Context-Sensitive Half-Time — why recovery depends on how long you infused

The idea: during a target-controlled infusion the plasma level is held constant, but the deep, poorly-perfused tissues (fat and muscle) keep loading up — the longer you run, the fuller that slow tank. When you stop, plasma falls partly by elimination and partly by continued redistribution into those tissues; once they are saturated, that sink is gone and recovery slows. The context-sensitive half-time is the time for the plasma level to fall by 50% after stopping — and it depends on the "context", i.e. how long you infused. Remifentanil is cleared so fast that it stays flat (~2–4 min) whatever the duration; fentanyl accumulates in the deep tank and its half-time climbs steeply. This is why terminal elimination half-life tells you little about real-world recovery.