Whilst this measurement provides a definitive value of how much anaesthetic agent is reaching the brain, it is impractical for clinical use because of the long duration and complexity of laboratory analysis that is required. Therefore, another approach can use the concentration of the volatile agent in the gases exhausted from the oxygenator in order to estimate the arterial blood concentration. This approach is analogous to using the end-tidal concentration that is expired from the lungs during routine general anaesthesia. Oxygenator exhaust, unlike arterial blood concentration, would be practical in a clinical scenario as the oxygenator exhaust from CPB can be attached to a standard anaesthetic gas monitor. Indeed, there is evidence that suggests measuring the oxygenator exhaust concentration of volatile anaesthetic agent is a good surrogate of arterial blood levels.