Effect-site targeting of volatile anaesthetic delivery

Ross Kennedy
Department of Anaesthesia, Christchurch Hospital and Christchurch School of Medicine and Health Sciences

The "target" used by anaesthetists for control of delivery of volatile anaesthetics is slowly moving away from the delivery system and towards the patient. We have moved from looking only at delivery (fresh gas flow and vaporisor dial setting) to using end-tidal (ET) levels to control anaesthetic depth. The next step is to move completely to a patient focus and use estimates of effect site concentration as the "target".

Estimates of effect site concentration are available in many propofol TCI devices. Similar values can be derived for many other drugs including volatile anaesthetics. We have confirmed that the half times for transfer between blood and effect site for volatile anaesthetics are around 3 - 3.5min, very similar to the values for propofol. These values are not greatly influenced by other drugs or surgery and in fact estimates of effect site concentration are not affected much by moderate changes in the half time. Based on this information we have developed a system which uses measured end-tidal concentrations to provide realtime estimates of effect site concentration allowing these to be used as the "target" for control.

Knowing the effect site concentration does not replace measures of effect, such as BIS. However these monitors only measure one effect (hypnosis) at one site and are a retrospective look at the balance between stimulus and anaesthetic "depth". Effect site targeting is a prospective tool which can be used to adjust the delivery of volatile anaesthetic to the effect site as the stimulus changes. This allows more accurate titration to patient needs, and may minimise possible side effects from excessive administration and improve recovery characteristics.

With suitable additional inputs (fresh gas flow and vaporisor setting) our system can also predict future effect site concentrations. This provides a simple form of target control which may help either maintain a constant effect site concentration or allow rapid, smooth changes in effect site concentration.

Even without these tools an understanding of the way the required effect site concentration changes with different stimuli and an appreciation of the interaction between end-tidal and effect site concentration may help improve the way we administer volatile anaesthetics.

Time of Presentation
Sunday 8 May 2005 - 1030-1200