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Aespire 7900
Ventilator theory
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6-18
General
The ventilator pneumatics are at the rear of the breathing system.
A precision valve controls gas flow to the patient.
During inspiration, this gas flow closes the exhalation valve and pushes the
bellows down.
During expiration, a small flow pressurizes the exhalation diaphragm to supply
PEEP pressure.
Volume and pressure measurements come from flow sensors in the flow
sensor module. Two tubes from each sensor connect to a transducer that
measures the pressure change across the sensor, which changes with the
flow. A third transducer measures airway pressures at the inspiratory flow
sensor.
Volume monitoring uses the expiratory flow sensor. The ventilator uses the
other sensor to adjust its output for changes in fresh gas flow, small leaks, and
gas compression upstream of the breathing circuit. There is no adjustment for
compression in the patient circuit. If necessary, add the compression loss to
the tidal volume setting (volume control mode). The average volume changes
from compression in the breathing circuit is small (0.5 to 1.25 mL/cmH
For better precision:
• When the fresh gas mixture includes Heliox, use the Heliox mode
(Ventilation setup menu). Heliox, used on some ANSI Models of the
Aestiva, changes the data collected by the flow sensors. When Heliox
mode is selected, the ventilator adjusts the flow sensor data to correct for
these changes.
• A small quantity of gas bleeds through a resistor to help keep the
pressure on the exhalation valve constant. At high airway pressures, this
can cause a slight hiss during inspiration.
Do not try to silence the pneumatic resistor. If it is blocked, the
ventilator can malfunction and cause patient injury.
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Always connect the expiratory flow sensor. If it is not connected,
the patient disconnect alarm cannot operate correctly.
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1009-0633-000
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