Fluke Biomedical VT Plus HF - Manual (Page 131)

Appendices
Gas Analyzer Tutorial
A
A-5
In the manual zero mode, if the user presses the ZERO/7 key when the Analyzer is at the
Flow screen, a message appears, indicating that the flow must be removed from the
currently selected flow port. The user must remove the flow so that a correct zero reading
can occur.
In the auto zero mode, the Analyzer uses the valves that switch between the high- and
low-flow ports to obtain a zero reading. It is assumed that there is no flow in the
unselected port. The purpose of this is to provide a means to zero the flow without having
to disconnect from the ventilator circuit (or other flow source being measured).
Note
When auto zeroing is selected, it is important that there is no flow on the
unselected flow port. For example, if the high flow range is selected, there
should not be any flow in the low flow port if auto zeroing is selected.
When auto zero is selected, the user may still push the ZERO/7 key to initiate a flow
zero. The valves still automatically switch to obtain the zero reading. Therefore, it is not
necessary for the user to disconnect the flow to zero. No message appears indicating that
the flow source should be removed.
With the auto zero setting enabled, the flow automatically zeros every 20 minutes. If
more frequent zeroing is desired, manually zero the flow using the ZERO/7 key. Because
some data is lost during a zero, ventilator parameter data for breaths affected by an auto
zero are discarded.
Airway pressure is also zeroed whenever flow is zeroed. Airway pressure can also be
independently zeroed from the airway pressure screen.
Breath Detection
To monitor ventilator parameters, the Analyzer must first determine when a breath
occurred and delineate the various phases of the breath. For example, the breath detection
algorithm determines the beginning and ending of the integration periods for calculating
tidal volumes.
The breath detection algorithm is implemented as a state machine. The flow waveform is
the input to the state machine. The state changes are based on flow waveform crossing of
predefined thresholds. A threshold is applied to both positive (inspiratory) and negative
(expiratory) flows. The breath detection threshold can be set to a high flow rate to reject
noise near zero flow.
Bi-directional, Inspiratory, Expiratory Breath Detect Modes
Figure A-1 shows the circuit connection for setting up the Analyzer in a bi-directional
mode. The gas flows in both directions through the Analyzer. This is the preferred setup
for using the Analyzer with a ventilator.