Gas Exchange

Technical description of Datex-Ohmeda’s module for gas exchange measurement

Patrick Sweeney

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Gas exchange measurements through the Datex-Ohmeda M-COVX module and the Datex-Ohmeda M-CAiOVX module measure the following parameters: O2 Consumption (VO2), CO2 Productions (VCO2), Energy Expenditure (EE) and Respiratory Quotient (RQ). The measurement technology is incorporated into a monitoring module for use in the S/5 series of modular monitors. This technology mirrors, but is not the same as, the Datex-Ohmeda Deltatrac Metabolic Monitor. The Deltatrac Metabolic Monitor uses a technique whereby gases are collected in a sample chamber for subsequent analysis, while the M series modules (M-COVX and M-CAiOVX) use mathematical integration of flow and time synchronized continuous gas sampling in order to provide the data in a continuous and non-invasive fashion.

Gas exchange measurements

Respiratory gas exchange measurements take into account the measurement of oxygen consumption (VO2) and carbon dioxide (VCO2). These parameters reflect the metabolic component of body systems and can be used to further calculate respiratory quotient (RQ) and energy expenditure (EE). The continuous and non-invasive measurement of respiratory gas exchange, also known as indirect calorimetry, is potentially valuable when employed for diagnostic and therapeutic purposes. The clinical applications of indirect calorimetry range from assessment of energy requirements in various physiological states to analysis of ventilation and oxygen transport in respiratory and hemodynamically compromised critical care patients. (1)

Measurement components

The D-lite + flow sensor

The D-lite + flow sensor is integral to the gas exchange measurement. This flow sensor in conjunction with the gas sampler is connected at the patient airway and provides the conduit for the gas exchange measurement. The D-lite + can be used with active humidification or in conjunction with an HME.

D-Lite + flow sensor

Sampling lines

The measurement of gas exchange also requires that the module (M-COVX) and (M-CAiOVX) use both flow sensor tubing line as well as a gas sampling line. The gas exchange measurement will only be accurate with a 2 meter gas sampling line. This is due to the synchronization and deconvolution of gas curves, which take place in the module.

Datex-Ohmeda M-COVX and M-CAiOVX modules

The Datex-Ohmeda compact airway modules for critical care (M-COVX) and for anesthesia (M-CAiOVX) incorporate the gas measurement capability. They also provide Patient Spirometry and lung mechanics measurement with M-CAiOVX.

Measurement technology

In order to provide an accurate breath-by-breath measurement of respiratory gas exchange the module must algorithmically integrate side stream gas concentrations (CO2 and O2) as well as the flows and volumes generated by each breath. This is done with the D-Lite + flow sensor in conjunction with the fast paramagnetic oxygen sensor and the infrared gas bench for CO2 measurement. Thus, the measurement can be thought of as a three stage technique whereby flows and concentrations are synchronized and gas volumes calculated. A more in-depth technical description of each measuring component is available in the airway gas module section of the service manual.

Basic data obtained

Oxygen consumption (VO2) and carbon dioxide production (VCO2) can be simultaneously measured by subtracting the amount of gas that is exhaled from the amount initially inhaled, This incurs multiplying each volume piece (dV) by the corresponding gas concentration fO2 or fCO2:

By employing inspiratory and expiratory minute volumes MVi and MVe, as well as the volume averaged inspiratory and expiratory concentrations, fi and fe, the above can then be written:

Synchronization of concentrations and flows

Due to the side stream principle of gas concentration measurement there is a significant lag time (1.5 s) in measurement due to the travelling time of the sample through the sampling line to the module. In contrast to this there is virtually no lag time when the flow signal is registered at the D-lite + flow sensor (< 10 ms). Therefore the module must algorithmically synchronize these concentrations and flows.

Haldane transformation

In order to ensure volume measurement results, which are less sensitive to errors the measurement technique incorporates Haldane Transformation.

This uses the fact that nitrogen is an inert gas and an individual will neither consume nor produce it. The amount of nitrogen in an inhaled sample of gas is equal to the amount of nitrogen in an exhaled sample of gas. This can be written as: fiN2 x MVi = feN2 x MVe. Since this is the case we can now write VO2 and VCO2 as:

with

Derived parameters in gas exchange

After the oxygen consumption and carbon dioxide production have been arrived at, the respiratory quotient (RQ) and the value for energy expenditure (EE) can be derived. The value for RQ is a straightforward calculation, which takes into account both measured parameters (VO2 and VCO2):

Energy expenditure is not a directly measured parameter but is calculated by using a modified Weir equation:

Reference

Takala J. Meriläinen P: "Handbook of Indirect Calorimetry and Gas Exchange" p.30, Datex-Ohmeda 876710-01

Last updated: 1 March 2001Created
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