Ventilator performance under hyperbaric conditions: A study of the Servo 900C ventilator

When critically ill individuals receive hyperbaric oxygen therapy (HBOT), adequate and reliable ventilation is of great importance. The Servo 900C ventilator is widely used in this setting but its performance at pressure has been incompletely characterised. Aims of the study: 1 To establish if the Servo 900C ventilator exhibits a reduction in delivered minute volume (MV) when used in synchronised intermittent mandatory ventilation (SIMV) mode at the maximum pressure commonly utilised in hyperbaric medicine practice. 2 If so, to calculate the correction factor(s) that needs to be applied to the ventilator settings in order to maintain delivery of the desired MV whilst at 283 kPa. Methods: Ventilation using the Servo 900C ventilator was simulated with a test lung at sea-level atmospheric pressure (101.3 kPa) and at a pressure of 283 kPa in a hyperbaric chamber. The ventilator was operated in synchronised intermittent mandatory ventilation (SIMV) mode. Gases were collected from the exhaust valve of the ventilator to measure minute volume during simulations. Results: Significant differences exist between the MV delivered at 101.3 kPa and that delivered at 283 kPa, for ventilator MV settings of 1.0, 10 and 15 L.min^-1. It is possible to fully compensate for the reduction in delivered MV at a desired setting of 1 L.min^-1 but full compensation cannot be achieved for set MVs of 10 and 15 L.min^-1. Conclusions: When using the Servo 900C ventilator in SIMV mode in a hyperbaric chamber at 283 kPa, it is possible to achieve delivery of desired MV at the lower settings. However, it is not possible to deliver MVs of 10 L.min^-1 or more at this pressure. This has implications for the ventilation of adult patients who concurrently require HBOT.