3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.1. Pharmacology principles
                      3.1.3.1.1. Pharmacokinetics
 3.1.3.1.1.6. Half-time and half-life 

Half-time and half-life

[SH4:p7]

Terminology

Elimination half-time vs elimination half-life

[Chris Flynn] T1/2 is important for calculating:

  • Duration of action
  • Time to steady state (4-5 T1/2)
  • Dosing intervals

Elimination half-time

Elimination half-time is time necessary for plasma concentration of a drug to fall to 50% during the elimination phase

  • ... is independent of the dose administered
  • Does not include the effect of redistribution

NB:

  • Looking at propofol [SH4:p156], I believe Stoelting uses elimination half-time as half-life during the beta phase... which could be a slow redistribution phase and is still different from actual elimination halflife quoted by other authors
    --> i.e. Elimination half-time is essentially halflife after the initial rapid redistribution phase, and is not about elimination at all.

Elimination half-life

Elimination half-life is time necessary to eliminate 50% of the drug from the body after an IV bolus

Difference arise between half-time and half-life

Where elimination from body doesn't parallel the plasma concentration
--> elimination half-time and half-life differ

Context-sensitive half-time

Context-sensitive half-time is time necessary for plasma concentration of a drug to fall to 50% after a continuous infusion of a specific duration

  • Context refers to the duration
  • Context-sensitive half-time generally increases with the duration of infusion
    * Due to less capacity available in inactive tissues for redistribution
  • No constant relationship to elimination half-time

Effect-site equilibration time

Effect-site equilibration time is the half-time of equilibration between the drug concentration in plasma and the drug effect
--> Impacts on the time of onset

Examples of short effect-site equilibration time

  • Remifentanil
  • Alfentanil
  • Thiopentone
  • Propofol

Examples of longer effect-site equilibration time

  • Fentanyl
  • Sufentanil
  • Midazolam

Decrement times

[From the web] [???] Need more authoritative source

  • Decrement time is the time required to decreased the concentration by a certain value after a continuous infusion of a certain duration
    * e.g. context-sensitivie half-time is essentially decrement time for 50% decrease
  • Decrement time is plotted on a graph
    * Horizontal axis = duration of infusion
    * Vertical axis = decrement time
  • Decrement time may be of more value clinically because it is rare that we wish to reduce the concentration of a drug by precisely 50%