Page 88 - Basic Monitoring in Canine and Feline Emergency Patients

reading, perform an arterial blood gas (see Chapter Patients breathing supplemental oxygen should
5). Patients who are cyanotic do not need to have have pulse-ox readings that are >98%. As men-
VetBooks.ir pulse oximetry performed as the observer has tioned in Section 4.1, pulse oximetry no longer
correlates with PaO once the PaO is significantly
already confirmed severe hypoxemia from visual
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examination. At that moment, these patients need
surrogate for lung function outside of this range.
emergent interventions, not exact measurements of greater than 100 mmHg and cannot be used as a
the degree of hypoxemia. Pulse oximetry values <95% indicate the need for
Any patient undergoing heavy sedation or general oxygen supplementation as they correlate with
anesthesia should have continuous monitoring with PaO values less than 80 mmHg (i.e. hypoxemia).
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pulse oximetry. Patients who are heavily sedated to Pulse oximetry can be inaccurate for a variety of
anesthetized are at high risk of hypoxemia, hypoven- reasons as discussed in the next section. Any abnor-
tilation, or apnea for a variety of reasons. Continuous mal result that does not fit with the patient’s pres-
monitoring with pulse oximetry gives visual and audi- entation or physical exam should be confirmed
tory clues to the person monitoring anesthesia that with an arterial blood gas analysis, as this is the
desaturation is occurring and therefore can lead to gold standard to diagnose hypoxemia. For more
life-saving interventions. It is important to remember information about interpretation of arterial blood
that patients breathing high concentrations of oxygen gas analysis, please see Chapter 5.
are very far to the right on the flat portion of the
OxyHb dissociation curve (see Fig. 4.3). As noted 4.5 Pitfalls of Pulse Oximetry
in Section 4.1, these patients can experience signifi-
cant loss of lung function and reductions in PaO Multiple factors can affect the accuracy of pulse oxi-
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long before they reach the sigmoidal portion of the metry in veterinary patients; these are outlined in
curve and the pulse-ox will actually start to drop. Table 4.2. In general, anything that interferes with the
Therefore, a reliably dropping pulse-ox in a patient pulse oximeter’s ability to detect an accurate pulsatile
breathing supplemental oxygen should be taken as a light signal may cause interference. Additional patient
late indicator of a very serious oxygenation problem factors that affect the accuracy of pulse oximetry are
and immediately investigated. the presence of abnormal species of Hb; pulse oxim-
etry cannot differentiate between MetHb, COHb, and
OxyHb. Interestingly, the application of gauze sponges
4.4 Interpretation of Results
between the tongue and pulse-ox transmission probe
Pulse oximetry values are reported as a percentage. has actually been shown to improve SpO readings in
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A normal SpO in a patient breathing room air both dogs and cats. It is hypothesized that the added
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(FiO = 21%) is generally between 96–98%, which thickness between the tongue and the LED and sensor
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corresponds to a PaO on the OxyHb dissociation on the probe improves contact pressure.
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curve of 85–100 mmHg (see Fig. 4.2). Hypoxemia
is defined as a PaO less than 80 mmHg and severe
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hypoxemia as a PaO of less than 60 mmHg. Hb alterations
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Therefore, patients with the correlating pulse-ox It is important to remember that the pulse oximeter
reading of <95% should be considered hypoxemic is an approximation of the percentage of Hb in arte-
and <90% severely hypoxemic. rial blood that is saturated with oxygen, NOT an
The pulse oximeter is generally accurate within ± approximation of the total arterial oxygen content.
2–3% of the measured SaO on an individual’s arte- As outlined in Box 4.1, severely anemic patients
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rial blood gas but becomes less accurate with wors- can have significantly low arterial oxygen content
ening hypoxemia. In patients with a true SaO <90%, despite fully saturated Hb.
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the pulse oximeter may vary by as much as ± 5%. Similarly, the presence of a normal amount of
Therefore, readings should be interpreted more as oxygen in the blood does not guarantee that tissues
trends within an individual patient and to classify are adequately oxygenated. For example, cyanide
general categories of hypoxemia rather than becom- toxicosis inhibits oxidative phosphorylation inside
ing fixated on a specific value. For example, a patient the cell, preventing extraction of oxygen from the
with a SpO of 85% might actually be as low as 80% blood into the cell. This will result in a high concen-
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or as high as 90%; either way, this patient is severely tration of oxygen in both venous and arterial blood,
hypoxemic and requires immediate intervention. as none leaves to enter the tissues yet the tissues are

80 K.A. Marshall and A.C. Brooks