Describe Context-Sensitive Half-Time in detail.

Definition

The context-sensitive half-time is the time taken for the plasma concentration of a drug to fall by half after stopping a continuous infusion — where the ‘context’ is the duration of the infusion. Unlike the elimination half-life, it takes account of the drug’s distribution into tissues during the infusion, and for most drugs it increases the longer the infusion has run.

The context-sensitive half-time rises with infusion duration for most drugs (e.g. Fentanyl) but stays almost flat for remifentanil — which is why it gives rapid, predictable offset.

Clinical Importance

It predicts how quickly a patient will recover after an infusion is stopped, and hence which drugs are suitable for TIVA. Remifentanil has an almost flat, very short context-sensitive half-time (it is broken down by tissue esterases independent of infusion duration), giving rapid offset however long it runs; propofol’s remains short; whereas drugs like fentanyl accumulate, so their half-time lengthens markedly with prolonged infusion.

• Context-sensitive half-time = time for concentration to halve after stopping an infusion of given duration.
• Accounts for tissue distribution; usually lengthens the longer the infusion runs.
• Predicts recovery after infusion → which drugs suit TIVA.
• Remifentanil: flat/short (esterase metabolism); propofol short; fentanyl accumulates.