Climatisation of anaesthetic gases using different breathing hose systems

Background. During general anaesthesia gas climate significantly is improve d by performance of low flow techniques. Gas climatisation, however, marked ly also will be influenced by the temperature loss at, and corresponding wa ter condensation within the hoses, factors which are related to the technic al design and material of the patient hose system. The objective of this pr ospective study was to investigate 1. how anaesthetic gas climatisation dur ing minimal flow anaesthesia is influenced by the technical design of diffe rent breathing hose systems in clinical practice. 2. to investigate, whethe r a sufficient gas climatisation also can be gained with higher fresh gas f lows if that hose system is used, proven beforehand to optimally warming an d humidifying the anaesthetic gases. Methods. Three different systems, a conventional two-limb hosing consisting of smooth silicone hoses, a coaxial hosing, and a hosing consisting of act ively heated breathing hoses, attached to a Drager Cicero EM anaesthesia ma chine,were used during minimal flow anaesthesia with a fresh gas flow of 0. 5 l/min. Gas temperature and absolute humidity were measured at the tapered connection between the inspiratory limb and the breathing system as well a s at its connection to the endotracheal tube. The best gas climatisation wa s observed if heated breathing hoses were used. Thus, using this hosing,add itionally gas temperature and humidity in the inspiratory limb were taken a t fresh gas flow rates of 1.0, 2.0 and 4.4 l/min respectively. Measurements were performed in all groups at all general anaesthesias lasting at least 45 minutes during the lists of eight different days each. Results. In minimal flow anaesthesia, with all hose systems likewise, gener ally an absolute humidity between 17 to 30 mgH(2)O/l is reached at the endo tracheal tube's connector during the course of the list. Only in the first cases of the day there was a short delay of 15 to 30 minutes before reachin g a humidity of at least 17 mgH(2)O/l. Only with heated hoses, however, hum idity frequently even exceeded 30 mgH(2)O/l. If conventional or coaxial hos ings were used, during minimal flow anaesthesia gas temperatures in an acce ptable range between 23 to 30 degrees C were measured at the tube connector . With heated hoses, however, warming of the gases was excellent with gas t emperatures between 28 to 32 degrees C. In minimal flow anaesthesia climati sation of the anaesthetic gases proved to be best if heated hoses were used . Thus, using heated hose systems another three trials with increasing fres h gas flow rates of 1.0, 2.0 and 4.4 l/min respectively were performed. Whe reas climatisation of the anaesthetic gases still was found to be optimal w ith a fresh gas flow of 1.0 l/min, the humidity dropped drastically to valu es lower than 17 mgH(2)O/l at 2.0 l/min and even down to 10 mgH(2)O/l at a flow rate of 4.4 l/min. Gas temperatures, however,turned out to be independ ent of the flow and remained at 28-32 degrees C, even at a flow as high as 4.4 l/min. Conclusions. Using conventional hose systems and coaxial hosings acceptable , but not optimal climatisation of the anaesthetic gases can be gained if m inimal flow anaesthesia is performed. The use of a coaxial hose system seem s to lead to improved climatisation in long lasting procedures only. In rou tine clinical practice, however, conventional and coaxial hose systems are similar in respect to the climatisation of breathing gases. Heated breathin g hoses performed markedly better in terms of climatisation of the breathin g gas than the coaxial and the conventional hose system. With this hosing n ot only sufficient but optimal moisture and temperature values are realized . Optimal climatisation, however, only can be gained if low flow anesthetic techniques with fresh gas flows equal or less than 1 l/min are performed. With higher fresh gas flow rates the humidity decreases markedly while high gas temperatures are maintained. It seems justified to assume, that ventil ation with warm but dry gases may result in increasingly drying out of the respiratory epithelium of the lower air ways. Heated hoses only should be u sed if low flow anaesthetic techniques are performed. While moisture conten t of the breathing gases mainly is influenced by the fresh gas flow rate, t emperature mainly is depending on the convectional loss of heat at the insp iratory limb of the hosing.