[The temperature-humidity profile of the PhysioFlex. Studies on a model]

Closed-system anaesthesia provides the best prerequisites for optimal warming and humidification of anaesthetic gases. The PhysioFlex anaesthesia machine fascilitates quantitative closed-system anaesthesia. Furthermore, its design may improve the climatization of the anaesthetic gases by revolving the system volume at 70 l/min, using a small soda-lime canister to allow optimal usage of the heat and moisture generated by CO2 absorption and by integrating all system components in thermally isolating housing. To determine the capacity of the PhysioFlex to climatize anaesthetic gases, we evaluated the heat and humidity profile at four characteristic places in the anaesthetic circuit under standardised conditions in a model.

In an air-conditioned room at 19-20 degrees C ambient temperature, the PhysioFlex was operated with a fresh gas flow of less than 500 ml/min, similar to quantitative closed-system anaesthesia in adults. With a respiratory rate of 10/min and a tidal volume of 600 ml, a humidifier was ventilated, that delivered humidity-saturated gas at 33-34 degrees C; 200 ml/min CO2 were added to the system at the humidifier to mimic the heat, moisture, and CO2 input of a patient into the anaesthetic circuit. A total of six series were performed, each starting with a cold and dry anaesthetic circuit. For 2 h the time-courses of temperature and humidity of the anaesthetic gases were measured at four distinct places: (1) in the soda-lime canister (M1); (2) at the outlet of the anaesthesia machine (M2); (3) at the inlet of the anaesthesia machine (M3); and (4) in the inspiratory limb close to the Y-piece (M4). Capacitive humidity sensors (VAISALA Type HMM 30 D without a protective cap) and very small thermocouples were used to measure relative humidity (rH) and temperature. The data were recorded at 5 min intervals. Due to the continuous gas stream in the system, the response time of the sensors, which is in the range of a few seconds, did not affect the accuracy of the measurement. With the temperature-dependent humidity content of 100% rH obtained from equation 1, absolute humidity was calculated.

The time courses of temperature and humidity at the different measuring points are depicted in Figs. 2 and 3, respectively. The steepest increase in temperature and humidity was observed at M1. Within 10 min 100% rH was achieved at all measuring points. Initially, there was a considerable temperature gradient between M1 and M2; this became gradually smaller, indicating system components with high heat capacities. There was only a small gradient between M2 and M4, indicating that there was only a small heat loss compared to the heat input. The recommended minimal climatization of the anaesthetic gases of 20 mg H2O/l [20] was obtained within 10 min at M4. During the whole measuring period heat and humidity increased in the system, reaching a maximum at M4 after 120 min with average values of more than 28 degrees C and 27 mg H2O/l, respectively.

With the PhysioFlex anaesthesia machine employing closed-system conditions, minimal climatization of anaesthetic gases was reached within 10 min. After a period of 120 min, the anaesthetic gases were nearly climatized to the extent recommended for long-term respiratory therapy. To date, no comparable temperature and humidity level has been reported with conventional anaesthesia machines. The time course of the gradient between M1 and M2 may give an opportunity for further optimising the system in reducing heat loss after the soda-lime canister, the active heat and moisture source in the circuit. At about 32 degrees C, the temperature in the soda-lime canister is 10-15 degrees C less than in conventional anaesthesia machines. Thus, the use of thermally instable volatile anaesthetics in the PhysioFlex under closed-system conditions may be less critical than in conventional anaesthesia machines under minimal-flow conditions.