Ketamine

A 78-year-old male with advanced dementia presents with a large incarcerated inguinal hernia. He is extremely confused, agitated and combative. He is being physically violent and despite his age and weighing only 60 kg he is requiring four theatre staff to prevent him from falling off the theatre table. He has already kicked one theatre support worker and attempts to secure venous access have failed, prompting further violent outbursts from the patient. It is your judgement that he requires a rapid sequence induction but that he currently poses a risk of harm to himself and others. It is your intention to provide sedation sufficient to tolerate intravenous cannulation whereupon you will pre-oxygenate and perform an intravenous rapid sequence induction. You request ketamine, 100 mg/mL, which you plan to deliver  intramuscularly. Which one of the following is the most suitable volume to administer?

a) 0.6mL

b) 1.2mL

c) 2.4mL

d) 4.2mL

e) 6.0mL

Answer: b

Explanation

Ketamine is a versatile drug with numerous applications but is often avoided because of concerns over its psychomimetic side effects. It may be delivered by virtually any route, but in the UK is licensed for intravenous and intramuscular use. It has a wide dose range according to delivery route and intended degree of depression of conscious level. For induction of anaesthesia, an intravenous dose of 1 to 2 mg/kg is quoted but 0.5 mg/kg may often be adequate in an older patient. For intramuscular induction of anaesthesia, 4 to 10 mg/kg is suggested. Sedation dose needs to accommodate the clinical state of the patient and the nature of the procedure to be facilitated (in terms of likely stimulation). A dose of 0.2 to 0.5 mg/kg is sufficient intravenously or 2 to 4 mg/kg intramuscularly. In this case, the patient is unwell, poses a risk of aspiration and only needs to tolerate intravenous cannulation and pre-oxygenation. Inadvertent induction of anaesthesia is a possibility. For this reason, the lowest dose of the intramuscular range is selected: 2 mg/kg, 120mg or 1.2mL of a 10% preparation.