Functioning of ICU ventilators under hyperbaric conditions - comparison ofvolume- and pressure-controlled modes

Objective: To evaluate the function of four currently available, not specif ically modified time-cycled ICU ventilators (EVITA 4, Oxylog 2000 HBO and M icrovent from Dragerwerk, Germany and Servo 900C, Siemens-Elema, Sweden) un der hygerbaric conditions using volume-controlled ventilation (VCV) and, if available, pressure-controlled ventilation (PCV). Design: All ventilators were studied on an electromechanical lung simulator consisting of a motor driven bellows (LS 1500, Dragerwerk, Germany) at nor mobaric (1 bar) and hyperbaric ambient pressures (1.3, 1.6, 1.9, 2.8 bar). Servo 900C and Microvent were additionally tested at 6 bar. Settings: Hyperbaric chamber. Measurements and results: During VCV the tidal volume (V-T) was set at 750 ml at normobaric conditions prior to starting hyperbaric exposure. During P CV the same V-T setting was achieved by adjusting the inspiratory pressure level. At each ambient pressure we registered airway pressure (measured ins ide the bellows) and flow (derived from the linear displacement of the bell ows) for a period of 1 min. From these data we calculated off-line V-T, ins piratory airway peak and plateau pressure (P-peak and P-plateau) and, durin g PCV only, peak inspiratory flow (V-max) and the time delay between onset of and peak inspiratory flow (V-delay). During VCV inspiratory flow and, co nsequently, V-T consistently decreased with increasing ambient pressure. In contrast, during PCV V-T remained stable at each condition despite a sligh t decrease in V-max. Conclusions: Whenever available, PCV should be preferentially used during h yperbaric oxygen therapy due to the stability of ventilator functioning. Ba sed on the specific ventilator properties at increasing ambient pressures, appropriate corrections should be possible which will allow the safe use of ICU ventilators even during VCV.