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38 Respiratory Monitoring in Critical Care 377

The adequacy of each of these sample types is dependent Exhaled carbon dioxide originates in the tissues and
VetBooks.ir on patient factors as will be discussed later. requires cardiac pumping activity to generate venous
return and CO 2 delivery. Cardiac arrest is associated
Arterial blood is often an ideal surrogate sample for
evaluating ventilatory status. Carbon dioxide is highly
soluble and diffusible and alveolar partial pressures typi- with an abrupt decline in ETCO 2 levels. Conversely, the
adequacy of cardiac compressions can be assessed by
cally reach equilibrium with those in the bloodstream as capnography or capnometry.
venous blood is arterialized. Diseases that result in Many modern capnography systems (e.g., NICO 2 ®)
impaired diffusion of respiratory gases (e.g., emphysema) include a flow disrupter and differential pressure trans-
often result in fatal hypoxemia before becoming suffi- ducers within the same disposable unit that is attached to
ciently severe to cause meaningful limitations in carbon the patient’s breathing circuit. This allows for monitor-
dioxide diffusion, although hypercapnia may develop in ing of respiratory mechanics as well as single‐breath cap-
these patients by other mechanisms. However, extremes nography (CO 2 plotted against cumulative exhaled
of ventilation–perfusion mismatching can result in a dis- volume instead of over time). Single‐breath capnography
sociation of arterial (PaCO 2 ) and alveolar (PACO 2 ) car- provides additional data relating to airway dead space,
bon dioxide tensions. Even with this limitation, PaCO 2 gas distribution, and perfusion that cannot be obtained
remains the preferred surrogate sample for defining the easily (or at all) with standard time‐plotted capnography.
adequacy of alveolar ventilation. The disadvantages of end‐tidal capnography are that it
Venous blood PCO 2 can be useful in the assessment of can add considerable apparatus dead space volume in
ventilatory status in certain settings. As shown in small (<2 kg) patients and promote rebreathing and
Table 38.1, the carbon dioxide tensions in venous and reduced alveolar ventilation. End‐tidal capnography also
arterial blood are not the same; however, in a hemody- becomes dissociated from arterial carbon dioxide ten-
namically stable patient the difference between the two sions when significant alveolar dead space is present. A
is suitably constant (~5–7 mmHg) so as to allow reason- larger gap between arterial and end‐tidal carbon dioxide
ably accurate prediction of what current arterial values tensions develops in the setting of pulmonary thrombo-
are likely to be. Venous values also define the maximal embolism or in cardiovascular instability/profound
value that may be present in arterial samples at that time. hypotension.
For example, if the PvCO 2 is 55 mmHg then the PaCO 2 is Hypercapnia and hypocapnia represent (and define)
likely to be 49 mmHg and will not be greater than inadequate or excessive alveolar ventilation. The abso-
55 mmHg. Unfortunately, the utility of venous blood lute values used to define these states will vary with the
samples for defining ventilatory status is strongly analyzer used and the species. Typically, normocapnia is
dependent on cardiovascular performance. As cardiac defined as PCO 2 of 35–45 mmHg in veterinary species.
output falls, the difference between PvCO 2 and PaCO 2
becomes progressively larger. In patients in shock or
other low‐flow states, venous samples are poor surrogate Assessment of the Adequacy
samples for the assessment of the adequacy of alveolar of Oxygenation
ventilation.
End‐tidal CO 2 (ETCO 2 ) is an important means of Hypoxemia is the term used to indicate inadequate par-
monitoring ventilation in critically ill animals and encom- tial pressure of oxygen in an arterial blood sample (PaO 2 ).
passes both capnometry (numeric value only) and cap- The values at which samples are considered to indicate
nography (waveform representation). Of the various hypoxemia vary slightly by region/elevation. At sea level,
surrogate markers for alveolar CO 2 , ETCO 2 , when it is PaO 2 of less than 80 mmHg and 60 mmHg are considered
working well, is the closest to actually measuring PACO 2 . to indicate hypoxemia and severe hypoxemia, respec-
End‐tidal plateau partial pressures of carbon dioxide tively. Hypoxemia does not always equate with reduced
largely reflect the levels in alveolar gas. End‐tidal CO 2 arterial oxygen content, however. Polycythemic patients
monitoring is the only tool that currently provides con- may have near‐normal arterial oxygen content (~20 mL
tinuous, real‐time assessment of the adequacy of alveolar O 2 /dL of blood) despite mild‐to‐moderate hypoxemia.
ventilation. In addition, respiratory rate can be deter- The increased hemoglobin concentration can allow for
mined via this methodology as well. Capnography has the the preservation of total content even when partial pres-
added benefit of allowing waveform inspection, which can sure of oxygen is reduced.
be used to monitor for rebreathing, airway obstruction/ Determination of the adequacy of oxygenation is rela-
bronchospasm, apnea, dyssynchronous alveolar emptying, tively straightforward. An arterial blood sample is
valve malfunctions, and many other events. obtained and the determination of whether hypoxemia is
Capnography and capnometry can also be useful in present is made. Alternatively, a surrogate can be used in
determining the status of the cardiovascular system. the form of pulse oximetry. With pulse oximetry, the