Function of different ICU ventilators under hyperbaric conditions

Introduction

ICU-ventilators are designed for use at normobaric conditions. Under certain circumstances patients need mechanical ventilation during hyperbaric oxygen therapy. The aim of the investigation was to analyse the function of four different ICU-ventilators [EVITA-4, Microvent, Oxylog 2000 HBO (Drägerwerk AG, Germany) and Servo ventilator 900 C (Siemens-Elema, Sweden] under 5 (normobaric, 1.3, 1.6, 1.9 and 2.8 ATA, EVITA-4 and Oxylog 2000 HBO) or 6 (additionally 6 ATA, Servo ventilator 900 C and Microvent) different atmospheric pressure conditions, regarding the difference of present and effectively applied volume and pressure from those set at the control panel.

Methods

We tested the following ventilator modes: volume (VCV) and pressure controlled ventilation (PCV, EVITA-4 and Servo ventilator 900 C only), the electromechanical lung simulator LS 1500 (Drägerwerk AG, Germany) consisting of a motor driven bellow.

During VCV tidal volume was set at (V_T) = 750 ml, respiratory rate (f) = 15/min, I:E ratio = 1:2, positive endexpiratory pressure (PEEP) = 0 cmH₂O. During PCV (f) = 10/min inspiratory pressure (P_insp) was set in order to achieve a V_t of 750 ml keeping all others ventilator parameters constant. The V_T applied by the ventilator was measured as the linear displacement of the bellows at each ambient pressure. (P_aw) was measured inside of the bellows.

Results

During VCV applied V_t of the ventilator decreases significantly with increasing ambient pressure while PCV with constant P_insp and PEEP V_t was stable at each depth.

Conclusions

Under hyperbaric conditions PCV should be preferred due to the stability of V_T applied. If PCV is not available, V_T needs to be adjusted during VCV depending on the actual ambient pressure.