The contribution of pharmacokinetics and pharmacodynamics to clinical anaesthesia care.

Defining dose requirement, pharmacokinetics and pharmacodynamics Clinical anaesthetists administer doses of drugs to obtain various degrees of drug effect. Some anaesthetic drug effects, such as muscle relaxation, are relatively easy to quantitate, whereas others such as anaesthetic depth are much more difficult. Anaesthetic dose requirement can be defined as the dose of drug that produces a defined degree of effect. For example, an EDs0 would represent the dose of a drug that results in 50 per cent of the maximal drug effect, whereas an EDgs is the dose that creates 95 per cent of the maximal drug effect. Alternatively, the dose requirement can be expressed as the dose that results in 50 or 95 per cent of a group of patients reaching a defined drug effect, i.e., no response to a specific stimuli. The dose requirement is determined by two pharmacological processes: pharmacokinercs, which is the relationship between dose and drug plasma concentration and pharmeeodynamics, the relationship between drug plasma concentration and degree of drug effect, Pharmaeokinetics characterizes those physiological or pharmacological processes that govern the resultant drug plasma concentration or "what the body does to the drug." Pharmacodynamics represents the relationship between drug plasma concentration, the drug concentration at the site of action and the actual occurrence of a measurable drug effect. Pharmacodynamics represents "what the drug does to the body."