Paediatric Anaesthesia 1999 9: 39–45 Evaluation of the efﬁciency of heat and moisture exchangers during paediatric anaesthesia MARCO LUCHETTI MD*, ANTONIA PIGNA MD, ANDREA GENTILI MD* AND GIUSEPPE MARRARO MD Department of Anaesthesia and Intensive Care, Fatebenefratelli and Ophtalmiatric Hospital, Milan, Italy; *Department of Anaesthesia and Intensive Care, Policlinico S. Orsola, Bologna, Italy Summary This study evaluates the efﬁciency of heat and moisture exchangers (HMEs) in allowing adequate humidiﬁcation and warming during anaesthesia in children. Eighteen paediatric patients undergoing anaesthesia were divided into two groups: group A ten patients: infants up to 10 kg→Hygrobaby HME; group B 8 patients: children above 10 kg→Hygroboy HME. The following parameters were evaluated: body temperature (bT), room temperature (rT), fresh gas temperature, HME warm-up time, inspired and expired gases temperature and humidity, conserving efﬁciency, and duration of anaesthesia. Gas temperatures were recorded by means of a recorder ﬁtted with four thermal probes. Humidity values were mathematically derived. The correlation between efﬁciency and rT, bT, and fresh gas temperature was computed. In both groups the inspired gases temperatures were below 30°C. Inspired absolute humidity was never more than 28 mgH 2 O·l -1 . The conserving efﬁciency was good (0.93 in both groups). A positive correlation was found between efﬁciency and fresh gas temperature. HMEs did not meet the minimum standards for humidity and heating during anaesthesia in children, although their conserving efﬁciency was found to be satisfactory. Keywords: humidiﬁcation; heat and moisture exchanger; anaesthesia Introduction inhaled air in the lungs, if completely saturated with water vapour at 37°C, is 43.4 mg·l -1 . In the The upper respiratory tract functions as a heat and spontaneously breathing patient inspired gases are moisture exchanger. Under normal physiological usually warmed in the upper airway to 34°C (1). conditions inhaled air reaches the lungs saturated During tracheal intubation and mechanical ventila- with water vapour at a temperature corresponding tion a substantial part of the upper respiratory tract to the body temperature. The absolute humidity of is bypassed, eliminating at least one-ﬁfth of the air conditioning surface area. Children and especially infants undergoing Correspondence to: Marco Luchetti, Department of Anaesthesia and anaesthesia are very prone to develop hypothermia, Intensive Care, Fatebenefratelli and Ophtalmiatric Hospital, Corso di Porta Nuova 23, 20121 Milano, Italy. due to the large surface area to body weight ratio 39  1999 Blackwell Science Ltd