Page 28 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

Chapter 2 · Emergency management of respiratory distress

pulse oximeter, therefore, unless it is reading an S aO 2 in Para eter Dog Cat
the high 90s, it is impossible to know whether P aO 2 is at a pH 7.36–7.42 7.24–7.40
VetBooks.ir erial blood pressure. As long as the pulse pressure is >20 pO 2 (mmHg) 85–95 84–96
safe level or whether the animal is about to desaturate.
Pulse oximeters do not provide any information on art-
29–42
pCO 2 (mmHg)
29–42
mmHg, pulse oximeters are able to determine a pulse rate.
–
It should also be noted that pulse oximeters amplify the [HCO 3 ] (mmol/l) 17–24 17–24
signal they receive; therefore, the flashing lights vary with S aO 2 (%) 97–100 97–100
the signal received and not with arterial blood pressure. TCO 2 (mmol/l) 19–26 19–26
Transmission probes can be placed over unpigmented Hb (g/l) 120–180 90–150
hairless skin or mucous membranes. Useful sites include Normal reference values for arterial blood gas analysis when
the tongue, toe web, ear pinnae, vulva and prepuce. 2.2 breathing room air (F iO 2 = 0.21 at sea level). F iO 2 = fractional
Probes designed to wrap around a toenail are very useful, inspired oxygen concentration; Hb = haemoglobin; pCO 2 = partial
as they are much less susceptible to movement artefact. pressure of carbon dioxide dissolved in plasma; pO 2 = partial pressure of
Reflection probes (which have the photodetector next to oxygen dissolved in plasma; S aO 2 = arterial haemoglobin oxygen
the light-emitting diodes) do not need two skin or mucous saturation; TCO 2 = total carbon dioxide.
membrane surfaces. Therefore, they can be placed at sites
Knowing the P aO 2 and P AO 2 allows calculation of the
such as in the rectum. However, this type of probe has alveolar–arterial oxygen difference, or P (A–a)O 2. This is use-
been associated with problems of inaccuracy. Movement,
ful because, in conjunction with other clinical information,
such as shivering, also greatly reduces accuracy, although it allows the cause of the hypoxia to be determined further
newer probes can overcome this.
(Figure 2.3). Normal P (A–a)O 2 should be <20 mmHg on room
Carboxyhaemoglobin (COHb) produces a falsely high
reading of S aO 2, because pulse oximeters measure COHb air. The value increases as F iO 2 increases. As a very rough
guide, P aO 2 (mmHg) should be approximately 4–5 times
as fully oxygenated haemoglobin. Therefore, pulse oximetry the inspired O 2 percentage.
cannot be used to monitor S aO 2 in animals that may have
been exposed to carbon monoxide. Methaemo globinaemia
can cause pulse oximeters to indicate low oxygen saturation. Causes of low P aO 2 Effect on P (A–a)O 2
Tissues such as cats’ tongues or ear pinnae may be too Cardiac/pulmonary right-to-left shunt Increased
thin for the probes (designed for human paediatric fingers) Decreased F iO 2 (e.g. altitude, gas supply failure) Normal
to measure. This can be compensated for by placing a
folded paper towel around the tissue between the probes. Ventilation–perfusion imbalance (e.g. pulmonary Increased
thromboembolism, general anaesthesia)
Hypoventilation (e.g. CNS trauma/disease, Normal
Blood gas analysis neuromuscular disease)
If blood gas analysis is to be performed for assessment iffusion impairment e.g. pulmonary fibrosis Increased
of respiratory disorders, an anaerobically drawn arterial congestive heart failure)
sample is essential. This may be taken from the meta tarsal, 2.3 Causes of low arterial oxygen partial pressure (P aO 2) and the
radial, auricular or femoral artery. Arterial stabs are painful effect on alveolar arterial o ygen difference (A–a)O 2). CNS =
and excessive bleeding is possible; the procedure is con- central nervous system; F iO 2 = fractional inspired oxygen concentration.
traindicated in patients with evidence of bleeding dis-
orders. Rapid (within 2 minutes) analysis is vital to ensure Oxygen saturation and oxygen content
accurate results; if a delay is inevitable, the sample should
be stored on ice. Ninety-eight percent of all oxygen in the blood is bound to
Venous blood can be used for acid–base analysis, but it haemoglobin; this does not exert a pressure at the oxygen
should be remembered that pH will be slightly lower and electrode. Neither S aO 2 nor P aO 2 indicates the oxygen con-
pCO 2 (partial pressure of carbon dioxide dissolved in tent of arterial blood (C aO 2).
plasma) slightly higher as long as perfusion is adequate.
The pO 2 (partial pressure of oxygen dissolved in plasma) C aO 2 = ([Hb] x 1.34 x S aO 2) + (0.003 x P aO 2)
may also give valuable clues to the patient’s underlying
disease process. Reference values are shown in Figure 2.2. (Note: 1.34 ml O 2 can bind to 1 g of haemoglobin; 0.003
The partial pressure of oxygen in arterial blood (P aO 2) is mEq/l/mmHg is the solubility coefficient of oxygen in
essentially an indicator of the adequacy and efficacy of plasma.) It is generally accepted that 50 g/l of deoxygen-
gas exchange in the lung. It is not an indicator of tissue ated haemoglobin is required to produce clinically evident
oxygen delivery, nor of blood oxygen content. P aO 2 repre- cyanosis. It is also possible for P aO 2 to be normal and C aO 2
sents the oxygen physically dissolved in the plasma; this is to be grossly abnormal, as in severe anaemia or in the
generally a little less than 2% of the total oxygen content presence of abnormal haemoglobins.
of the blood. P aO 2 can only really be evaluated relative to
P AO 2 (partial pressure of oxygen in the alveolus), which can
be calculated from the alveolar gas equation:
Diseases associated with
P AO 2 = F iO 2 (PB – 47) – 1.2 (P aCO 2)
respiratory distress
Where F iO 2 = the fractional inspired oxygen concentra-
tion (if the patient is on room air (21% O 2), F iO 2 = 0.21) and Diseases of the conducting airway
PB = atmospheric pressure (usually 760 mmHg). The Diseases of the conducting airway (pharynx, larynx or
equation assumes that the patient’s respiratory quotient trachea) causing respiratory distress are associated with
(RQ) is 0.8 and that P ACO 2 = P aCO 2 (for the vast majority of loud respiratory sounds heard without the stethoscope.
cases this is true). Problems in these areas include oedema, infection, foreign

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