2006 Annual Scientific Meeting - CLICK FOR PROGRAMME

TIVA increases the incidence of awareness

A R Aitkenhead

University of Nottingham

There are a number of reasons why it can be argued that awareness is more likely to occur during TIVA than during anaesthesia with inhaled anaesthetic agents.

First, it is not possible to monitor the concentration of intravenous anaesthetic agents in the brain in real time. In contrast, it is possible to estimate with considerable accuracy the blood concentration; the brain concentration is similar during stable anaesthesia. This allows the anaesthetist to be able to minimise the risk of awareness in situations in which blood pressure is very low (e.g. in a shocked patient). In addition, it acts as an early warning of failure of delivery of the inhaled agents to the patient, irrespective of the cause.

Second, the pharmacokinetic principles of intravenous and inhalational drugs are fundamentally different. The rates of distribution of drug from the blood into the various pharmacokinetic compartments, and to the effect site, vary enormously among individuals (for both intravenous and inhalational drugs). However, with intravenous drugs, a predetermined dose is administered by the anaesthetist. The blood concentration decreases as the drug is distributed into the other compartments unless drug delivery to the blood is matched to loss into other compartments. As loss cannot be predicted with any degree of accuracy, matching input to output can be no more than a chance event. In contrast, a known concentration of inhaled anaesthetic is added to the inspired gas mixture, and the concentrations in blood and in the other compartments increases towards that concentration. There is, in effect, an unlimited supply of drug, so that if the anaesthetist determines that a specific blood concentration is required, that can be maintained at a constant level; in a patient with large pharmacokinetic compartments or rapid distribution, the amount of drug taken up by the body increases automatically, without any risk of the blood and brain concentrations decreasing to values which might be associated with awareness.

Third, a proportion of TIVA users invent ‘recipes’ which are not based on scientific evidence of efficacy, and are fundamentally unsound. Examples which I have encountered are individuals who have used published infusion regimens, but who have failed to appreciate that these regimens were validated only when given with nitrous oxide and an opioid analgesic, and individuals who have not started the intravenous infusion until after transfer into the operating theatre. Techniques such as these will result in predictably inadequate brain concentrations of anaesthetic agent to prevent awareness when surgery starts. 

Fourth, although some departments have several consultants who use TIVA techniques and teach them regularly to trainees, others do not, resulting in trainees learning by experimentation. Patients should not be the subject of experiments except in rigorously conducted clinical trials.