TARGET CONTROLLED INFUSIONS DURING ANESTHESIA; PHARMACOLOGICAL PRINCIPLES

⏩TCIs are used for sedation and as an alternative to inhalational anaesthesia. 

⏩Here an infusion, controlled by a microprocessor-driven pump that alters the infusion rate to maintain a user-defined target plasma concentration ( as guided by certain pharmacokinetic models e.g. modified Marsh model and Schnider models for propofol and the Minto model for remifentanil.) delivers the drug.

⏩ Previous models used the plasma concentration (Cp) to target. Recent models target an effect site concentration (Ce). The 'effect site compartment' reflects the hysteresis between plasma concentration and the clinical effect observed.

⏩They basically work based on the principle of the bolus, elimination and transfer regime:

1️⃣An initial bolus dose to fill the central compartment with the drug (B)

2️⃣An infusion of constant rate, equal to the elimination rate (E)

3️⃣An infusion that compensates for transfer of drug to the peripheral tissues (T)

.......forming the 'BET' regimen

⏩ The Marsh model assumes that the central compartment volume is directly proportional to the weight of the patient only. It does not use age as part of its calculation, but will not function if an age less than 16 is entered. 

⏩ The more recent Schnider model calculates lean body mass using age, height and weight and calculates doses and infusion rates accordingly. 

⏩ Consequently , there is difference, in the size of the central compartment when calculated using different models

REMEMBER👇🏾

⏩The plasma and effect-site concentrations are calculated ; not measured (as compared with end tidal agent monitoring). 

⏩The hemodynamic impact of the drug will depend upon the maximum plasma concentration achieved and so in elderly patients, a large overshoot in the plasma concentration and haemodynamic compromise can happen, when a bolus is given: be cautious

⏩In morbidly obese patients with a BMI > 42, with increasing body weight the calculation of Lean Body Weight used in the Schnider model paradoxically decreases. This decrease leads to a higher estimated clearance and thus a larger initial bolus dose and higher infusion rate will be administered by the pump.

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Reference: Hill SA. Pharmacokinetics of drug infusions. Contin Educ Anaesth Crit Care Pain. 2004; 4(3): 76–80.