Intravenous Anaesthetics Inducing Agents

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Chapter: Essential pharmacology : General Anaesthetics

These are drugs which on i.v. injection produce loss of consciousness in one armbrain circulation time (~11 sec); are generally used for induction because of rapidity of onset of action.



These are drugs which on i.v. injection produce loss of consciousness in one armbrain circulation time (~11 sec); are generally used for induction because of rapidity of onset of action. Anaesthesia is then usually maintained by an inhalational agent. They also serve to reduce the amount of maintenance anaesthetic. Supplemented with analgesics and muscle relaxants, they can also be used as the sole anaesthetic.


Thiopentone sod.


It is an ultrashort acting thiobarbiturate, highly soluble in water yielding a very alkaline solution, which must be prepared freshly before injection. Extravasation of the solution or inadvertent intraarterial injection produces intense pain—necrosis and gangrene may occur.


Injected i.v. (3–5 mg/kg) as a 2.5% solution, it produces unconsciousness in 15–20 sec. Its undissociated form has high lipid solubility— enters brain almost instantaneously. Initial distribution depends on organ blood flow— brain gets large amounts. However, as other less vascular tissues (muscle, fat) gradually take up the drug, blood concentration falls and it back diffuses from the brain: consciousness is regained in 6–10 min (t½ of distribution phase is 3 min).


On repeated injection, the extracerebral sites are gradually filled up—lower doses produce anaesthesia which lasts longer. Its ultimate disposal occurs mainly by hepatic metabolism (elimination t½ is 7–12 hr), but this is irrelevant for termination of action of a single dose. Residual CNS depression may persist for > 12 hr. The patient should not be allowed to leave the hospital without an attendant before this time.


Thiopentone is a poor analgesic. Painful procedures should not be carried out under its influence unless an opioid or N2O has been given; otherwise, the patient may struggle, shout and show reflex changes in BP and respiration.


It is a weak muscle relaxant; does not irritate air passages. Respiratory depression with inducing doses of thiopentone is generally transient, but with large doses it can be severe. BP falls immediately after injection mainly due to vasodilatation, but recovers rapidly. Cardiovascular collapse may occur if hypovolemia, shock or sepsis are present. It does not sensitize the heart to Adr, arrhythmias are rare.


Thiopentone is a commonly used inducing agent. It can be employed as the sole anaesthetic for short operations that are not painful.


Adverse Effects


Laryngospasm occurs generally when respiratory secretions or other irritants are present, or when intubation is attempted while anaesthesia is light. It can be prevented by atropine premedication and administration of succinylcholine immediately after thiopentone. Succinylcholine and thiopentone react chemically—should not be mixed in the same syringe.


Shivering and delirium may occur during recovery. Pain in the postoperative period is likely to induce restlessness; adequate analgesia should be provided. Postanaesthetic nausea and vomiting are uncommon.


It can precipitate acute intermittent porphyria in susceptible individuals—contraindicated.


Other Uses


Occasionally used for rapid control of convulsions.

Gradual i.v. infusion of subanaesthetic doses can be used to facilitate verbal communication with psychiatric patients and for ‘narcoanalysis’ of criminals; acts by knocking off guarding.


PENTOTHAL, INTRAVAL SODIUM 0.5, 1 g powder for making fresh injectable solution.


Methohexitone sod. 


It is similar to thiopentone, 3 times more potent, has a quicker and briefer (5–8 min) action. Excitement during induction and recovery is more common. It is more rapidly metabolized (t½ 4 hr) than thiopentone: patient may be roadworthy more quickly.




Currently, propofol has super-seded thiopentone as an i.v. anaesthetic, both for induction as well as maintenance. It is an oily liquid employed as a 1% emulsion. Unconsciousness after propofol injection occurs in 15–45 sec and lasts 5–10 min. Propofol distributes rapidly (distribution t½ 2–4 min). Elimination t½ (100 min) is much shorter than that of thiopentone due to rapid metabolism.


Intermittent injection or continuous infusion of propofol is frequently used for total i.v. anaesthesia when supplemented by fentanyl. It lacks airway irritancy and is particularly suited for outpatient surgery, because residual impairment is less marked and shorter lasting. Incidence of postoperative nausea and vomiting is low; patient acceptability is very good. Excitatory effects and involuntary movements are noted in few patients. Induction apnoea lasting ~1 min is common. Fall in BP due primarily to vasodilatation with less marked cardiac depression occurs consistently, and is occasionally severe, but short lasting. Bradycardia is also frequent. Maintenance anaesthesia with propofol produces dose-dependent respiratory depression which is more marked than with thiopentone. Pain during injection is also frequent; can be minimized by combining with lidocaine.


Dose: 2 mg/kg bolus i.v. for induction; 9 mg/kg/hr for maintenance.


PROPOVAN 10 mg/ml and 20 mg/ml in 10, 20 ml vials.


In subanaesthetic doses (2.4 mg/kg/hr) it is the drug of choice for sedating intubated patients in intensive care units. However, it is not approved for such use in children; prolonged sedation with higher doses has caused severe metabolic effects and heart failure even in adults.




It is another induction anaesthetic, which has a briefer duration of action (4–8 min) than thiopentone; produces little cardiovascular and respiratory depression, but motor restlessness and rigidity is more prominent as are pain on injection or nausea and vomiting on recovery. It is a poor analgesic and has not found much favour.



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