This viva is relevant to Section B(v) of the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the concepts of effect-site and context sensitive half time".
"Half-life (t½) is the time required to change the amount of a drug in the body by one-half during elimination" - College answer to Question 11 from the second paper of 2012
"Half-life is the time take for the amount of the drug in the body (or the plasma concentration) to fall by half." - Birkett, 2009
t½ = 0.693 × Vd /CL
Half life is dependent on a first-order elimination rate.
If the drug is eliminated by first-order kinetics, one can plot the concentration on a linear scale over time and achieve a familiar concentration/time curve, with predictable halving of the concentration with each passing time interval. This does not work if the elimination of the drug occurs at a constant rate which is independent of concentration.
"Context-sensitive half time is the time required for a 50% decrease in the central compartment drug concentration after an infusion of the drug is ceased; where the "context" is the prior duration of drug infusion." - Hughes Glass and Jacobs (1992)
Context-sensitive half time is the concept which relates the drug distribution into and out of tissue compartments to the change in plasma concentration after sustained infusion. It is defined as the time required for the plasma concentration to fall to half of the value at the time of stopping an infusion.