Propofol TCI modelling for procedures under sedation
Marsh model
Schnider model
Implication for sedation
Concentration target/recommended mode
Plasma concentration
Sedation- 1.5–5 μgm/ml
Effect site concentration- brain
Central compartment volume for initial dose
15.9 l (for 70 kg male)
4.27 l (independent of age and weight)
Initial doses smaller for Schndier model – Less hemodynamic instability
Fixed initial dose in Schnider model irrespective higher weight or age- poorer predictability
Infusion rates estimated immediately after bolus
Generally higher for all patients
Lower
Lesser dose may reflect into better hemodynamic stability (especially in moribund patients)
Over pressure technique (higher initial bolus)
Can be performed manually
Inherent part of protocol- gradual higher dose
Faster onset with Schnider model
Rapid increase in depth available- e.g., bronchoscopies etc.
Initial infusion rate estimation based on
Total body weight
Total body weight and Lean body mass (calculated value)
Studies show poor predictability with increasing BMI in Schnider model
Marsh model may overestimate total dose
Effect site equilibration constant (KeO)
0.26
0.456
Predicted time to peak effect (TTPE) is smaller with Schnider- Rapid onset
More hemodynamic instability in elderly – as drug overshoots higher
Estimated rate of drop of concentration
Independent for age
Age accounted
Elderly would have smaller level fall- maintenance may be overestimated
Evidence based overall total dose
Higher
Lower
No Direct comparison available- wake up times may be better in Schnider model
Many studies have evaluated the benefits of TCI based propofol delivery systems and the well-known benefits include
- (a)
Hemodynamic stability- Studies have shown that with the use of TCI based propofol delivery the total amount of propofol consumed is lower than that administered via manual boluses or fixed rate continuous infusions. This decreased amount of propofol delivered translates into better preserved hemodynamic stability. Patients sedated in intensive care units have shown to need fewer vasopressors with the use of propofol TCI [4].
- (b)
Decreased airway intervention- Evidence suggests that with the use of TCI based sedation the overshoot and undershoot of propofol concentration is less likely. The overshoot of concentration (when given as bolus) has the propensity to cause apnea or airway obstruction. Hsu et al. demonstrated that these episodes in patients undergoing endoscopy were significantly lower with the use of propofol TCI in comparison to fixed infusion of propofol.
- (c)
Faster recovery- one of the aim of developing technology is to improve the safety and also lower the associated costs. As already stated, the total amount of propofol injected can be decreased with TCI use the patient wakeup times are actually smaller. Lin et al. showed that patients sedated with propofol TCI for bronchoscopic procedures had smaller discharge times with lower complication rates [5]. These rapid turn overs can reflect in terms of saving vital operating room time that can have long term economic impact [6].
- (d)
Better patient and endoscopist satisfaction towards the procedure- Propofol based TCIs maintains a steady state of concentration and prevents changes in patient’s depth of sedation. This helps to avoid procedural interruptions and thus improves the endoscopist satisfaction. On the other hand, frequent episodes of light sedation can lead to patient awareness and thus cause poor patient satisfaction. TCI being able to maintain a steady state of sedation helps in both the above goals. These aspects of propofol TCI used were demonstrated by Fanti et al. during gastroenterological sedation procedures [7].
- (e)
Improved efficiency of the sedation provider- This is an indirect benefit. Manual titration of propofol dose requires constant vigilance. Frequent adjustments in infusion doses can divert the attention from monitoring. TCI after initial bolus automatically adjusts its dose to maintain a steady state of sedation. This can allow the sedation provider to focus on patient monitoring more effectively.
Presently, “Open TCI Pumps” are available in the market. They allow the operator to not only choose what modelling for a particular drug to use but also allow to alter the drug. Thus in a single TCI system in addition to propofol an additional drug can be used as well (in a separate syringe). This would cut down hospital costs allowing to buy single device to run TCI for drugs like propofol and remifentanil. A few of these open TCI systems include -CATIA, IVA-SIM (University of Bonn, Germany), STANPUMP (Stanford University, USA), STELPUMP (University of Stellenbosch, South Africa) etc.
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