The Temperature and Humidity in a Low-Flow Anesthesia Workstation With and Without a Heat and Moisture Exchanger

BACKGROUND: The Drager Primus anesthesia workstation has a built-in hotplate to heat the patient's exhaled gas. The fresh gas flow is mixed with the heated exhaled gas as they pass through the soda lime canister. A heat and moisture exchanger (HME) may also be used to further heat and humidify the inhaled gas. In this study we measured the temperature and humidity of the inhaled gas coming from the Drager Primus with or without a HME. METHODS: Thirty female patients were randomly divided into 2 groups and their lungs ventilated by the Primus Drager anesthesia workstation with or without a HME. The humidity and temperature of the inhaled gas were measured 15, 30, 60, 90, and 120 minutes after connecting the patient to the breathing circuit. RESULTS: After 120 minutes of ventilation with a low-flow breathing circuit, the temperatures of inhaled gas were 25 degrees C +/- 1 degrees C and 30 degrees C +/- 2 degrees C without and with HME, respectively, with a statistically significant difference between groups (P < 0.001) with 95% confidence interval (CI) of 3.80 degrees C to 6.40 degrees C; and the absolute humidity values of the inhaled gas were 20.5 +/- 3.6 mgH(2)O.L(-1) and 30 +/- 2 mgH(2)O.L(-1) without and with HME, respectively, with a statistically significant difference between groups (P < 0.001) with 95% CI of 7.37 degrees C to 13.03 degrees C. CONCLUSIONS: The Primus anesthesia workstation partially humidifies the inspired gas when a low fresh gas flow is used. Insertion of an HME increases the humidity in inhaled gas, bringing it close to physiological values. (Anesth Analg 2011; 113: 534-8)