Page 79 - Basic Monitoring in Canine and Feline Emergency Patients

VetBooks.ir 4 Pulse Oximetry

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Kristen A. MArshAll * And AiMee C. BrooKs 2
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1 University of Tennessee, Knoxville, Tennessee, USA; Purdue University
College of Veterinary Medicine, West Lafayette, Indiana, USA

4.1 Physiology sickle cell anemia. Alpha- and β-subunit mutations
likely exist in veterinary patients, but are poorly
Oxygen (O ) is carried in the blood in two forms: described.
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free gas dissolved in the plasma measured as a par- The ‘heme’ portion of Hb is a porphyrin ring
tial pressure (PO ) and molecules reversibly bound bound centrally to ferrous iron (Fe ; Fig. 4.2). The
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to hemoglobin (Hb). Greater than 98% of the total chemical structure of porphyrins contain many con-
oxygen in the blood is bound to Hb within red jugated double bonds, which allow these compounds
blood cells. The percentage of all available to absorb light within the visible spectrum, giving
Hb-binding sites that are saturated with oxygen in them color. This is why many similar porphyrins such
arterial blood is called the SaO . When a pulse as myoglobin (red) and chlorophyll (green) are vary-
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oximeter is used to estimate this saturation it is ingly colored, as are the breakdown products of
called the SpO . In this chapter, we will be focusing heme (bilirubin; yellow, biliverdin; green, stercobilin;
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on the portion of oxygen that is bound to Hb. For brown). Variation in the conformation of the por-
more information about measurement and inter- phyrin affects the wavelength of visible light that it
pretation of PO , please see Chapter 5. absorbs. For Hb, this creates the visible difference
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Each hemoglobin protein is composed of four between red arterial blood (containing oxygen, por-
protein subunits, with each subunit containing a phyrin, and Fe ) and purple venous blood (contain-
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heme and a globin portion (Fig. 4.1). The globin por- ing Fe and porphyrin only). This variability in light
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tion is made up of chains of amino acids that form absorption between oxygenated Hb (OxyHb) and
spirals (helices), which give the protein its shape. deoxygenated Hb (DeOxyHb) is the basis for pulse
Different types of globins will contain different oximetry and is how the eye can distinguish the pres-
amino acid sequences and lengths and are labeled by ence of DeOxyHb as cyanosis.
Greek letters. A complex of two α chains and two β Cyanosis is defined as a bluish discoloration of
chains is the most common globin configuration in mucous membranes from the presence of an
adult mammals. Each globin chain is attached to a abnormal amount of DeOxyHb. Detection of cya-
heme group that can interact with oxygen. Therefore, nosis is subjective and dependent on variables such
a fully saturated Hb protein is capable of carrying as ambient lighting (harder to detect with fluores-
four oxygen molecules. cent lighting) and the visual acuity of the observer.
Hb exists in several different forms – which In general, it is necessary to have a DeOxyHb
variety is determined by the amino acid composi- concentration of ≥5 g/dL, consistent with a SpO
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tion of the α and β subunits. For example, adult of <85%, to be able to visually detect cyanosis.
human Hb contains two α and two β subunits, Therefore, cyanosis may not be detected in severely
while fetal human Hb contains an α subunit and a anemic animals, especially if they do not have
γ-type β subunit. Abnormalities in the protein more than 5 g/dL of total Hb present in their body.
structure of Hb subunits as a result of genetic These patients would likely die from hypoxemia
abnormalities can cause disease states in humans. before enough DeOxyHb is present for cyanosis to
The most commonly understood example of this is be observed. A general rule of thumb is that

* Corresponding author: kmarsh23@utk.edu

© CAB International, 2020. Basic Monitoring in Canine and Feline Emergency Patients 71
(eds E.J. Thomovsky, P.A. Johnson and A.C. Brooks)