Fuzzy logic and decision-making in anaesthetics.

The task of the anaesthetist is to control the continuum between consciousness and unconsciousness, pain and analgesia, muscle activity and relaxation—inhibition of activation and enhancement of inhibition.1 During an operation, an anaesthetized patient is part of a ‘feedback circuit’ (Figure 1). Changes in variables such as blood pressure and respiratory rate are monitored and stability is restored by adjustments to ventilation and drug dosage.2 The decision-maker and controller in this loop is the anaesthetist, who will make an individual judgment on how best to respond to, say, low blood pressure, tachypnoea or a decreasing oxygen saturation. Computer programs employing ‘fuzzy logic’ are intended to imitate human thought processes in these complex circumstances but to function at greater speed. A simple computerized system might be based on the rule ‘if X then do Y’. The drawback of such programs is that a large number of rules are needed to deal with every possible situation. In addition, if two or more indices are being measured the rule then becomes ‘if X and Y, then Z’ and the number of rules multiplies vastly. Fuzzy logic works by drastically reducing the number of rules and using proportionate amounts of each rule; and it can also ‘learn’ by assessing responses to changes in output. It thus opens the way to automation in circumstances that would be difficult or impossible to model with simple linear mathematics.