Decrement time

Decrement time

• Estimate of the time course of the decline of plasma or effect site concentration after drug administration is stopped
• Determined by three simultaneous exponential processes
  • Rapid transfer from the central compartment to and from the rapidly equilibrating compartment (muscle)
  • Slow transfer from the central compartment to and from the slowly equilibrating compartment (fat)
  • Elimination of drug
• Complex calculation
  • Each process occurs at a different rate and has a different rate constant
  • Each process relative contribution to the decrement time changes over time
  • Thankfully modern TCI pumps take care of this challenging arithmetic for you
• 50% decrement time = context-sensitive half-time (CSHT)
• The decline in plasma concentrations is sensitive to the ‘context’ – meaning the duration and dosage of the drug prior to stopping the infusion.
• The higher the concentration, the longer the decrement time will be until a prefixed level is reached
• TCI pumps have an inbuilt decrement function that displays the estimated time it would take, after stopping the infusion, for the current plasma concentration to decline to a chosen plasma concentration
• TCI default decrement concentration setting is usually
  • Propofol : 1.5 μg.ml−1
  • Remifentanil: 1 ng.ml−1
  • These values can be changed easily to allow for individual patient characteristics
  • Different patients will regain consciousness at different plasma and effect-site concentrations
• Slow induction of anaesthesia will allow you to correlate loss of consciousness and effect-site concentrations for your patient
  • Neural inertia may explain why the return of consciousness occurs at a lower effect-site concentration than at induction
• Most drugs show context-sensitive pharmacokinetics
  • The longer the infusion the longer the decrement time
  • Exception: Remifentanil is metabolised by plasma esterase
  • Remifentanil’s decrement time remains relatively constant approx 4 minutes regardless of the duration of the infusion

Caution: decrement time is given on most TCI pumps to plasma concentraion not effect site concentration. The gradient between plama and effect site is greatest at the start of the infusion and after the infusion is discontinued