Monitoring of patient response to the anaesthetic drugs is an attractive improvement for achieving a correct balance of sedation level, increasing the chance of success in the right procedure of anaesthesia. Nowadays, there are no commercial tools able to offer real-time monitoring of anaesthetics, indeed, there is still a lack in sensing technologies able to maintain high performances in long term monitoring within a portable miniaturised hardware system. To overcome these limitations, we are here presenting the innovative concept of a portable pen-device able to sense anaesthetic compounds over time. This study is based on an electrochemical sensor to be fully integrated into a complete pen-shaped point-of-care for the monitoring of anaesthesia delivery. The design of the system is based on a bio-inspired event-based approach that is guaranteeing low complexity, low power consumption and is therefore suitable to be scaled to fit the barrel of a pen. An exhaustive comparison between the proposed system and a lab instrument proves that the presented approach obtains comparable performances in terms of sensitivity and resolution with the ones obtained by expensive commercial instrumentation, meanwhile, the results show a 95% power consumption reduction and a 92% area decrease w.r.t. previously presented implementation.