Drugs in concentrations

Drugs in concentrations

When a substance is dissolved in a liquid it forms a solution. The volume of a solution is expressed in litres or millilitres. 1litre = 1000mls.

The substance dissolved is known as the solute. The amount of solute in a solution is expressed as a concentration. The amount of solute may be described by its mass (grams or milligrams per litre) or by its amount (moles per litre or millimoles per litre).

If the solute has a known chemical formula eg. salt (NaCl), then it is preferable to use mol/l or mmol/l. If the solute does not have a defined chemical composition (such as a protein), then mg/l or g/l is used.

Some solutions such as local anaesthetics and thiopentone that are used in anaesthesia on a daily basis are expressed as a percentage eg. lignocaine 2% and thiopentone 2.5%. When using drugs prepared in this way it is necessary to calculate the number of mg in ml of solution. This is easiest done by multiplying the percentage of the solution by 10:

• 2% lignocaine x 10 = 20mg/ml
• 0.5% bupivacaine x 10 = 5mg/ml
• 2.5% thiopentone x 10 = 25 mg/ml

The maths behind this calculation is as follows:

• A 2.5% solution means that there is 2.5g of thiopentone in 100ml:
  2.5g in 100ml
  = 2500mg in 100ml
  = 25mg in 1 ml (25mg/ml)

Some solutions such as adrenaline (epinephrine) may be expressed as 1:1000 or 1:10 000 or 1:100 000. This means that in a 1:1000 adrenaline ampoule there is one part adrenaline to 1000 parts solution. To work out how many milligrams of adrenaline are present:

• 1:1000 solution adrenaline
  = 1g adrenaline in 1000ml solution
  = 1000mg adrenaline per 1000ml solution
  = 1mg per ml
• 1:10 000 solution adrenaline
  = 1g adrenaline in 10 000ml solution
  = 1000mg per 10 000ml
  = 1mg per 10ml which can also be expressed as 100mcg per ml