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VII. THEORY OF OPERATIONS
The Crossvent-2i+ is a time cycled, volume or pressure limited ventilator. Its basic principle of operation is extremely
simple. Supply gas, either air, oxygen, or a blended mixture, is connected to the Crossvent-2i+ inlet fitting.
NOTE: For blended gas, the Bio-Med Devices blender should be used.
For pneumatic schematics, refer to the Schematic section of this manual.
Gas flows first to an internal pressure regulator that provides output gas regulated to approximately 16 psi. This is used
both for patient gas and to set PEEP and PIP levels.
From the regulator the gas flows to a normally closed, 2-way pilot valve operated by a miniature solenoid valve (valve A).
The gas exits the pilot valve and goes to an electronically encoded flow valve. The encoding is accomplished via a
precision potentiometer. Since the upstream (supply) pressure is constant and much greater than the downstream
(patient) pressure, changes in downstream pressure may be neglected. Therefore, since the supply pressure is accurately
regulated, the flow rate becomes a function solely of the flow valve setting. The length of time that gas flows is the
inspiratory time. The volume of gas that flows during the time it is on is the Tidal Volume and is equal to the on-time
(inspiratory time) multiplied by the flow rate.
From here the gas goes into the patient circuit through the patient connector. Pre-set pressure relief valves in the manifold
limit the maximum pressure, and allow the patient to draw in ambient air if the entire system becomes inoperative.
During the period of time when valve A is open and gas flows, solenoid valve B is actuated, allowing gas from the PIP
valve to pressurize the diaphragm of the exhalation valve. This assures that all gas will flow to the patient. At the end of
inspiration, valve A closes and gas flow ceases. Simultaneously, valve B is de-energized, connecting the PEEP valve signal
to the exhalation valve diaphragm. This allows the patient to exhale to atmosphere and the pressure in the patient circuit
to fall to PEEP or atmospheric pressure.
A low flow flush system is provided to prevent humidity from traveling back up the pneumotach sensing lines (if used) and
damaging the pressure transducer. This is accomplished with two solenoid valves, D1 & D3. A third solenoid valve, D2,
is used to zero the pressure transducer to compensate for drift. During inspiratory, these solenoids actuate. A very low
flow is passed through solenoids D1 & D3 and out the pneumotach tubes. At the same time, the transducer ports are
shunted through solenoid D2. This zeros the transducer by equalizing the pressure across it. During expiratory, these
solenoids are de-energized and the pressure differential from the pneumotach is then passed through D1 and D3 to the
transducer.
NOTE: The flush circuit activates every 30 seconds, but may activate more frequently when the unit is
initially turned on. If the inspiratory time is greater than .250 seconds, the flush circuit lasts for the
duration of the I-time. For shorter inspiratory times, then the flush is fixed at .280 seconds and
therefore continues into the expiratory phase of the breath.
Solenoid E2 provides a base flow of 2.5 lpm during expiratory when using Flow Trigger.