The term premedication is used to describe the administration of drugs before operation, with the general aims of lessening anxiety and fear and contributing to the ease and safety of the anesthetic.
Purposes of Premedication
- To alleviate anxiety and fear
- To prevent pain during vascular cannulations, regional anesthesia procedures or positioning
- To facilitates smooth induction of anesthesia and reduce the dose of anesthetic required
- To reduce the volume and acidity of gastric contents
- To reduce secretions especially salivary and bronchial
- To prevent undesirable reflexes, e.g. bradycardia
- To provide anti-asthma and anti-allergy therapy if relevant
- To reduce post-operative nausea and vomiting
Drugs Used for Premedication
- Barbiturates: pentobarbital (Nembutal) and secobarbital (seconal) are intermediate-acting barbiturates that are sedative and hypnotic but not analgesic; in dosage of 1-4 mg/kg ( up to 200 mg) IM, they may depress respiratory derive but have little effect on cardiovascular function. They may be used alone or in combination with narcotics or other sedatives or anticholinergics. The onset of effects is within 30 minutes; duration is 2-3 hours. These agents should not be used in suspected cases of acute intermittent poryphyria
- Benzodiazepines: Diazepam (Valium) and lorazepam (Ativan) are effective tranquilizers that rarely produce significant cardiovascular or respiratory depression when used at recommended dosage in healthy patients. Diazepam (5-15 mg PO ) and lorazepam (1-4 mg IM or PO) may be used alone or in combination with other classes of premedications.
- Droperidol ( Inapsin) is a butyrophenone that produces long acting sedation ( peak is at 30-60 minutes after IM injection) in dosage of 0.03-0.14 mg/kg IM, alone or in combination with a narcotic. Droperidol has a potent antiemetic effect and low doses (1.25-2.5 mg IV in adults) seem to prevent postoperative nausea and vomiting. Droperidol produces mild systemic vasodilatation from alpha-adrenergic blockade and occasionally, extra pyramidal symptoms from its anti-dopaminergic effects.
- Phenothiazines: Promethazine (phenergan) and chlorpromazine (largactil) in doses 25- 75 mg IM, are mild tranquilizers, antiemetics and H1 blockers; they potentiate the sedating and analgesic properties of narcotics. Lower doses (25 mg IM) are selected when given in combination with narcotics.
- Morphine has both analgesic and sedative properties. It causes little cardiac depression, although it may produce hypotension through histaminic mediated vasodilatation. Morphine depresses the medullary response to carbon dioxide and causes respiration to slow and deep. Other effects include papillary constriction and sometimes nausea and vomiting. Occasionally, morphine causes spasm of sphincter Oddi, so it is prudent to avoid its use in the presence of biliary tract disease. The typical dosage range is 0.1-0.2 mg/kg IM, often in combination with antisialogogue or antiemetic.
- Meperidine (Demerol): is analgesic that is one-tenth as potent as morphine (1.0-1.5 mg/kg IM is used for premedication). It is less sedating than morphine, so it is often combined with promethazine. Meperidine is vagolytic and may increase heart rate; compared to morphine; it seems to produce les hypotension, nausea, and vomiting and sphincter Oddi spasm.
- Anticholinergics Atropine, hyoscine (scopolamine) and glycopyrrolate are commonly used anticholinergics as preoperative medication. Atropine - as premedication atropine is given for its antisialogogue properties, in dosage of 0.4- 0.6 mg IM for full-sized adults. Decreasing oral secretions may reduce the likelihood of laryngospasm during Ketamin induced anaesthesia or when using irritating inhalational agents like diethyl ether. Most patients find the dry mouth sensation unpleasant and atropine use in patients with chronic bronchitis or cystic fibrosis may lead to inspissation of mucus with plugging of small airways. Atropine premedication is employed in children to prevent the bradycardia associated with laryngoscopy and succinylcholine use; however, such protection is more reliably achieved when atropine is administered IV immediately prior to induction. In young children atropine may also produce flushing and exacerbation of fever.
- Scopolamine (0.3-0.4mg IM in adults) is a centrally active anticholinergic with potent sedative, antisialogogue, and antiemetic properties, particularly when used in combination with morphine. Scopolamine occasionally produce delirium, hallucinations and prolonged somnolence due to "central anticholinergic syndrome"; such manifestation are usually reversible with physiostigmine ( 1-2 mg IV)
- Glycopyrrolate (Robinul) acts as antisialogogue and inhibits cardiac vagal reflexes during induction and intubation when used at doses of 0.2- 0.3 mg IM or IV. It is also efficacious in reducing the volume and acidity of gastric fluid, and thus it provides some protection against pulmonary aspiration and thus it has the best drying action and the least central effect.
C. Antacid Drugs (Prophylaxis for Pulmonary Aspiration)
- Sodium citrate (a non-particulate antacid) 30 ml of 0.1M po can be used pre-operatively to counteract the acidity of the gastric contents. It takes approx. 10 minutes to work and its effects last approx. 20 minutes.
- Histamine H2 blockers i.e. ranitidine 300mg ( PO- 50-200 mg or IV 50 mg and Cimetididine 300 mg Po or 150 mg IV can be given at least 1 hour prior to surgery also decrease the acidity of gastric contents. Metoclopramide 10mg given at the same time may benefit. Omeprazole 40mg given 2-6 hours prior to surgery also helps reduce gastric acid
- Anti-emetics: Phenothiazines, butyrophenones, antihistamines, metoclopramide and hyoscine can be used to help reduce incidence of post-operative nausea and vomiting.
Routes of Administration
- Oral administration: This has become much more popular in recent years. It has the advantage of avoiding an injection and is often favored by pediatric anesthetists. It is suitable where only a sedative, tranquillizer or analgesic tablet is required. Its disadvantage is that absorption is slow and not dependable and the tablets or syrup have to be given 1-2 hours pre-operatively. Tablets may be taken with a sip of water.
- Intramuscular injection: Premedicant drugs such as narcotic analgesics and anticholinergic drugs are usually given intramuscularly. IM injection is also used when the patient cannot take anything by mouth e.g. intestinal obstruction or vomiting.
- Intravenous injection: Occasionally premedication is given intravenously in the operating theatre before the induction of anesthesia. It is given in the following situations:
a. In patients requiring emergency surgery when time cannot be allowed for the 1M injection to take effect.
b. In patients presenting for elective or routine surgery who for some reason have not been given premedication.
c. In patients who have an intravenous infusion running and where it is decided to avoid an intramuscular injection.
d. In the shocked patient where intramuscular absorption is slow. Often no premed is given here.
e. In patients with a bleeding tendency where intramuscular injections may be associated with bruising.
f. Rectal administration: This route is rarely used. Patient acceptance is low.
No Premedication: There is a place for no premedication in anaesthesia. Very ill and frail patients fall into this category.
Choice of Pre Medications
Choice of Drug and Dosage
The choice and dose of drugs will depend on a variety of conditions. This includes the following
- Weight of patient
- Nature of surgery (long? painful?)
- Regional or general anesthetic (relaxant with IPPV or spontaneous breathing)
- Degree of apprehension
- Anticipated post operative pain
Important Points Regarding Choice of Drugs
- Narcotics such as pethedine or morphine should be used (unless otherwise contraindicated) if the patient is in pain, e.g. from a fractured femur.
- Narcotics cause respiratory depression and should not be used in head injured patients unless facilities for pre and post-op long-term mechanical ventilation are available.
- A sedative premedication should be avoided in patients with raised intracranial pressure.
- Narcotics are avoided in Caesarean sections and operative obstetrics, e.g. forceps delivery. They can be given when the baby is born.
- Narcotic premedication is not used in children under the age of twelve months.
- Smaller doses of narcotics are used in
- Poor risk patients from whatever cause
- The elderly
- The very young
- Patients who should not be too drowsy at the end of the operation, e.g. patients who have had surgery on the upper airway, those with full stomachs, diabetic patients in who prolonged drowsiness may mask a hyper or hypoglycemic state.
- Pethedine and atropine are prescribed 30-45 minutes pre-operatively.
- In asthmatics a good premedication is a combination of pethedine, promethazine and atropine. A salbutamol nebulizer 30 minutes prior to surgery is often beneficial.
- In regional anaesthesia tranquilizers such as midazolam or diazepam may be used. A narcotic such as pethedine may be used to provide analgesia so that the patient will be comfortable when the local anaesthetic begins to wear off.
- Chlorpromazine (Largactil) or other benzodiazepines (diazepam) are good drugs to use in premedicating patients receiving ketamine anaesthesia. The hypertension and hallucinations that follow ketamine are minimised. In conclusion, a variety of drugs are available for premedication. The drugs selected must be tailored to the patient's needs.