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2.3 Principles of Operation
For active inspiration, the fl ow control valve is opened to provide a specifi c gas
fl ow into the bellows assembly. Simultaneously, the expiratory solenoid closes
and pressure is generated in the bellows assembly producing an inspiratory
fl ow to the patient.
The fl ow and pressure are measured and monitored by the microprocessor
feedback system.
Expiration occurs when the fl ow control valve is closed and the expiratory
solenoid opens and releases the gas from the bellows assembly.
In Pressure Control Mode, the set pressure is achieved during inspiration
and maintained at that level by allowing a controlled bypass through the
expiratory valve. This allows the required pressure level to be maintained while
compensating for any fresh gas fl ow into the patient circuit.
For expiration, the expiratory solenoid is opened which releases the gas from
the bellows assembly.
During all modes of ventilation, an autozero is periodically applied to the fl ow
sensors just prior to a breath being delivered. At this point, there is no fl ow
through the sensors, which ensures that the measured values are maintained
as accurately as possible regardless of environmental variations.
A safety valve is present in the drive pressure exhaust
75 cmH
2
O. Its function is to protect the patient against pres-
sures exceeding 75 cmH
2
O ± 2 cmH
2
O by relieving the drive
pressure on the bellows.
NEEP is not supported by this machine but patient gener-
ated pressures may be measured to - 10 cmH
2
O, at which
point an alarm will sound.
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