Rome, June 2012
Contents
Bias Flow Correction in Quark RMR
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Learn more
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What is Bias Flow ?
Does Bias Flow impact on VO2 and
VCO2 measurements ?
How does the Bias Flow detection
work ?
How to calibrate Bias Flow with
Quark RMR ?
Useful Links
COSMED Homepage
http://www.cosmed.com
Quark RMR w/ICU
http://www.cosmed.com/quarkrmricu
In-Depth Notes
The Quark RMR ICU Option includes a simplified testing procedure that allows the
system to automatically identify the Bias Flow of the ventilator, either before or
during the metabolic test. This solution avoids the need to compensate, divert or
turning off Bias Flow during indirect calorimetry measurements.
What is Bias Flow ?
Bias Flow is a common feature of new generation ventilators. It minimizes the work
of breathing by allowing the patient to tap into a continuous gas flow rather than
initiate flow through a delivery circuit.
Most of the ventilators use a Bias Flow based detection system when operating
according to spontaneous breathing modes. A continuous (Bias) flow circulates
within the breathing circuit of the ventilator; its decrease from the base value is used
to rapidly detect the beginning of the spontaneous inhalation of the patient and to
trigger the activation of the vent.
Does Bias Flow impact on VO2 and VCO2 measurements ?
Bias Flow can greatly complicate metabolic measurements because the system
must be able to differentiate between gas delivery to the patient’s lungs and gas
continuously flowing through the circuit. As a matter of fact, Quark RMR can measure
VO2, VCO2, RQ and RMR in patient mechanically ventilated only if the eventual Bias
Flow of the ventilator can be detected and measured.
The latest AARC guidelines (“AARC Clinical Practice Guideline: Metabolic
measurement using indirect calorimetry during mechanical ventilation—2004
Revision & Update”. Respiratory Care, September 2004 Vol 49 n. 9) state among the
limitations of procedure the following:
7.4 Inaccurate measurement of REE and RQ during open circuit measurement
may be caused by
7.4.3 Inability to separate inspired and expired gases due to bias flow
from flow triggering systems, IMV systems, or specific ventilator
characteristics
Therefore, it is important to detect the Bias Flow and provide an automatic correction
on the metabolic measurements.
How does the Bias Flow detection work ?
Usually the ventilator allows the user to change two parameters:“Bias Flow value” or
“Bias Flow sensitivity”.
The presence of a Bias Flow affects the measurement of the expired volumes of air,
O2 and CO2 calculated by the Quark RMR. An automatic correction is performed by
the PC software according to the value set by the user (Vt).
Similarly other measured parameters such as expired oxygen volume (O2exp) and
exhaled Carbon Dioxide Volume (CO2exp) are also corrected according to the value of
Bias Flow specified by the user.
The correct specification of the Bias Flow guarantees the accuracy of the
measurements.
• If Bias Flow is significantly smaller than what specified, Vt and all values are
underestimated;
• If Bias Flow is significantly higher than what specified, it is possible that no
breaths are detected;
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How to calibrate Bias Flow with Quark RMR ?
Bias Flow calibration can be executed before starting the test and/or at the beginning
of the test. During this calibration the software detects the Bias Flow that adds to the
expiratory flow, so that it deducts it and measure the correct Vt expired.
During the calibration the Bias Flow should be adjusted to match the Vt of the Quark
RMR with that one of the ventilator (Vte). Ventilator should be calibrated before.
Once selected the ICU test option, the “Bias Flow” calibration screen is prompted.
The graph shown below represents the actual flow measured by the turbine
flowmeter at the exhalation port of the ventilator.
Bias Flow should be set to the value measured at the end of exhalation phase.
An “Autolearn” function has been added on the calibration menu prior the test in
order to help in the detection of Bias Flow (software searching automatically the
minimum of flow over a period of 10 seconds)
Bias flow (the green line in the picture below) must be adjusted to get the VT of
the Quark similar to Vte of the Ventilator. Modifying the Bias Flow even the step
detection is modified. User must pay attention that all steps are detected.
If the specified bias flow it too high, breath volumes is underestimated.
Bias Flow can be checked and recalibrated during the test by accessing Event/
Check Bias Flow
For more infos on Bias Flow Calibration and on the ICU Option, please refer to the
Quark RMR User Manual.
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