Effects of heat and moisture exchangers on minute ventilation, ventilatory drive, and work of breathing during pressure-support ventilation in acute respiratory failure

Objective: To 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. Design: Prospective, randomized trial. Setting: Intensive care unit of a university hospital. Patients: Fourteen patients with moderate acute respiratory failure, receiving pressure-support ventilation. Interventions: Patients were assigned randomly to two treatment groups, in which two different heat and moisture exchangers were used: Hygroster®(DAR S.p.A., Mirandola, Italy) with higher deadspace and lower resistance (group 1, n = 7), and Hygrobac-S® (DAR S.p.A.) with lower deadspace and higher resistance (group 2, n = 7). Patients were assessed at three pressure-support levels: a) baseline (10.3 ± 2.4 cm H₂O for group 1, 9.3 ± 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 Results: At 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 ± 3.2 to 15.0 ± 2.6 L/min for group 1, and 11.8 ± 3.6 to 14.2 ± 3.5 L/min for group 2); static intrinsic positive and-expiratory pressure (2.9 ± 2.0 to 5.1 ± 3.2 cm H₂O for group 1, and 2.9 ± 1.7 to 5.5 ± 3.0 cm H₂O for group 2); ventilatory drive, expressed as P_0.1 (2.7 ± 2.0 to 5.2 ± 4.0 cm H₂O for group 1, and 3.3 ± 2.0 to 5.3 ± 3.0 cm H₂O for group 2); and work of breathing, expressed as either power (8.8 ± 9.4 to 14.5 ± 10.3 joule/min for group 1, and 10.5 ± 7.4 to 16.8 ± 11.0 joule/min for group 2) or work per liter of ventilation (0.6 ± 0.6 to 1.0 ± 0.7 joule/L for group 1, and 0.8 ± 0.4 to 1.1 ± 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. Conclusions: The 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 H₂O) may compensate for the increased work of breathing.