## Weights and measures

In this book doses are given in the metric system, or in international units (IU) when metric doses are impracticable.

Equivalents: 1 litre (1 or L) = 1.76 pints 1 kilogram (kg) = 2.2 pounds (lb)

Abbreviations: 1 gram (g)

1 milligram (mg) (1 x 10 3 g) 1 microgram3 (1 x 10"6 g) 1 nanogram3 (1 x 10~9 g) 1 decilitre (dL) (1 x 10"11) 1 millilitre (mL) (1 x 10"31)

Home/domestic measures. A standard 5 ml spoon and a graduated oral syringe are available. Otherwise the following approximations will serve: 1 tablespoonful = 14 ml (or mL) 1 dessertspoonful = 7 ml (or mL) 1 teaspoonful = 5 ml (or mL)

' Spell out in full in prescribing.

Percentages, proportions, weight in volume

Some solutions of drugs (e.g. local anaesthetics, epinephrine/adrenaline) for parenteral use are labelled in a variety of ways: percentage, proportion, or weight in volume (e.g. 0.1%, 1:1000,1 mg per mL). Also, dilutions may have to be made by doctors at the time of use. Such drugs are commonly dangerous in overdose and great precision is required, especially as any errors are liable to be by a factor of 10 and can be fatal. Doctors who do not feel confident with such calculations (because they do not do them frequently) should feel no embarrassment,4 but should recognise that they have a responsibility to check their results with a competent colleague or pharmacist before proceeding.

4 Called to an emergency tension pneumothorax on an intercontinental flight, two surgeons, who chanced to be passengers, were provided with lignocaine 100 mg in 10 ml (in the aircraft medical kit). They were accustomed to thinking in percentages for this drug and 'in the heat of the moment' neither was able to make the conversion. Chest surgery was conducted successfully with an adapted wire coat-hanger as a trocar ('sterilised' in brandy), using a urinary catheter. The patient survived the flight and recovered in hospital. Wallace W A 1995 Managing in-flight emergencies: A personal account. British Medical Journal 311: 374.

Clinical pharmacology