(Model K) Page 13
2.2 Pneumatic System
The pneumatic system comprises of the tubing and electro-mechanical valves necessary to
provide the gas in conventional 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 positive pressures nozzles are controlled by two electronically controlled
pressure regulators. The negative nozzle is controlled by an electronically controlled
solenoid valve.
The other positive nozzle (the mean jet) is used to generate steady pressures in ventilation
(CPAP or PEEP pressures). These three nozzles (or jets) are used in various combinations
to generate all ventilation modes.
2.2.3 Conventional ventilation
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.
This provides a smooth rise in pressure and allows user adjustable rise times. The fall of the
inspiratory wave is also controlled by the forward jet pressure regulator to bring the pressure
down quickly and smoothly. 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.
Note that all jet pressures actually sum in the exhalation block, for example, to ventilate a
patient with a PEEP pressure of 5 mbar and a PIP pressure of 30 mbar, the mean jet will be
set to generate a continuous circuit pressure of 5 mbar and the forward jet will be set to
generate a circuit pressure varying between zero (exp phase) and 25 mbar (insp. phase).
Since the jet pressures will sum, this will result in the desired patient pressure.
See “Pneumatic Unit Schematic” on page 199.
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