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


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Abstract

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 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 H2 O for group 1, 9.3 plus minus 1.3 cm H2 O for group 2); b) 5 cm H2 O above baseline; and c) 5 cm H2 O 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 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 H2 O for group 1, and 2.9 plus minus 1.7 to 5.5 plus minus 3.0 cm H2 O for group 2); ventilatory drive, expressed as P0.1 (2.7 plus minus 2.0 to 5.2 plus minus 4.0 cm H2 O for group 1, and 3.3 plus minus 2.0 to 5.3 plus minus 3.0 cm H2 O 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.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 H sub 2 O) may compensate for the increased work of breathing.(Crit Care Med 1996; 24:1184-1188)

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