Page 120 - Basic Monitoring in Canine and Feline Emergency Patients

or against expired lung volume during a respiratory These devices are best utilized to confirm proper
cycle (volume capnogram). endotracheal intubation. The device is placed at the
VetBooks.ir efficiency of ventilation. It is the most commonly end of the endotracheal tube (ETT). If the color
The time capnogram gives an indication of the
does not change, there is a high chance the ETT
used method because it is simple to use, evaluates
tioned. It is important to wait for six breaths before
both parts of the respiratory cycle (inspiration and tube is in the esophagus and it needs to be reposi-
expiration), and is an excellent tool for patient deciding if the ETT is properly placed.
monitoring at bedside. The volume capnogram has
its own benefits too: it allows a breath-by-breath
quantification of the volume of lung ventilated, as Quantitative capnometry
well as the measurement of alveolar dead space, and There are multiple physical methods that can be
physiological dead space. This means that the clini- used to quantitatively measure CO : (i) infrared
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cian can assess the degree of ventilation/perfusion spectroscopy; (ii) molecular correlation spectros-
(V/Q) mismatch. Different physiological concepts copy; (iii) Raman spectrography; (iv) photo acous-
can be used (i.e. Bohr or Enghoff approach) to tic spectrography; (v) mass spectrography; and (vi)
quantify the global V/Q mismatches. This method chemical colorimetric analysis spectrography.
also allows separation of shunt fraction (blood The most commonly used principle is the meas-
passing by unventilated alveoli) and true dead space ure of infrared light; therefore, this method will be
(air not available for gas exchange). The major limi- described in more detail in this chapter. This prin-
tations of volume capnograms include the require- ciple uses the application of the Beer and Lambert
ment to use a mechanical ventilator and other Laws and infrared waves. This means that the
elaborate equipment and it only provides informa- quantity of infrared (IR) waveforms absorbed is
tion regarding the expiratory phase of the breathing proportional to the concentration of the infrared-
cycle (as the expired CO is plotted against expired absorbing substance. In other words, the quantity
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lung volume). of infrared waveforms absorbed by CO molecules
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This chapter will only focus on time capnograms. is proportional to the concentration of CO present
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A time capnograph provides a graphic represen- in the expired gas. The higher the CO concentra-
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tation (waveform) denoting the relationship of the tion the higher the quantity of IR absorbed. This
ETCO concentration to time. It also provides a concept might seem difficult to follow, but it will be
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measured value of the maximum level of ETCO at described in more detail below.
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the end of each breath. Changes in size, shape, and A capnograph using infrared spectroscopy con-
distribution of the waveforms are suggestive of tains three main components: (i) infrared source;
underlying pathologic conditions while changes in (ii) sample chamber; and (iii) infrared detector.
the level of measured ETCO reflect disease sever-
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ity as well as response to treatment.
Capnometry is further classified based on the The infrared source
technique used to measure CO concentration: As the name implies, the infrared source emits
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● ● Qualitative capnometry: provides a color-coded infrared waves. The infrared waves are absorbed by
different gases present in the anesthesia circuit.
(colorimetric) capnogram representing the pres- Each gas absorbs infrared waves of different wave-
ence of CO in the patient’s breath. lengths. For example, CO absorbs infrared wave-
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● ● Quantitative capnometry: provides a numeric lengths of 4.25 micrometers, nitrous oxide absorbs
value for the measured CO concentration. It infrared wavelengths of 4.5 micrometers, while
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can also provide a graphic form which is called oxygen does not absorb infrared light.
‘quantitative capnography.’
Why is this important? Because knowing the
wavelength absorption of a certain gas (in our case
the CO ), a device can be designed with an infrared
Qualitative capnometry 2
source that will only emit that specific wavelength
Qualitative (colorimetric) capnographs are hand- and only measure the concentration of that gas. If the
held devices which contain a pH-sensitive impreg- source were to emit a wide spectrum of wavelengths,
nated paper. If the exhaled gas contains CO the then it might measure the concentration of other
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color of the paper changes from purple to yellow. gases that are not of importance and the final reading
112 L.A.M. Ilie