The effect of a heat and moisture exchanger on gas flow in a Mapleson F breathing system during inhalational induction

Heat and moisture exchangers (HMEs) humidify, warm and filter inspired gas, protecting patients and apparatus during anaesthesia. Their incorporation into paediatric anaesthetic breathing systems is recommended. We experience d delays in inhalational induction whilst using a Mapleson F breathing syst em with an HME. We have demonstrated that the HME significantly alters gas flow within the breathing system. Approximately half of the fresh gas flow is delivered to the patient, the remainder being wasted into the expiratory limb of the breathing system. We suggest that the HME should be removed fr om the Mapleson F breathing system until inhalational induction is complete , or that the reservoir bag is completely occluded until an effective seal is obtained with the mask.