Page 90 - Basic Monitoring in Canine and Feline Emergency Patients

underestimate the true SaO depending on the does not fit with the measured pulse oximetry read-
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patient’s clinical status. In this setting, a PaO MUST ing. Assuming the patient is not also severely ane-
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VetBooks.ir be measured to determine the amount of oxygen in mic, she would be expected to be cyanotic at a SpO
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of 45% but her mucous membranes are pink.
the plasma available to tissues. In order to defini-
Despite a heart rate that is ‘close’ to that of the
tively diagnose MetHb, co-oximetry (discussed
below) or a MetHb assay should be performed. patient, the poor correlation with clinical signs
should make the practitioner consider this reading
erroneous. This does not automatically mean we can
Co-oximetry
assume the patient is oxygenating well, simply that
Certain pulse oximetry units have the ability to we do not know her true level of oxygenation.
measure abnormal Hb variants. The sensors on Repeating the reading on a different tissue bed, mak-
these units emit the standard wavelengths of light ing sure the probe is in place long enough to see good
at 660 nm and 940 nm, but also emit two (or more) signal quality, troubleshooting for issues such as out-
additional wavelengths at 500 nm and 1400 nm. lined in Table 4.2, or measurement of blood oxygena-
These expanded wavelengths allow for the differ- tion (PaO ) via arterial blood gas analysis could be
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entiation between MetHb, COHb, OxyHb, and considered in this patient to further assess whether or
DeOxyHb. These units are commercially available, not hypoxemia is the cause of these clinical signs.
although not widely used.
More commonly, co-oximetry is performed as
part of arterial blood gas analysis, if the blood gas Case study 2: SpO in a house fire patient
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analyzer has spectrophotometric capabilities. To A 3-year-old female spayed domestic shorthaired
obtain the co-oximetry reading, the blood sample is cat presents after being rescued from a house fire.
hemolyzed and between four and eight different Her vital signs are as follows: temperature 101.0°F
wavelengths of light are projected through the sam- (38.3°C), pulse 220 beats/minute, respiratory rate
ple to measure OxyHb, DeOxyHb, COHb, and 60 breaths/minute, mucous membranes pink, capil-
MetHb. The analyzer will typically display each as lary refill time <2 seconds. She is obtunded and
a % of the total Hb measured and calculate the recumbent with increased respiratory effort. She
percent oxygen saturation (SaO ). has no obvious external burns but smells strongly
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of smoke. Her pulse-ox reads 98% with a strong
signal matching her heart rate.
4.6 Case Studies Does this patient require supplemental oxygen?
Given her history of recently being in a house
Case study 1: Do you trust this SpO reading?
2 fire, carbon monoxide, cyanide toxicity (commonly
An 11-year-old female, spayed mixed breed dog released from burning materials), and smoke inha-
presents for increased respiratory rate and effort. lation are all possible concerns. While primary lung
On physical examination she is quiet, alert and damage caused by smoke inhalation would nor-
responsive, and is ambulatory with increased bron- mally lower the SpO , this may be masked by arti-
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chovesicular sounds bilaterally. Her vitals include a ficial elevation of the SpO caused by COHb or
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temperature of 101.3°F (38.5°C), a heart rate of cyanide toxicosis.
190 beats/minute and a respiratory rate of 70 You perform a standard arterial blood gas and
breaths/minute. Her mucous membranes are pink find a pH of 7.3, a PaO of 90 mmHg, PaCO of
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−
and moist. Your assistant reports that they per- 32 mmHg, bicarbonate (HCO ) of 15.2 mEq/L,
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formed a pulse oximetry measurement on your base excess −9.9, and a SaO of 96%. Lactate is
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patient which revealed a SpO of 45% and a heart moderately elevated at 4.0 mg/dL.
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rate of 180 beats/minute. Does this patient require supplemental oxygen?
Do you believe this value? What would you do to The normal PaO tells you that the lungs are func-
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troubleshoot to assess the accuracy of this value? tioning well and severe compromise from smoke
This is a significantly low SpO reading, indicat- inhalation is unlikely at this point in time (but should
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ing very severe hypoxia barely compatible with life. be reassessed over time). However, if this analyzer is
This patient, while exhibiting signs consistent with not performing co-oximetry, it cannot differentiate
respiratory distress (increased lung sounds and res- OxyHb from COHb, so the SaO given by the ana-
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piratory rate), is conscious and ambulatory which lyzer may be confounded just like the pulse oximeter.
82 K.A. Marshall and A.C. Brooks