Pre Anesthesia drugs and medicines

Pre anesthesia drugs and medicines is the term applied to the use of drugs prior to the administration of an anesthetic agent, with the objective of making anesthesia safer and more agreeable to the patient. The reasons for using pre anesthesia drugs are:

1. For sedation, to reduce anxiety and apprehension without producing much drowsiness.
2. To obtain an additive or synergistic effect so that induction could be smooth and rapid and the dose of the general anesthetic could be reduced.
3. To counteract certain adverse effects of the anesthetic drug used such as salivation, bradycardia and vomiting.
4. To relieve pre- and post-operative pain.
5. To suppress respiratory secretions and to reduce reflex excitability.

There is no single drug which can achieve all these objectives of pre anesthesia drugs and hence usually a combination of drugs is used.

It must be emphasized; however, that factors other than drugs can favorably affect preoperative psychological preparation and a preoperative visit by the anesthesiologist and a sympathetic discussion with the patient about the events of the next day in itself have a high therapeutic value.

The drugs commonly used for pre anesthesia drugs are

1. Opioids
2. Sedatives and tranquillizers
3. Ant muscarinic drugs
4. Antiemetics

Opioids as pre anesthesia drugs

Opioid analgesics such as morphine (10- 15 mg. IM), pethidine (50-100 mg. 1M) and buprenorphine (300 mcg. IM), are commonly employed for their sedative and analgesic properties.

Buprenorphine has longer duration of action than morphine and pethidine.  They also reduce the amount of general anesthetic required.

Disadvantages of Opioids as pre anesthesia medicines are

They may depress respiration and may cause respiratory arrest. Further, drugs like morphine increase the tone of smooth muscle such as bronchial muscles.

In emphysema or in kyphoscoliosis where the pulmonary reserve is already low, use of opioids may precipitate pulmonary insufficiency.

They may cause vasomotor depression, and may decrease the ability of circulation to respond to stress. They often delay the awakening as their clinical effect lasts for 4-6 hours.

Morphine may induce vomiting, anti-diuresis and can interfere with pupillary reactions.

Pethidine by its vagolytic action may produce tachycardia.

Both these drugs are histamine liberators.

Sedatives and tranquillizers as pre anesthetic medication

Benzo­diazepines (diazepam, nitrazepam) are preferred because of their safety, muscle relaxant property and less respiratory depression. They also provide amnesia.

Diazepam in doses of 5 to 20 mg. has been most widely used. It is active orally and can also be given parenterally, though its action is less predictable by this route. Other tranquillizer compounds used belong to phenothiazine and butyrophenone groups. They exhibit similar properties.

Phenothiazines possess sedative, antiarrhythmic, antiemetic and anti-histaminic properties. They, however, can potentiate the effects of barbiturates and morphine and may cause respiratory depression and hypotension.

Phenothiazines commonly employed are promethazine andtrimeprazine (Vallergan). They can be given orally as well as parenterally.

Dexmedetomidine is a imidazole derivative causes analgesia and sedation with very little respiratory depression by its central and peripheral alpha2 adrenergic receptor agonist action. Its main adverse effects are hypotension and bradycardia. It is used IV for short term sedation of critically ill adults and as anesthetic adjunct.

Antimuscarinic drugs as pre anesthesia drugs

Atropine (600 mcg. IM) is generally combined with morphine to block the vagal actions so as to reduce salivary and respiratory secretions and to prevent parasympathetically induced reflex hypotension and bradycardia. It may thus lessen the possibility of cardiac arrhythmias during the induction stage. Due to blockade of cardiac vagal action, atropine may produce tachycardia.

It causes initial vagal stimulation by its central action; use of atropine in combination with neostigmine to reverse the neuromuscular blockade by the muscle relaxant d-tubocurarine, therefore, may cause cardiac arrest.

Synthetic anti-muscarinic such as glycopyrrolate (Pyrolate), a long acting quaternary amine, can be used instead of atropine for this purpose and are sometimes preferred because of their less central actions and less tendency to cause excessive tachycardia.

Antiemetics as pre anesthesia drugs

The commonly used phenothiazines such as promethazine and trinieprazine have antiemetic properties and thus may help to prevent the post-operative nausea and vomiting. This advantage should, however, be weighed against the possible hypotension following these drugs. Other drugs used are cyclizine, 50 mg., trimethobenzamidc 200 mg and benzquinamide 25-50 mg.

Other drugs used as pre anesthesia drugs

In addition to the use of above mentioned drugs, proper pre evaluation and specific premedication are needed in patients with special problems such as chronic lung disease, emphysema, ischemic heart disease, diabetes mellitus, hypertension, under nutrition and in debilitated and old people. Antibiotic prophylaxis may be needed.

The risk of stopping long term medication before surgery is often greater than the risk of continuing it during surgery.

This applies particularly to glucocorticoids, analgesics, anti parkinsonian drugs, anti-glaucoma drugs, and thyroid or anti thyroid drugs.

On the other hand, it is advisable to discontinue combined oral contraceptive pills 4 weeks before major surgery; monoamine-oxidase inhibitors 2 weeks before surgery; and lithium 2 days before surgery.