Penlon AV-S, Service Manual, Page: 86 / 94

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APPENDIX 3
AV-S Ventilator Spirometry System
Ventilator Spirometry Measurement
The AV-S ventilator drive gas and
spirometry system uses a total of three
mass flow gas sensors to monitor and then
independently measure the gas flows within
the ventilator and breathing system.
This ensures that correct volumes are
delivered to the patient.
These monitors are measuring firstly in the
ventilator delivery control system, and
secondly in the patient breathing system.
During use of the ventilator the user will set
a required tidal volume and at the first
breath the ventilator will use its pre-
calibrated delivery flow rate valve settings
to set the proportional delivery valve
position to deliver the requested tidal
volume.
To confirm that the correct flow rate (tidal
volume) is being delivered by the ventilator
delivery system an internal flow sensor (a
Honeywell AWM43300V mass flow sensor),
monitors the delivered flow rate and makes
adjustments every 30 ms using proportional
regulation.
As this sensor is always measuring the
known drive gas rather than breathing
system gas the volumes measured will
always be independent of breathing system
gas composition. This system ensures
accurate delivery volume from the ventilator
control unit.
To monitor for correct delivery volumes in
the breathing system there are two
breathing system mass flow sensors
(Honeywell AWM 720P1 spirometers).
One sensor is located in the inspiratory
limb, and one in the expiratory limb.
Measurements are taken from these
sensors to determine the actual delivered
and exhaled gas volumes in the breathing
system. This enable measurements to be
made to compensate for fresh gas flow,
compliance losses and possible breathing
system leaks.
During the inspiratory cycle the inspiratory
flow sensor measures the gas volume
delivered to the patient.
The flow sensor output is read at least
every 2 msec and then five sets of readings
are averaged and the averaged value is
sent every 10 ms to the processor for
calculation of the volume delivered to the
patient.
This delivered volume will consist of the
volume delivered from the ventilator bellows
plus the fresh gas flow from the anaesthetic
machine fresh gas supply, minus any
compliance loss and minus any leak.
This gives a total actual inspired tidal
volume.
A similar measurement method is used for
the exhaled volume. During the exhalation
period the measured exhaled volume is
subtracted from the inspired volume, and at
the end of exhalation.
A negative (more gas coming out) volume
indicates that fresh gas has increased the
delivered volume.
A positive volume (less gas coming out)
indicates a leak in the circuit.
The ventilator control system will then
adjust the next delivered tidal volume, up to
a maximum of 100 ml. This will bring the
delivered volume to exactly as set.
If the variation between set and delivered is
greater than the maximum rate of change
allowed, the adjustment will occur gradually
over several breaths.
The displayed volume is the average of the
inspiratory and expiratory volumes. If this
value is less or more than 50% of set
volume, a low or high volume alarm is
given.
Breathing System Gas Composition
Gas flow measurements are affected by the
breathing system gas composition. To
compensate for these effects the ventilator
has
a) a gas composition setting whereby the
user is able to select the gasses being
delivered, i.e. oxygen/air, oxygen/nitrous
oxide etc,
b) an oxygen monitor;
Thus the ventilator knows the overall
oxygen concentration and the majority of
the remaining gas composition.
APPENDIX