Welch Allyn Duet CO2 Module - User Manual - Manual (Page 10)

Duet CO
2
Module OEM Implementation Manual
Page 10 Confidential Welch Allyn OEM Technologies
Measurement Calculation
Measurements provided to the host system include ETCO
2
, Inspiratory CO
2
(InsCO
2
), or baseline CO
2
,
and respiratory rate (RR). These three measurements are collectively referred to as breath data. A
proprietary breath algorithm is used to calculate the breath data.
Breath Algorithm
Duet’s breath algorithm incorporates an initial learning period which, based on certain assumptions of
CO
2
waveform morphology, establishes CO
2
reference points for threshold determinations. A sliding
window is used to detect a stable maximum, or ETCO
2
value, and a baseline, or InsCO
2
value.
Thresholds are updated in real time with each breath. A signal averaging technique is used to calculate
the RR based on this set of measurements. By incorporating these adaptive and signal averaging
techniques, the breath algorithm effectively reports accurate CO
2
measurements while maintaining a
high level of noise immunity.
Measurement Compensation
IR absorption in the CO
2
wavelength band can be affected by a number of factors that alter the CO
2
measurement. The Duet Module automatically compensates for some of these factors while others
may be disabled by the host system.
These factors include
• water vapor
• pressure broadening
• gas mixing
• oxygen, nitrous oxide and desflurane or O
2
/N
2
O/desflurane
• Body Temperature, ambient Pressure and Saturated with water vapor or BTPS
Water Vapor
Water vapor compensation accounts for the effect that water vapor has IR absorption characteristics of
CO
2
molecules. During normal mainstream or sidestream operation, CO
2
measurements are adjusted
mathematically to compensate for this effect.
The host may choose to disable this compensation when performing a dry gas measurement in which
the gas does not contain water vapor. Dry gas procedures may include steady state measurements
and verification procedures. Steady state measurements are performed only when background CO
2
, or
CO
2
present in the immediate environment, is measured. An example of a steady state measurement
is measuring the CO
2
content inside an incubator. Verification procedures use calibrated gas that is
free of water vapor, or dry, as well.
The water vapor compensation is ON by default and may be enabled or disabled via a host system
command.
Pressure Broadening
Pressure broadening compensation accounts for the effects the barometric pressure has on CO
2
molecule distribution and is used in both mainstream and sidestream mode operation.
The pressure broadening compensation is ON by default and cannot be disabled by the host system.