ObjectiveTo evaluate the effect of two commonly used heat and moisture exchangers on respiratory function and gas exchange in patients with acute respiratory failure during pressure-support ventilation.DesignProspective, randomized trial.SettingIntensive care unit of a university hospital.PatientsFourteen patients with moderate acute respiratory failure, receiving pressure-support ventilation.InterventionsPatients were assigned randomly to two treatment groups, in which two different heat and moisture exchangers were used: Hygroster Registered Trademark (DAR S.p.A., Mirandola, Italy) with higher deadspace and lower resistance (group 1, n equals 7), and Hygrobac-S Registered Trademark (DAR S.p.A.) with lower deadspace and higher resistance (group 2, n equals 7). Patients were assessed at three pressure-support levels: a) baseline (10.3 plus minus 2.4 cm H2O for group 1, 9.3 plus minus 1.3 cm H2O for group 2); b) 5 cm H2O above baseline; and c) 5 cm H2O below baseline. Measurements obtained with the heat and moisture exchangers were compared with those values obtained using the standard heated hot water humidifier.Measurements and Main ResultsAt baseline pressure-support ventilation, the insertion of both heat and moisture exchangers induced in all patients a significant increase in the following parameters: minute ventilation (12.4 plus minus 3.2 to 15.0 plus minus 2.6 L/min for group 1, and 11.8 plus minus 3.6 to 14.2 plus minus 3.5 L/min for group 2); static intrinsic positive end-expiratory pressure (2.9 plus minus 2.0 to 5.1 plus minus 3.2 cm H2O for group 1, and 2.9 plus minus 1.7 to 5.5 plus minus 3.0 cm H2O for group 2); ventilatory drive, expressed as P0.1 (2.7 plus minus 2.0 to 5.2 plus minus 4.0 cm H2O for group 1, and 3.3 plus minus 2.0 to 5.3 plus minus 3.0 cm H2O for group 2); and work of breathing, expressed as either power (8.8 plus minus 9.4 to 14.5 plus minus 10.3 joule/min for group 1, and 10.5 plus minus 7.4 to 16.8 plus minus 11.0 joule/min for group 2) or work per liter of ventilation (0.6 plus minus 0.6 to 1.0 plus minus 0.7 joule/L for group 1, and 0.8 plus minus 0.4 to 1.1 plus minus 0.5 joule/L for group 2). These increases also occurred when pressure-support ventilation was both above and below the baseline level, although at high pressure support the increase in work of breathing with heat and moisture exchangers was less evident. Gas exchange was unaffected by heat and moisture exchangers, as minute ventilation increased to compensate for the higher deadspace produced in the circuit by the insertion of heat and moisture exchangers.ConclusionsThe tested heat and moisture exchangers should be used carefully in patients with acute respiratory failure during pressure-support ventilation, since these devices substantially increase minute ventilation, ventilatory drive, and work of breathing. However, an increase in pressure-support ventilation (5 to 10 cm H sub 2 O) may compensate for the increased work of breathing.(Crit Care Med 1996; 24:1184-1188)