COMPARISON OF THE EFFECTS OF HEAT AND MOISTURE EXCHANGERS AND HEATED HUMIDIFIERS ON VENTILATION AND GAS-EXCHANGE DURING WEANING TRIALS FROMMECHANICAL VENTILATION

Heat and moisture exchangers (HME) are increasingly used to warm and h umidify inspired gases in intubated ventilated patients, But these dev ices add dead space that may alter Lte alveolar ventilation, This coul d impair the efficiency of spontaneous ventilation (SV) during weaning trials from mechanical ventilation. Fifteen patients were tested with an HME (Hygrobac-DAR) and a heated humidifier (HH) (Fischer-Paykel MR 450) in a random order during weaning trials in SV with inspiratory p ressure support, Minute ventilation VE, tidal volume), and respiratory rate were recorded and arterial blood was sampled for blood gas analy sis with each device. The HME gave a significantly greater VE than the HH (9.3+/-0.8 L/min vs 8.1+/-0.8 L/min; p<0.005), because of increase d respirator). rate (21+/-2/min vs 19+/-2/min; p<0.05). Tidal volume w as unchanged for HME and HH (470+/-32 mt vs 458+/-39 mL). The higher P aCO2 with HME than with HH (44+/-2 mm Hg vs 42+/-2 mm Hg;, p<0.005) re vealed an insufficient alveolar ventilation response to the increase i n dead space, Arterial Po-2 rose nith the HME, but not significantly a bove the HH values (103+/-6 mm Hg vs 97+/-6 mm Hg; p=0.055), possibly because of a positive end-expiratory pressure effect of the HME. The n eed to increase VE in SV when an HME is used should be taken into acco unt during difficult weaning from mechanical ventilation.