End-tidal Control software for use with Aisys closed circuit anaesthesia systems for automated gas control during general anaesthesia

Introduction

Approximately 2.4 million patients in the UK had general anaesthesia in 2007 (NICE 2012). Most patients had a combination of intravenous and inhaled agents to induce and maintain general anaesthesia. Maintenance of inhalational anaesthesia needs an anaesthetist to continually monitor end‑tidal (or expired) oxygen and anaesthetic concentrations. The anaesthetist manually adjusts the vaporiser settings, which control the concentration of anaesthetic and fresh gas (oxygen, air, and optionally, nitrous oxide) flow rates to provide adequate anaesthesia.

There are risks associated with manually controlled maintenance of inhalational anaesthesia. These include risk of hypoxia, hypercapnia (also known as hypercarbia), and over- or under‑dosage of anaesthetic (Baum and Aitkenhead 1995), with the latter potentially leading to patients regaining a level of consciousness (Schober and Loer 2006). The 5th National Audit Project by the Association of Anaesthetists of Great Britain and Ireland and the Royal College of Anaesthetists reported 153 cases of accidental awareness during general anaesthesia (including people anaesthetised by inhalational anaesthesia as well as other techniques) in the UK in 2011, representing a risk of 1 in 15,414 general anaesthesia procedures (Pandit et al. 2013).

Automating the process of monitoring and adjusting gas concentrations shortens anaesthetic induction and results in steadier arterial and brain anaesthetic concentrations, stabilising the level of anaesthesia (Sieber et al. 2000). It also minimises the amount of fresh and anaesthetic gas wasted, reducing both healthcare costs and environmental burden (Nunn 2008).