RESISTIVE PRESSURE OF A CONDENSER HUMIDIFIER IN MECHANICALLY VENTILATED PATIENTS

Objectives: Heat and moisture exchangers (or ''nose'' humidifiers) are commonly used to aid in the humidification of inspired gases of mecha nically ventilated patients. These devices add resistance to the venti lator circuit that has heretofore not been quantified in critically il l patients. Accordingly, we determined the resistive pressures associa ted with new and old (but < 24 hrs in the circuit) humidifiers in 23 c ritically ill, mechanically ventilated patients.Design: Prospective st udy. Setting: Adult medical and surgical intensive care units at a uni versity center. Patients: Twenty-three critically ill, mechanically ve ntilated patients using a condenser humidifier between the wye and the endotracheal tube. Interventions: Peak and plateau airway pressures w ere determined with the humidifier in place. These measurements were r epeated without the humidifier, then after insertion of a fresh humidi fier into the circuit. In five patients, measurements were repeated af ter humidifiers had remained in place for a full 24 hrs. Measurements and Main Results: The new humidifiers increased the resistive pressure of the ventilator circuit by 4.8 +/- 2.6 cm H2O compared with no humi difier (p < .01) and had a mean resistance of 4.2 +/- 1.5 cm H2O/L/sec . Old humidifiers increased resistive pressure by 6.3 +/- 3.6 cm H2O c ompared with no humidifier (p < .01) and had a mean resistance of 5.1 +/- 1.8 cm H2O/L/sec. The resistive pressure doubled from 3.4 +/- 1.2 to 7.0 +/- 1.8 cm H2O (p < .01) in five patients in whom the humidifie rs were left in the ventilator circuit for a full 24 hrs. Conclusions: The humidifier adds a significant resistance to the ventilator circui t which may lead to incorrect assessment of respiratory system mechani cs, to inappropriate therapy (e.g., bronchodilators), or to difficulty in weaning from mechanical ventilation.