GE auisys cs2, User'S Reference Manual

Page: 199 / 268

Ventilator theory
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. If the maximum pressure
(pmax) is reached during inspiration, the ventilator will cycle to
expiration.
Volume and flow 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.
The ventilator uses the data from the flow sensors for volume-related
numerics and alarms. The ventilator also uses the flow sensors to
adjust its output for changes in fresh gas flow, small leaks, and gas
compression upstream of the breathing circuit. There is adjustment
for compression in the patient circuit.
In volume ventilation modes, certain alarm conditions prevent the
automatic adjustment of ventilator delivery based on measured flow
values. In these cases, ‘TV accuracy decreased. Adjust manually.’
displays above the ventilator setting area of the screen. When this
message shows, the ventilator may not be able to deliver within the
accuracy range specified. When this occurs, the tidal volume must
be manually adjusted until the volume delivered reaches the desired
level. If compensation stops for a number of breaths, the condition
causing the hold shows as an alarm. Automatic volume
compensation resumes when the alarm conditions are resolved.
For better precision a small quantity of gas bleeds through a resistor
to keep pressure on the exhalation valve constant. At high airway
pressures, this can cause a slight hiss during inspiration.
Always connect the expiratory flow sensor. If it is not
connected, the patient disconnect alarm cannot operate
correctly.
O2 monitoring theory of operation
O2 monitoring measures O2 concentration in the patient circuit. The
O2 concentration measured from the O2 cell is shown on the
ventilator display.
The O2 cell is an electrochemical device (galvanic cell). Oxygen
diffuses through a membrane into the cell and oxidizes a base metal
electrode. This oxidation produces an electrical current proportional
to the partial pressure of the oxygen at the electrodes sensing
surface. The base metal electrode gradually wears out from the
oxidation process.
WARNING
11 Specifications and theory of operation
2067226-001
11-23