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Target Site

Both plasma and effect-site targeting can be used with TCI. Gain a better understanding of which target site to choose for your patients.

Blood vs. Effect-site control

  • The effect-site concentration lags behind the plasma concentration because of the blood-brain barrier
  • TCI systems can reduce this delay by raising the plasma concentration beyond the intended effect-site concentration. This then increases the gradient between plasma and effect site to “pressure” the drug into the effect site more quickly. Most drug TCI models now incorporate this option. sitetarget
  • Caveats
    • PK models can only predict the concentration achieved in plasma and effect site.
    • The plasma concentration can be verified by taking blood samples to compare the predicted with the actual concentration so that the precision and bias can be calculated.
    • However, the effect-site concentrations can not be measured but are estimated from the clinical effect. Models, therefore, often vary considerably in predicting effect-site concentrations.
    • Your patient’s pharmacokinetics and pharmacodynamics may also vary from the model
    • Using effect-site targeting results in higher plasma concentrations and may increase adverse effects e.g. hypotension
    • Careful titration and monitoring are required

Impact of Ke0

  • Keo is a rate constant that describes the time lag between Cp and Ce
  • Larger Ke0 = Shorter time lag
  • Incorporating a smaller Ke0 results in a larger bolus to achieve the same Ce when compared to the same model using a larger Ke0
  • TCI systems using the Marsh model often incorporate a larger Ke0 than originally intended to reduce the impact of a large bolus dose at induction
  • This is still more accurate than the Schnider effect site model which overestimates the effect-site concentration much more than Marsh
  • The Eleveld model, clinically, seems to be the most accurate in estimating effect-site
  • It is difficult to know the “right” Ke0 – it is model specific and there are also inter-individual differences
  • The following graphs reveal the difference between a small and large Ke0 TCI system on effect-site target at induction Ke0

Ke0

  • Caveats
    • If you are confused with Ke0, don’t worry!
    • When we talk about the TCI models and dosing, we will give you more guidance on models and techniques
  • Ke0 is the parameter that should be considered when predicting loss and recovery of consciousness based on the effect-site concentration
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