Work of breathing imposed by different ventilators under hyperbaric conditions

Introduction

The continuous increase in gas density caused by compression is likely to affect the work of spontaneous breathing (WOB) under hyperbaric conditions. In intubated patients breathing spontaneously through demand flow ventilator systems WOB is additionally influenced by the WOB component imposed by the ventilator (WOB_imp). Since the gas density dependent components of the ventilators available (eg inspiratory and expiratory valves) have been designed for use under normobaric conditions, WOB_imp may differ significantly with increasing ambient pressure. Our aim was to analyze WOB as well as WB_imp with four different ventilators under hyperbaric conditions in a lung model study.

Methods

spontaneous breathing was simulated by the electromechanical lung model LS 1500 (Drägerwerk AG, Germany) consisting of a motor driven bellows. Lung model settings were: spontaneous tidal volume (V_T) = 500 ml, respiratory rate (f) = 20/min, compliance of the respiratory system (CRS) = 50 ml/cm H₂O and resistance (R) = 5 cm H₂O×s/l. The ventilators EVITA-4, Microvent, Oxylog 2000 HBO (Drägerwerk AG, Germany) and Servo ventilator 900C (Siemens-Elema, Sweden) were tested in the modes CPAP (0 cm H₂O) and Pressure Support Ventilation (PSV, 5 and 10 cm H₂O over CPAP except of the Oxylog 2000 HBO that do not provide a PSV-mode) under 5 (1, 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 respectively. The motor driving electrical signal modulated by the linear displacement of the bellows was used as pleural pressure (P_pl), the displacement of the bellows as flow (V) signal. Airway pressure (P_aw) was measured inside the bellows. WOB and WOB_imp were calculated by integration of the inspiratory P_pl and P_aw drop over V respectively.

Results

Conclusion

WOB and WOB_imp, increased significantly with atmospheric pressure. Due to the marked differences between the ventilators the properties of each one needs to be known in order to assess the impact on WOB.