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6 Mathematics and pharmacokineticsAgain, it is important to realize that the actual blood level achieved by TCI pump
in any individual patient is not necessarily the exact target level; there are pharma-
cokinetic variations between patients. In addition, this target level may or may not
be appropriate for the stage of surgery. While a convenient aid to the anaesthetist,
therefore, the infusion must be adjusted to effect, just as the vaporizer setting is
adjusted during volatile anaesthesia.
Some TCI pumps have adecrement timedisplayed for propofol, which is a cal-
culated value that is the predicted time for the plasma level to fall to a value of 1.2
μg.ml−^1 (by default). At this level the patient is expected to awaken. However, there
is a number of reasons why this time may be longer, particularly the use of hypnotics
other than propofol (especially opioids).Safety of TIVA and TCI in clinical practice
Delivering anaesthesia by the inhalational route to a spontaneously breathing patient
has an inherent feedback that provides some degree of autoregulation for depth of
anaesthesia. If the patient is too deep, the minute volume falls and delivery of the
inhaled anaesthetic is reduced. Conversely, if the patient is too light, more volatile is
inhaled and anaesthesia deepens. No such protection occurs during volatile anaes-
thesia in a paralyzed patient and this is also the situation for all patients anaes-
thetized with TIVA and TCI. Inadvertent discontinuation of the infusion will result
in the patient waking up. This technique must therefore be used carefully to avoid
awareness. Various measures may be taken to reduce this:
The infusion should be either via a dedicated intravenous cannula or by a dedicated
lumen of a multi-lumen central line, and it should be in view at all times so that
adisconnection may be noticed. Ideally a non-refluxing valve should be used for
each infusion.
The use of midazolam in a small dose (2 mg) as an adjunct to anaesthesia reduces
the incidence of awareness. Co-induction with propofol and midazolam allows a
lower initial propofol target to be set for induction.
Using oxygen in nitrous oxide rather than air to give an additional analgesic and
anaesthetic effect (not strictly TIVA).
The pumps used for TCI infusions have two duplicate sets of circuitry to calculate
the infusion rate and predicted effect site levels. If the two independent calculations
do not agree, the pump will alarm. One advantage of using TIVA/TCI over con-
ventional intravenous induction followed by volatile anaesthesia is that there is no
‘twilight period’ between the offset of intravenous anaesthetic effects and the onset
of volatile anaesthesia. This period characteristically occurs at the time of intubation.
Itshould always be remembered that the effect site and/or plasma concentrations
displayed on a TCI /TIVA pump remain a guide to the anaesthetist. More recent
work has led to the development of a TCI pump for children and, as was mentioned
above, a TCI pump that targets effect-site concentration rather than plasma concen-
tration. Although targeting the effect site seems more appropriate, it is important to