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

To evaluate the function of four currently available, not specifically modified time-cycled ICU ventilators (EVITA 4, Oxylog 2000 HBO and Microvent from Drägerwerk, Germany and Servo 900C, Siemens-Elema, Sweden) under hyperbaric conditions using volume-controlled ventilation (VCV) and, if available, pressure-controlled ventilation (PCV).

All ventilators were studied on an electromechanical lung simulator consisting of a motor driven bellows (LS 1500, Drägerwerk, Germany) at normobaric (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.

Hyperbaric chamber.

During VCV the tidal volume (VT) was set at 750 ml at normobaric conditions prior to starting hyperbaric exposure. During PCV the same VT setting was achieved by adjusting the inspiratory pressure level. At each ambient pressure we registered airway pressure (measured inside the bellows) and flow (derived from the linear displacement of the bellows) for a period of 1 min. From these data we calculated off-line VT, inspiratory airway peak and plateau pressure (Ppeak and Pplateau) and, during PCV only, peak inspiratory flow (Vmax) and the time delay between onset of and peak inspiratory flow (Vdelay). During VCV inspiratory flow and, consequently, VT consistently decreased with increasing ambient pressure. In contrast, during PCV VT remained stable at each condition despite a slight decrease in Vmax.

Whenever available, PCV should be preferentially used during hyperbaric oxygen therapy due to the stability of ventilator functioning. Based 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.