Intravenous agents

Thiopentone

1.    Most commonly used barbiturate for induction of anaesthesia and it is often combined with inhaled anaesthetics.

2.       Pharmacokinetics:

a.       Given i.v. as a single dose, rapidly induces hypnosis and anaesthesia without analgesia.

b.       Being highly lipid soluble, it crosses the blood-brain barrier easily.

c.    It re-distributes rapidly to all body tissues which explains its ultra-short duration of action. (t1/2 9min).

d.       Extensively metabolized in the liver. (t1/2 60min).

3.       Advantages:

a.    In a single dose, usually exerts its cerebral depressant effect within 30s.

b.       Effect persists for about 4 – 7 min.

c.       Anaesthesia is induced rapidly, pleasantly and without excitement.

4.       Disadvantages:

a.    Has insignificant analgesic action.

b.       Cannot be used alone as an anaesthetic agent.

c.       Large and repeated doses results in prolonged anaesthesia and delayed recovery characterized by somnolence and respiratory and circulatory depression.

5.       Contraindications:

a.       Allergy to barbiturates.

b.       Dyspnoea or obstructive respiratory disease.

c.       Status asthmaticus.

d.       Addison’s disease.

e.       Hepatic dysfunction.

f.       Myxoedema.

6.       Precautions:

a.       Equipment for resuscitation and endotracheal intubation should be available and ready for use.

b.       Patient must always lie supine as even a small overdose can cause hypotension.

7.       Adverse effects:

a.    A short period of apnoea may follow i.v. injection.

b.       Rapid injection may result in severe hypotension and hiccoughs.

c.       Coughing, sneezing, or laryngeal spasm may occur during induction.

8.    Drug interactions:

a.       Other cerebral depressants, e.g. opioids, may augment the action of thiopentone.

b.       Antihypertensives or diuretics may augment hypotensive effect.

Ketamine

1.    A phencyclidine derivative which in anaesthetic produces a trance-like state called dissociative anaesthesia: sedation, amnesia, dissociation, analgesia.

2.       Pharmacokinetics:

a.       Highly lipid soluble, rapidly distributed to body tissues with short duration of action.

b.       Undergoes hepatic metabolism with both urinary and biliary excretion.

3.       Effects:

a.       Cardiovascular stimulation with increased plasma catecholamines.

b.       Increases in heart rate, blood pressure and cardiac output.

c.       Increases cerebral blood flow, oxygen consumption and intracranial pressure.

4.       Advantages:

a.       Anaesthesia persists for up to 15min after a single i.v. injection and is characterized by profound analgesia.

b.    It is less likely than other anaesthetic agents to induce vomiting.

c.       Since it does not induce hypotension the patient does not have to remain supine.

d.    It is of particular value in children and poor-risk patients, and also in asthmatic patients, because it rarely induces bronchospasm.

5.       Disadvantages:

a.       Produces no muscular relaxation.

b.       Tends to raise heart rate, intracranial and intraocular pressure.

c.       Emergence phenomenon during recovery: disorientation, sensory and perceptual illusion and vivid dreams.

6.       Uses:

a.       Subanaesthetic doses of ketamine may be used to provide analgesia for painful procedures of short duration such as dressing of burns, radio-therapeutic procedures, marrow sampling and minor orthopaedic procedures.

b.    May be used for the induction of anaesthesia or for both induction and maintenance for short-acting diagnostic and surgical interventions, including dental procedures.

c.    Of particular value in children requiring frequent repeated anaesthetics.

7.       Dosage and administration:

a.       Administration of ketamine should always be preceded by premedication with atropine to reduce salivary secretions.

b.       Premedication with diazepam reduces subsequent requirement for ketamine and the incidence of emergence reactions.

8.       Contraindications:

a.       Moderate to severe hypertension.

b.       Congestive heart failure or a history of stroke.

c.       Acute or chronic alcohol intoxication.

d.       Cerebral trauma.

e.       Intracerebral mass or haemorrhage.

f.       Psychiatric disorders such as schizophrenia.

9.    Use in pregnancy:

a.       Ketamine is contraindicated in pregnancy before term, since it has oxytocic activity.

b.    It  is also contraindicated in patients with eclampsia or pre-eclampsia.

c.    It may be used for assisted vaginal delivery.

d.    It is better suited for use during caesarean section; ketamine results in less fetal and neonatal depression than do other anaesthetics.