Benzodiazepines like diazepam (5–10 mg oral) or lorazepam (2 mg or 0.05 mg/kg i.m. 1 hour before) have become popular drugs for preanaesthetic medication because they produce tranquility and smoothen induction; there is loss of recall of perioperative events (especially with lorazepam) with little respiratory depression or accentuation of postoperative vomiting. They counteract CNS toxicity of local anaesthetics and are being used along with pethidine/fentanyl for a variety of minor surgical and endoscopic procedures.

 

Midazolam is a good amnesic with potent and shorter lasting action; it is also better suited for i.v. injection, due to water solubility.

 

Promethazine (50 mg i.m.) is an antihistaminic with sedative, antiemetic and anticholinergic properties. It causes little respiratory depression.

 

2. Opioids

 

Morphine (10 mg) or pethidine (50–100 mg), i.m. allay anxiety and apprehension of the operation, produce pre and postoperative analgesia, smoothen induction, reduce the dose of anaesthetic required and supplement poor analgesic (thiopentone, halothane) or weak anaesthetics (N2O). Postoperative restlessness is also reduced.

 

Disadvantages They depress respiration, interfere with pupillary signs of anaesthesia, may cause fall in BP during anaesthesia, can precipitate asthma and tend to delay recovery. Other disadvantages are lack of amnesia, flushing, delayed gastric emptying and biliary spasm. Some patients experience dysphoria. Morphine particularly contributes to postoperative constipation, vomiting and urinary retention. Tachycardia sometimes occurs when pethidine has been used.

 

Use of opioids is now mostly restricted to those having preoperative pain. When indicated, fentanyl is mostly injected i.v. just before induction.

 

3. Anticholinergics

 

Atropine or hyoscine (0.6 mg i.m./i.v.) have been used, primarily to reduce salivary and bronchial secretions. Need for their use is now less compelling because of the increasing employment of nonirritant anaesthetics. However, they must be given before hand when ether is used. The main aim of their use now is to prevent vagal bradycardia and hypotension (which occur reflexly due to certain surgical procedures), and prophylaxis of laryngospasm which is precipitated by respiratory secretions. Hyoscine, in addition, produces amnesia and antiemetic effect, but tends to delay recovery. Some patients get disoriented; emergence delirium is more common. They dilate pupils, abolish the pupillary signs and increase chances of gastric reflux by decreasing tone of lower esophageal sphincter (LES). They should not be used in febrile patients. Dryness of mouth in the pre and postoperative period may be distressing.

 

Glycopyrrolate (0.1–0.3 mg i.m.) is a longer acting quaternary atropine substitute. It is a potent antisecretory and antibradycardiac drug; acts rapidly and is less likely to produce central effects (see Ch. No. 8).

 

4. Neuroleptics

 

Chlorpromazine (25 mg), triflupromazine (10 mg) or haloperidol (2–4 mg) i.m. are infrequently used in premedication. They allay anxiety, smoothen induction and have antiemetic action. However, they potentiate respiratory depression and hypotension caused by the anaesthetics and delay recovery.

Involuntary movements and muscle dystonias can occur, especially in children.

 

5. H2 Blockers

 

Patients undergoing prolonged operations, caesarian section and obese patients are at increased risk of gastric regurgitation and aspiration pneumonia. Ranitidine (150 mg) or famotidine (20 mg) given night before and in the morning benefit by raising pH of gastric juice; may also reduce its volume and thus chances of regurgitation. Prevention of stress ulcers is another advantage. They are now routinely used before prolonged surgery.

 

The proton pump inhibitor omeprazole/ pantoprazole is an alternative.

 

6. Antiemetics

 

Metoclopramide 10–20 mg i.m. preoperatively is effective in reducing postoperative vomiting. By enhancing gastric emptying and tone of LES, it reduces the chances of reflux and its aspiration. Extrapyramidal effects and motor restlessness can occur. Combined use of metoclopramide and H2 blockers is more effective.

 

Domperidone is nearly as effective and does not produce extrapyramidal side effects.

 

After its success in cancer chemotherapy induced vomiting, the selective 5HT3 blocker Ondansetron (4–8 mg i.v.) has been found highly effective in reducing the incidence of post anaesthetic nausea and vomiting as well (see Ch. No. 47).

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