[Are humidity filters necessary in the inspired air in the breathing circuit? A new in vivo method of measuring humidity in the air breathed].

Humidification of inspiratory gases under anaesthetic conditions still is a matter of controversial discussion. Physiological humidification and heating of breathing air are preconditions for mucociliary clearance, pulmonary cleaning and defence mechanisms. These functions of the upper respiratory tract are eliminated by application of artificial airways. In general the humidification of inspiratory gases should not remain under 70% of relative air humidity at 37 degrees C. Under clinical conditions it is problematic to ensure sufficiently rapid and reproducible measurements of humidity during breathing cycles. We developed a measuring method that enables to make these measurements without big mechanical device. Aim of this investigation was to measure air humidity in typical semiclosed systems during anaesthesia and semiopen CPAP-respiration. The necessity and efficiency of a heat and moisture exchanger (HME) was to be investigated as well. After approximately 5 minutes there was an inspiratory relative air humidity not below 70% at 28 degrees C (19 mg H2O/l humid air) within the breathing circuit with CO2 double-absorber. By using an HME it is possible to increase relative air humidity within this system to 86% at 29.5 degrees C (25 mg/l). After one hour's respiration with this system without HME a relative humidity of 87% at 30 degrees C (26 mg/l) is reached after replaced HME. Initial relative humidity in a semiopen CPAP-system is about 12% at 28 degrees C (3 mg/l). This is increased to 85% at 29.5 degrees C (25 mg/l) after 15 minutes respiration with HME.(ABSTRACT TRUNCATED AT 250 WORDS)