Fresh gas flow is not the only determinant of volatile agent consumption: a multi-centre study of low-flow anaesthesia†   J.F. Coetzee, L.J. Stewart  British Journal of Anaesthesia  Volume 88, Issue 1, Pages 46-55 (January 2002) DOI: 10.1093/bja/88.1.46 Copyright © 2002 British Journal of Anaesthesia Terms and Conditions

Fig 1 Box and whisker plots of mean expired partial pressures measured in patients at sea level and altitude. Mean FA=mean expired partial pressure obtained by averaging the end-tidal partial pressures of anaesthetic agent (kPa) recorded every 5 s. Sea=sea level; Alt=altitude greater than 1300 m. Explanation of the plots: the ends of the ‘box’ illustrate the 25th and 75th percentiles and the horizontal line within the box depicts the median value. The ‘notch’ represents the 95% confidence interval of the median. The ‘whiskers’ illustrate the range. British Journal of Anaesthesia 2002 88, 46-55DOI: (10.1093/bja/88.1.46) Copyright © 2002 British Journal of Anaesthesia Terms and Conditions

Fig 2 Hydraulic analogue model of uptake and distribution of volatile anaesthetic agent from a circle breathing system (modified from Mapleson16). Agent partial pressures are represented by the heights of fluid in the cylinders. The anaesthetic breathing circuit is the two cylinders on the left (FD and FI) that deliver drug to the central cylinder, the alveoli (FA), from which drug is distributed to peripheral compartments. Vapour is introduced into the system from the cylinder on the far left (FD), which is kept full at all times. The partial pressure of the delivered vapour is represented by the height of cylinder FD and that of the inhaled vapour by the height of fluid in cylinder FI. Excess vapour vented from the breathing system is depicted by spill-over from cylinder FI. Fresh gas flow rate and excess vapour spill-over are represented by the associated arrows. FD=delivered (fresh gas) partial pressure; FI=inspired partial pressure; FA=alveolar partial pressure; FGF=fresh gas flow. The upper illustration depicts vapour delivered at a low partial pressure (short cylinder FD) by a high fresh gas flow rate (thick outlet pipe from FD) with large amounts of excess agent drug spilling out of the breathing system at FI. The lower illustration depicts vapour delivered at a high partial pressure (tall cylinder FD) at a low fresh gas flow rate (narrow outlet pipe from FD) with little excess agent being vented. Inhaled and alveolar partial pressures are thereby maintained at the same levels as in the upper illustration. The dotted lines in the lower illustrate that if the partial pressure of FD is reduced while maintaining a low fresh gas flow, the inspired (and therefore alveolar) partial pressures decrease to below targeted values. British Journal of Anaesthesia 2002 88, 46-55DOI: (10.1093/bja/88.1.46) Copyright © 2002 British Journal of Anaesthesia Terms and Conditions