18. Carbon Dioxide Measurement Flashcards
How is CO2 measured in solution?
Carbon dioxide (CO2) is routinely measured in anaesthetic practice:
> Arterial blood gas analysis
(partial pressure of CO2 in the blood)
> Capnography (percentage CO2 in expired gas).
CO2 is measured in solution using the
Severinghaus or CO2 electrode.
Fig. 67.1 The Severinghaus CO2 electrode
Fig. 67.1 The Severinghaus CO2 electrode
> Measurement of PCO2 is based upon [H+] measurement.
CO2 + H2O
↔ H2CO3
↔ H+ + HCO3-
> The CO2 electrode is a
modified pH electrode in
contact with sodium
bicarbonate solution.
It is kept separate from the blood
sample to be analysed by a
semi-permeable membrane.
> CO2 is able to diffuse across
this membrane and
react with H2O to
generate H+ ions.
The resulting change in [H+]
is measured by the glass
electrode (potentiometric electrode).
> Response time of the system is in the
order of 2–3 min.
> It requires calibration prior
to use with gas of known CO2 concentration.
> The temperature of the system
must be maintained at 37 °C.
How is CO2 measured in a gas mixture?
Capnography has become an
integral part of monitoring in anaesthesia.
Monitors can use infrared spectrography, mass spectrography, Raman spectrography, photoacoustic analysers or colorimetric devices
to measure carbon dioxide in the
respiratory gases and
then give a numerical reading (capnometry)
and a waveform (capnography).
We will look at the most commonly used
monitor, the infrared analyser.
Infrared analyser
Infrared analyser
> Diatomic gas molecules
(i.e. containing two or more different atoms)
absorb infrared radiation.
> Each diatomic gas absorbs radiation
of a particular wavelength.
> By measuring the proportion of
infrared radiation absorbed by a gas
mixture,
the partial pressure of a
diatomic gas can be inferred.
> An infrared beam passes through a
filter to obtain the required frequency
of light absorbed by the gas of interest.
> The infrared beam splits and passes
through reference and sample gas chambers.
> Sample and reference chamber windows
are made of crystal
(silver bromide or sapphire)
as glass absorbs infrared.
> CO2 absorbs infrared radiation
and emergent beams are compared by
photoelectric cells (the detector).
> The analyser is calibrated
using air (assumed zero CO2)
and a known concentration of CO2
(gas cylinder) or electronically (step input voltage).
> Analysis is affected by the
barometric and extraction pressure in
system –
a change in atmospheric pressure directly
influences the reading
of capnographs since CO2
concentration is measured as partial pressure.
> A water vapour trap is required
(water has high infrared absorbance).
> Hygroscopic tubing is needed.
If you were allowed only one
anaesthetic monitoring device,
what would you chose?
Most anaesthetists would probably request
capnography because of the
amount of information that can be
gained from capnogram analysis:
> Confirmation of endotracheal intubation
> Detection of rebreathing (inadequate fresh gas flow)
> Detection of obstructive expiratory airflow
> Detection of inter-breathing in a ventilated patient
> Sudden fall in end-tidal CO2
may indicate low systemic blood pressure,
circulatory arrest or pulmonary embolism
> End-tidal CO2 provides an
estimation of arterial PaCO2
> Detection of malignant hyperpyrexia
Which parameters are measured
directly by an arterial blood gas
analyser?
Direct + Derived
Which parameters are measured directly by an arterial blood gas analyser? Directly measured: > PaO2 > PaCO2 > pH Derived: > HCO3 – > base excess > O2 saturations
