(Model J) Page 15
2.2 Pneumatic System
The pneumatic system comprises of the tubing and electro-mechanical valves necessary to
provide the gas in conventional and oscillatory ventilation modes. The two gas controlling
functions are blending and pressure generation
2.2.1 Blending
The method used for blending air and oxygen in known proportions is to pressure regulate
the two supplies (air and oxygen) so they produce equal flow rates and then allow each
supply into a mixing chamber for a time period equivalent to the proportions required. For
example, delivering oxygen at a set flow rate into a mixing chamber for 1 second and air at
the same flow rate for 2 seconds will result in a mixture of 1 part oxygen to 2 parts air
(resulting in a mix of 47.3%).
2.2.2 Pressure Generation
There are three nozzles within the exhalation block in the pneumatic subsystem. One for
generating negative pressure in the patient circuit and the other two for generating positive
pressure. The pressures generated from the three nozzles are controlled by three
electronically controlled pressure regulators. The negative and one of the positive nozzle
pressures can also be switched on and off rapidly with in-line (high speed) solenoid valves.
The other positive nozzle (the mean jet) is used to generate steady pressures in ventilation
(CPAP or PEEP pressures in conventional modes and mean pressures in HFO modes).
These three nozzles (or jets) are used in various combinations to generate all ventilation
modes.
2.2.3 Conventional ventilation
In non HFO modes the negative (or reverse) jet is used in a steady mode to provide a small
amount of flow to offset the inadvertent patient circuit pressure generated from the fresh gas
flow of 8 lpm. The mean jet is also used in a steady mode to generate the base pressure
level (CPAP or PEEP) and the forward jet is used to generate the PIP pressure during the
inspiratory phase. The rise time of the inspiratory phase is controlled by dynamically
controlling the forward jet pressure regulator rather than switching a steady pressure with
the high speed valves.
This provides a smooth rise in pressure and allows user adjustable rise times rather than
abrupt changes and pressure “ringing” which can result from high speed switching. The fall
of the inspiratory wave is also controlled by the forward jet pressure regulator to bring the
pressure down quickly and smoothly, using the high speed valves to do this would again
result in inadvertent ‘ringing’ which causes difficulties for the monitor subsystem in trying to
detect a patient breath attempt by monitoring the pressure alone. Once the pressure has
been brought close to the base pressure, after about 100 ms, the forward jet solenoid is
switched off to prevent any further artefact causing false triggering.
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