THE EFFECT OF A HEAT AND MOISTURE EXCHANGER ON HUMIDITY IN A LOW-FLOWANESTHESIA SYSTEM

The heat and humidity in a low-flow breathing system was measured in o rder to study the inherent humidifying properties of the system at low fresh gas flows (<1 and 2l.min(-1)) and whether a heat and moisture e xchanger could compensate for the loss of heat and humidification occu rring at higher fresh gas flows (51.min(-1)) in these systems. Sixty p atients were randomly divided into three groups (<1, 2 and 5l.min(-1) fresh gas flows) with a heat and moisture exchanger and three groups w ithout a heat and moisture exchanger in the breathing system. Thirty m inutes after the start of anaesthesia a control measurement was perfor med, after which a heat and moisture exchanger was inserted into the b reathing system of the three groups randomly allocated to have one. Th ree more measurements were performed at 10, 30 and 60 min after contro l. At low fresh gas flows the humidifying properties of the low-flow b reathing system are adequate (i.e. provide an absolute humidity > 20 m g.l(-1)) but at a fresh gas flow of 51.min(-1) there is a need for a h eat and moisture exchanger for adequate humidification of the inspired gas.