EFFICIENCY OF AN ENCLOSED AFFERENT RESERVOIR BREATHING SYSTEM DURING CONTROLLED VENTILATION

We describe an enclosed afferent reservoir (EAR) breathing systemdeveloped by Ohmeda and designed to operate efficiently in spontaneousand controlled ventilation. The efficiency of the system wasevaluated by calculating the fractional utilization of freshgas in 10 ASA I–III patients during anaesthesia with controlledventilation. Maximum efficiency occurred when the minute ventilationto fresh gas flow ratio was greater than 1.5. Under these conditions,fractional utilization was relatively constant with a valueof 0.73 (95% confidence interval 0.69–0.78). The minimumfresh gas flow for use during controlled ventilation was determinedin another eight ASA I–III patients when the minute volumeto fresh gas ratio was greater than 1.5. In view of an increasedarterial to end-tidal carbon dioxide partial pressure differencein patients in the first part of the study (1.03 kPa), normocapniawas defined as an end-tidal carbondioxide partial pressure of4.3 kPa. Normocapnia was achieved with a mean fresh gas flowof 66 ml kg^–1 min^–1, while 70 ml kg^–1 min^–1produced mild hypocapnia.