2 Theory of Operation
1505-1018-000 08/07
2-7
Following the total flow sensor are the free-breathing check valve {11} and the
inspiratory effort valve {12}.
During normal operation, the inspiratory effort valve is open, allowing the
free-breathing check valve to admit flow if the patient draws a significant
amount of inspiratory pressure, causing the airway pressure to become more
negative than -0.5 cm H
2
O. The free-breathing check valve allows the patient
to spontaneously breathe in case of a ventilation delivery failure.
On occasion, to assess the patient’s tolerance to be weaned from the
ventilator, clinicians can determine the amplitude of inspiratory effort that the
patient can create. During this “procedure”, the inspiratory effort valve is
closed, effectively locking out the free breathing valve from the patient circuit.
Next in the flow path is the O
2
sensor {13}. The sensor is used to monitor the
O
2
concentration produced by the combined O
2
and Air flows.
The O
2
sensor uses the paramagnetic principle (oxygen molecules are
attracted in magnetic fields) to measure the oxygen concentration. The sensor
includes two nitrogen-filled glass spheres mounted on a suspension
containing a conductive coil that is located in a non-uniform magnetic field.
When the system is disturbed by an impulse of current, the suspension begins
to oscillate, inducing an EMF into the coil. The oscillation period of the
induced EMF is dependent on the partial pressure of oxygen surrounding the
suspension.
As sample gas fills the sensor, oxygen that is present in the sample is
attracted into the strongest part of the magnetic field. This congregation of O
2
molecules alters the natural oscillation frequency of the suspension.
Calculations based on the difference between the oscillation period for an
oxygen sample and that for nitrogen, and readings from the absolute pressure
transducer, determine the measured O
2
percentage.
O
2
Flow
Sensor
Flow
Sensor
Total Flow
Inspiratory
Effort
Valve
Free Breathing
Check Valve
H
T
H
T
Pneumatic
Resistor
O2 Sensor
Auxi
T
Flow
Valve
Flow
Valve
Expiratory Press
P
H
T
{13}
O
2
Air
{11}
{12}
Summary of Contents for Engstrom Carestation
Page 1: ...Engström Carestation Technical Reference Manual ...
Page 12: ...Engström Carestation x 08 07 1505 1018 000 ...
Page 52: ...Engström Carestation 2 26 08 07 1505 1018 000 ...
Page 100: ...Engström Carestation 5 26 08 07 1505 1018 000 ...
Page 124: ...Engström Carestation 7 16 08 07 1505 1018 000 ...
Page 188: ...Engström Carestation 8 26 08 07 1505 1018 000 ...
