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Gas exchange
Stopping the gas exchange measurement
1. Remove the gas exchange parameter windows from the screen.
You can continue measuring airway gases and Patient Spirometry with the same
module.
Gas exchange measurement basics
Gas exchange measurement description
The E-COVX and E-CAiOVX modules with gas exchange option enable monitoring of
O
2
consumption (VO
2
), CO
2
production (VCO
2
), energy expenditure (EE) and respiratory
quotient (RQ).
To provide an accurate breath-to-breath measurement of respiratory gas exchange,
the E-COVX and E-CAiOVX modules algorithmically integrate sidestream gas
concentrations (CO
2
and O
2
) as well as flows and volumes generated by each breath.
This is done with the D-lite(+)/Pedi-lite(+) flow sensor in conjunction with the fast
paramagnetic oxygen sensor and the infrared gas bench for CO
2
measurement.
Due to the sidestream measurement principle, there is a delay of approximately 2.5
seconds in the measurement, caused by the traveling time of the sample through
the sampling line to the module. The module algorithmically synchronizes these
concentrations and flows.
To obtain the oxygen consumption of a patient, the gas exchange module measures
the amount of oxygen that is inhaled, and subtracts the exhaled amount from it.
Carbon dioxide production is measured by subtracting the amount of inhaled carbon
dioxide from the amount of exhaled. These amounts can be obtained by multiplying
each measured volume sample by the corresponding gas concentration.
To ensure volume measurement results that are less sensitive to errors, the Haldane
transformation is applied. The Haldane transformation is based on the assumption
that nitrogen is an inert gas, and an individual will neither consume nor produce it,
except in the case of air emboli. Therefore, the amount of nitrogen inhaled is equal
to the amount exhaled.
How to interpret the gas exchange values
Though the measurements can be made easily, accuracy and reproducibility of
results requires understanding of the basic principles of the measurement and
related physiology. Furthermore, gas exchange, or indirect calorimetry, is sensitive to
measurement errors; the need for routine procedures of quality control is therefore
emphasized. Despite accurate measurement, several clinical and physiological factors
influence the results of gas exchange measurements and should be considered in
interpretation. In this respect, the relationship between ventilation and gas exchange
is of crucial importance. Any acute change in alveolar ventilation will be immediately
reflected in CO
2
production, which will not measure the metabolic production of CO
2
,
until a new steady state has been achieved. Similar, but shorter, transient will also
be seen in O
2
consumption. Analogously, acute changes in tissue perfusion may
influence both tissue oxygen uptake and removal of CO
2
from the tissues.
Pulmonary gas exchange measurement means monitoring of oxygen consumption
(VO
2
) and carbon dioxide production (VCO
2
). Based on these measurements, it is
possible to calculate the respiratory quotient, RQ, which is the ratio between CO
2
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