The curriculum is a bit vague on TIVA, asking for 'advantages and disadvantages' of the technique, correct set up of TCI for induction/maintenance and use in middle ear surgery.
The 2018 SAQ on TIVA (70% pass rate) saw examiners mostly critique candidates' handwriting, as well as lament a lack of knowledge of TCI systems.
Propofol; either as a ml/hr infusion (e.g. ICU) or according to models such as Bristol, Schnider, Marsh or Eleveld
Remifentanil; Minto or Eleveld models
Alfentanil; Maitre model
Ketamine; Domino model
Other drugs are used on intensive care units to facilitate a form of TIVA in critically unwell patients e.g. fentanyl, midazolam, morphine but often as simple ml/hr infusions
May cause greater haemodynamic instability or cardiac dysfunction, including bradycardias
AAGBI guidelines state must use depth of anaesthesia monitoring if using TIVA + NMBA, but some surgeries make using BIS impossible/invalid
Risk of awareness
Risk of delivery failure if poor or difficult venous access
Actual blood level of drug may be different to calculated plasma concentration; generally within 25% but still requires titration
Contraindicated in those with mitochondrial disease
Target controlled infusions (TCI) use micro-processor controlled infusion pumps pre-programmed according to a pharmacokinetic model for a given drug e.g. propofol's 3-compartment model
They allow selection of a target site concentration required for a certain effect (sedation, anaesthesia)
TCI pumps have other advantages such as providing a decrement time
Loss of consciousness typically occurs at a Ce of 1μg/ml although naturally this has inter-patient variability
Bristol
This is actually a manual TIVA model which targets a blood concentration of 3μg/ml within 2mins
It is based on patients who received temazepam pre-medication, 3μg/kg fentanyl and 1mg/kg propofol induction
One the provides a variable rate propofol infusion:
10mg/kg/hr for 10mins
8mg/kg/hr for 8mins
6mg/kg/hr thereafter
Compared to the target controlled infusions below, it tends to use more propofol with consequent greater haemodynamic effects and a slower wake-up
Schnider TCI
1998 model based on a study of 24 patients
Uses weight, age, height and gender
Allows plasma- and effect-site concentration targeting
Tends to underestimate plasma propofol level during the recovery phase
Marsh TCI
1991 model based on data from the 1987 Gepts study (18 patients)
A 'linear' model based purely on weight (lean body mass), although not suitable for those <16yrs old so also asks for the patient's age.
Delivers a much large initial propofol bolus than Schnider due to a large central compartment size in the model
The standard Marsh model only allows plasma-site targeting although the modified Marsh model allows effect-site targeting
The synergy between propofol and opioid TCI can provide both highly effective surgical anaesthesia and a propofol-sparing effect
Remifentanil
Remifentanil is an ultra-short acting synthetic opioid commonly used as a TCI alongside propofol
Available models: Minto, Eleveld
Its context-insensitive half-time make it a popular choice for TCI, as well as its synergy with propofol
Its short-acting nature necessitate the use of additional opioids towards the end of a case, while concerns remain over remifentanil-induced hyperalgesia
Alfentanil
Alfentanil is another short-acting synthetic opioid which became more popular as a choice for TCI during periods of remifentanil shortages
Available models:
Maitre: weight, age, and sex covariates
Scott: no covariates so the same dose is infused regardless of demographics
It benefits from having synergy with propofol, as well as a low lipid solubility reducing the rate of accumulation compared to fentanyl
Similar time to onset of effect as remifentanil, but longer offset when used as a TCI (BJA, 2022)
Although its context-sensitive half-time is predictable, it does rise to reach a CSHT of 1hr after 8hrs infusion
This is longer than propofol's CHST, therefore may require down-titrating towards the end of surgery
In some cases, 1-2mg of alfentanil are mixed with propofol in a single syringe, which is then used as a propofol TCI (a.k.a 'dirty TIVA')
Sufentanil
Sufentanil is a potent synthetic opioid which can be used as a TCI
Available models: Gepts
It benefits from a shorter CSHT than alfentanil, but still requires down-titration at the end of the case
Fentanyl
TCI models (McClain, Shafer) for fentanyl are not commercially available, so if an infusion is desired a manual regimen is required
An example manual regimen is:
3μg/kg bolus
2μg/kg/hr infusion for 30mins
1.5μg/kg/hr infusion for the next 120mins
1μg/kg/hr infusion thereafter
Infusion stopped 30mins before skin closure
The CSHT of fentanyl rises rapidly after 1hr, making it a poor choice for TCI unless rapid emergence is not required
Its longer duration may make it suitable if significant post-operative pain is anticipated
Other, non-opioid adjuncts may be used alongside propofol TIVA to provide both multi-modal analgesia and a propofol-sparing effect
Drug
Regimen
Reduction in propofol requirements
Dexmedetomidine
1μg/kg loading over 5-10mins 0.2 - 0.7μg/kg/hr infusion
20-50%
Ketamine
0.1 - 1mg/kg loading 0.1 - 0.2mg/kg/hr infusion
20-40%
Magnesium
20 - 50mg/kg loading over 20mins 6 - 20mg/kg/hr infusion
15-20%
Lidocaine
1 - 1.5mg/kg loading 1 - 2mg/kg/hr infusion
10-20%
If total body weight is used in the models then a greater induction dose will be delivered
Equally a higher-than-normal infusion rate will be delivered
Following NAP5, it was suggested using total body weight is probably better than ideal or lean body weight to reduce the risk of accidental awareness
SOBA say that one should use lean body weight for induction but then total body weight or ABW40 for maintenance
Model issues with high BMI
Marsh
A body weight of up to 150kg is allowed
However, entering actual body weight may lead to a significant overdose due to the linear nature of the model
Anecdotally, there is concern using lean body weight may conversely cause underdosing, and ABW40 is often used instead
Schnider
Calculates LBW as part of the model
Paradoxically smaller initial bolus dose when BMI exceeds 42kg/m2 (men) or 35kg/m2 (women)
Requires the use of 'fudge factors' e.g.
Increasing patient height until an allowable actual body weight can be put in
Use Servin's formula; input weight as IBW + 0.4(actual - ideal BW)
Minto
TBW probably better than LBW/IBW
However the pump won't allow extremely high BMI to be entered
Often the correct (total) weight is entered but a fudged height is used to bypass this
Alarms
High infusion pressure alarm
Low infusion pressure alarm
End-of-infusion warning (5mins)
End-of-infusion alarm
Disengagement of driver warning
Low battery or mains disconnection warning
Occlusion alarm
Syringe drivers
Displays drug and concentration on-screen to reduce risk of wrong-drug infusion
Confirms syringe driver and type to ensure correct rate of infusion
Syringe driver service record alerts to aid regular maintenance
Use a TCI system checklist
Regular TCI pump maintenance
Cannula-related factors:
Affixed firmly to patient's skin
Use anaesthetist-inserted cannula rather one from the ward
Visible so that disconnection, leakage or 'tissued' cannula are readily detected
Check infusion site if pump alarms
Routine checking of cannula during long cases
Syringe- and line-related
Only use Luer lock syringes
Standard order of syringes in the pumps (to prevent erroneous up- or down-titration of the incorrect agent)
Remifentanil syringe only labelled after drug added