In any basic pharmacology course, we’re taught the concept of an “elimination half-life”, or the time it takes to reach 50% of a drug’s initial concentration. With more advanced multi-compartmental models of pharmacokinetics, the concept of a “context-sensitive half-time” more accurately describes the interplay between a drug’s distribution in the body’s compartments, metabolism, and excretion to reach the same 50% concentration endpoint.
A context-sensitive half-time describes the relationship that the duration of a drug infusion has on its apparent half-life. When we first start an infusion, the plasma and tissue concentrations are both zero. With the initiation of the medication (let’s say a propofol drip), the plasma concentration increases and some of the propofol is rapidly metabolized by the liver. High levels of plasma propofol will subsequently transfer to vessel rich organs, fatty tissue, etc. based on concentration gradients.
When the propofol infusion is stopped, plasma concentrations will begin to fall. This will favor transfer out of the peripheral tissues back into the plasma. Now propofol will continue exerting its effect even though the intravenous infusion was stopped. Coupling this with ongoing metabolism, distribution, and excretion, one can see that there are many factors playing into pharmacokinetics. The context-sensitive half-time reflects the cumulative effects of these factors.
This concept is marked with lipophilic drugs like thiopental, diazepam and fentanyl. In the case of fentanyl, after an infusion of 2-3 hours, its context-sensitive half-time skyrockets due to the depot of fentanyl in peripheral tissues. This is why I’m against using fentanyl drips for sedation in patients requiring mechanical ventilation. If the drip has been running for a while, it will take MANY hours before the patient is ready to extubate after discontinuing the sedation.
Drugs like remifentanil are unique because, as shown in the figure, its context-sensitive half-time is independent of the duration of infusion. In other words, it doesn’t matter if you run remifentanil for 2 hours or 10 hours, it has a context-sensitive half-time of 3-4 minutes. Remember this tells you nothing about whether the plasma level you achieve is within the drug’s therapeutic window (ie, is it actually providing analgesia in the case of remifentanil?)
Next time you run an infusion of ANYTHING, remember the concept of context-sensitive half-times! 🙂