Effect of Ventilation Equipment on Imposed Work of Breathing
ABSTRACT
Objective:
To determine the imposed work of different ventilation systems at 3 levels of pressure support.
Design: Laboratory study.
Setting: Teaching hospital respiratory laboratory.
Participants: Healthy, human volunteers.
Interventions: Measurement of imposed work of breathing (WOBi) in six ventilators with alteration of ventilatory settings, humidification device, and triggering mechanisms.
Results:
At 0 cmH20 CPAP and 0 cmH20 PSV, clinically significant (> 0. 1 joules per litre or J/L) WOBi occurred in all systems. At 5 cmH20 pressure support ventilation (PSV) median WOBi ranged from 0.01 to 0.11 J/L. Removal of the pleated membrane heat and moisture exchanger (HME) significantly reduced WOBi (0.38 vs. 0. 11 J/L, p < 0. 0001). Drawover humidification marginally increased WOBi (0. 16 vs. 0. 11 J/L, p = 0. 0001). Flow triggering reduced WOBi with the Servo (p < 0.0001) and Bennett ventilators (p = 0.001) but not with the Bear 1000 ventilator.
Conclusions:
Up to 7 cmH20 of PSV may be required to reduce WOBi related to the ventilator, circuit and humdification devices. This pressure support does not address the additional resistive effect of the endotracheal or tracheotomy tube. Higher levels of PSV may therefore be required to offset WOBi. (Critical Care and Resuscitation 2001; 3: 148-152)
Key words:
Monitoring, physiologic, artificial ventilation, work of breathing
