ANESTHESIA CIRCUITS, HUMIDITY OUTPUT, AND MUCOCILIARY STRUCTURE AND FUNCTION

We compared the effects of humidity delivered by the circle system at low fresh gas flows (FGF) with conventional two-limb and coaxial circu it on the structure and function of the tracheobronchial epithelium in dogs. Animals were anaesthetized and mechanically ventilated using an anaesthesia ventilatory to maintain normocarbia. Group 1 (control) an imals received a FGF equal to the required minute ventilation mimickin g an open circuit technique. Group II and III animals had FGF set at 2 0% of the required minute ventilation. Group II used a two-limb circui t and Group III used a coaxial circuit. Relative humidity and temperat ure of inspired gases were measured at baseline and hourly afterwards. In the first experiment, biopsies of the tracheobronchial tree were o btained bronchoscopically at baseline and then hourly for six hours. M icroscopic examination of these samples allowed calculation of mean ci liary length. In the second experiment, tracheal mucus flow velocity ( TMFV) was measured at baseline and hourly afterward, using a cinebronc ho-fibrescopic method. Delivered absolute humidity was greatest with l ow FGF and the coaxial circuit, followed by low FGF and a conventional circuit, and high FGF (15+/-1.4 vs 9+/-0.8 vs 5+/-0.4 mg H2O, P<0.01) after two hours. Mean cilia length (mu m) and TMFV (mm/min) fell duri ng the first hour in all three groups. At hour two TMFV returned to ba seline in Group III and was significantly greater than Groups I and II (0.8+/-0.4 vs 8.6+/-1.1 vs 15.4+/-2.1, P<0.001). Mean ciliary length demonstrated a similar pattern with reductions from baseline in all th ree groups for the first two hours. Groups II and III had an increase in cilia length beginning at hour three and were both significantly gr eater than Group I at hours 3 through 6 (1.3+/-0.5 vs 3.2+/-1.1 vs 4.2 +/-0.8, P<0.001). Alterations in tracheobronchial structure and functi on result from exposure to dry gases and are amplified by the duration of exposure. Our findings suggest a minimum of 12 to 15 mg H2O/l is n ecessary to prevent these alterations. In this study, the combination of low FGF and a coaxial anaesthesia circuit reached this minimum thre shold more quickly than a conventional two-limb circuit.