Introduction

Anesthesia can depress cardiovascular and respiratory functions. Therefore, it is extremely important to monitor vital signs in anesthetized patients. Pulse oximeters are one of the most employed monitors during anesthesia of dogs and cats. During the 2000s, the widespread use of pulse oximeters to monitor anesthetized patients was associated with a reduced risk of anesthetic-related deaths in cats.¹

Pulse oximetry provides pulse rate (PR) and arterial blood saturation with oxygen (SpO₂) automatically, non-invasively and continuously. Pulse oximetry monitors are easy to use, even by a non-experienced user. In dogs and cats, the pulse oximeter probe is usually placed on the tongue, but other suitable sites are the lip, interdigital membrane, digit, foreskin and vulva. Once the probe is adequately placed, the readings are displayed within a few seconds. Pulse oximeters are available worldwide and are reasonably priced. Nevertheless, pulse oximeters present limitations and there are some factors that affect their accuracy; these should be taken into consideration when interpreting PR and SpO₂ readings displayed by the monitor.

The Oxyhemoglobin Dissociation Curve

Pulse oximeters do not actually measure arterial hemoglobin oxygen saturation (SaO₂). The latter is measured by co-oximeters using arterial blood samples. Nevertheless, SpO₂ readings displayed by pulse oximeters are accurate estimates of SaO₂, mostly within the range of 80% to 95%.²

The partial pressure of oxygen dissolved in arterial blood plasma has been named PaO₂. The PaO₂ represents the ability of lungs to oxygenate blood. The most important factor influencing PaO₂ is the inspired fraction of oxygen (FiO₂). In healthy dogs and cats, the expected PaO₂ is approximately five times the FiO₂. At sea level, the FiO₂ of room air is 21%. Therefore, PaO₂ values around 100 mm Hg are expected in normal dogs and cats breathing room air. Increasing the FiO₂ will increase the PaO₂, and values of as much as 500 mm Hg can be expected in patients anesthetized with inhalation anesthetics receiving FiO₂ close to 100%.²

There is a direct relationship between PaO₂ and SaO₂ (and consequently SpO₂) such that increasing PaO₂ results in increases in SaO₂ (and SpO₂). However, this relationship is not linear and takes the form of a sigmoid curve: the oxyhemoglobin dissociation curve. The slope of the curve is steep for PaO₂ values until 60 mm Hg^3,4; at this point, SaO₂ is approximately 85% in dogs and cats⁴. At PaO₂ values greater than 60 mm Hg, the slope of the curve is reduced and becomes flat at values in the range of 80 mm Hg, which correlates with SaO₂ values of approximately 93%^3,4. Normal SaO₂ and SpO₂ values in healthy dogs and cats breathing room air (FiO₂ =21%) are approximately 97%.

Clinical Applications

Pulse oximeters are useful in providing a visual, continuous, value for PR. In anesthetized dogs and cats, the anesthesia practitioner can easily place the probe at a suitable site and identify increases or decreases in PR. Therefore, bradycardia and tachycardia can be rapidly identified and treated as required.

Most general and dissociative anesthetics depress ventilation. In dogs and cats breathing room air, respiratory depression by anesthetics can result in hypoxemia. The latter has been defined as PaO₂ <80 mm Hg and severe hypoxemia is considered to exist when PaO₂ <60 mm Hg.² Monitoring of SpO₂ with pulse oximetry can anticipate the diagnosis of hypoxemia in anesthetized patients and indicate the need for oxygen supply. A decrease in SpO₂ <97% may indicate hypoventilation due to anesthetics. A decrease in SpO₂ <93% is suggestive of PaO₂ values <80 mm Hg and decreases in SpO₂ <85% are indicative of severe, life-threatening, hypoxemia (PaO₂ <60 mm Hg). Diagnosis of hypoxemia by a pulse oximeter can be considered much safer than simply observing cyanosis of mucous membranes. Depending upon the hemoglobin concentration of the animal, cyanosis may only be observed at PaO₂ <40 mm Hg, which is undoubtedly life-threatening. In addition, in patients presenting anemia, cyanosis may not be observed even at very low PaO₂ values.²

Limitations of Pulse Oximetry

First, pulse oximetry cannot identify hypoventilation in patients breathing an enriched oxygen mixture. Under these circumstances, patients with a normal lung function are unlikely to be hypoxemic; the PaO₂ will be in the range of 100 to 500 mm Hg and SpO₂ readings will always be close to 100%.² However, hypoventilation may result in hypercapnia and respiratory acidosis that may be unnoticed.

Second, pulse oximetry is also unable to identify mild to moderate degrees of lung dysfunction in patients breathing an enriched oxygen mixture. A dog breathing 100% oxygen should have a PaO₂ of approximately 500 mm Hg.² If this dog has a PaO₂ of 100 to 200 mm Hg, it is likely that some degree of lung dysfunction is present, but the pulse oximeter reading will still be high (≥97%). Therefore, in patients breathing enriched oxygen mixtures, the pulse oximeter reading may only decrease when a severe lung dysfunction is present, leading to PaO₂ <100 mm Hg.

Third, pulse oximeters may provide inaccurate readings due to motion artifact, tissue pigment, tissue thickness, peripheral vasoconstriction and hypovolemia. All these factors may reduce signal strength and quality.^2,5 In clinical practice, before accepting the SpO₂ reading, it is important to evaluate the PR and the plethysmographic curve (provided by most pulse oximeters). For accurate readings, the pulse oximeter should display a clear plethysmographic curve and the PR reading should be correct. If the displayed PR is wrong and/or if it does not display a plethysmographic curve, the SpO₂ reading should be discarded.

Final Considerations

In animals breathing room air, if the SpO₂ reading is 97% or higher and the PR reading is correct, than everything must be all right. Conversely, in animals breathing an enriched oxygen mixture, a 97% reading alone may not be considered as a measure of adequate ventilation and lung function because de pulse oximeter is not a reliable monitor to detect changes in these variables in patients breathing high FiO₂.

References

1.  Brodbelt DC, Pfeiffer DU, Young LE, Wood JL. Risk factors for anaesthetic-related death in cats: results from the confidential enquiry into perioperative small animal fatalities (CEPSAF). Br J Anaesth. 2007;99(5):617–623.

2.  Haskins SC. Monitoring anesthetized patients. In: Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, eds. Veterinary Anesthesia and Analgesia. Ames, IA: Wiley Blackwell; 2015:86–113.

3.  Clerbaux T, Gustin P, Detry B, Cao ML, Frans A. Comparative study of the oxyhaemoglobin dissociation curve of four mammals: man, dog, horse and cattle. Comp Biochem Physiol Comp Physiol. 1993;106(4):687–694.

4.  Cambier C, Wierinckx M, Clerbaux T, Detry B, Liardet MP, Marville V, Frans A, Gustin P. Haemoglobin oxygen affinity and regulating factors of the blood oxygen transport in canine and feline blood. Res Vet Sci. 2004;77(1):83–88.

5.  McMorrow RC, Mythen MG. Pulse oximetry. Curr Opin Crit Care. 2006;12(3):269–271.