Pulse Oximetry and Oxygenation
Introduction
Pulse oximeters are a widely available piece of anaesthesia patient monitoring equipment used in veterinary practices. They are inexpensive, easy to use, and provide a non-invasive method to gain real-time information on oxygen transport, accompanied by an audible pulse rate. The use of pulse oximetry has been shown to reduce the risk of anaesthesia related deaths (Brodbelt et al., 2008).
Oxygen Transport
Oxygen is transported from the lungs into the blood where it is released into the tissue for cellular metabolism. Oxygen diffuses into the plasma and then binds to the protein haemoglobin found in red blood cells, however, a very small amount of oxygen remains dissolved in plasma.
Each red blood cell carries millions of haemoglobin molecules; when the four subunits of the haemoglobin molecule are each carrying an oxygen molecule, the haemoglobin become ‘saturated’ and is referred to as “oxyhaemoglobin”, conversely when haemoglobin is not carrying oxygen it is referred to as “deoxyhaemoglobin”.
The pulse oximeter uses a light absorption technique to determine how much of the haemoglobin in arterial blood is saturated with oxygen (SpO2).
The Pulse Oximeter Probe
The most common probe used is the transmission probe; this clip design holds a thin piece of tissue between it and emits an infrared light and a red light from one side of the clip to a receiver on the other side.
Placement of the probe should be on pigment free tissue where contact can be made: mucous membranes like the tongue and lip are commonly used, however the prepuce, vulva, ear or webbing between the toes can be used too. If the patient has dark or thick fur, readings may be very difficult to obtain or they may be inaccurate.
The probe selectively reads the (pulsatile) arterial blood flow of the tissue; oxyhaemoglobin absorbs more infrared light and deoxyhaemoglobin absorbs more red light at different wavelengths. The pulse oximeter then gives a reading based on the difference between these two absorbances. For example, if an SpO2 reading is 97%, it means that 97% of the red blood cells that were read by the probe had haemoglobin molecules that were fully saturated with oxygen, and 3% were not.
What is Pulse Oximetry telling us?
Pulse Oximetry provides information on oxygen transport; its relationship to the Oxyhaemoglobin Dissociation Curve and the Partial Pressure of Arterial Oxygen (PaO2) is used to presume there are normal levels of oxygen in the blood for cellular metabolism in the tissues.
In a healthy patient breathing room air, a normal PaO2 is 80-100mmHg which correlates to an SpO2 of 95%-99%. The PaO2 measures the amount of oxygen that is dissolved in plasma after the red blood cells are fully saturated. PaO2 is what drives the diffusion of oxygen into the tissues, so think of SpO2 as the oxygen reservoir until it is released into the plasma.
Hypoxemia is defined as a PaO2 is less than 60mmHg which correspond to an SpO2 <90% (Schauvliege, 2016), and may occur during anaesthesia for the following reasons:
- A low Fraction of Inspired Oxygen (FiO2) is being delivered to the patient due to an insufficient fresh gas flow supply, a breathing system that is not correctly assembled, an exhausted oxygen supply, the use of nitrous oxide or a kinked/long endotracheal tube.
- The patient may hypoventilate under anaesthesia due to the respiratory depression caused by the anaesthesia drugs administered. Assessing and potentially decreasing anaesthetic depth should be the first consideration, changing patient positioning or supplementing breaths through Positive Pressure Ventilation should also be considered.
- Pulmonary disease can cause ventilation/perfusion (V/Q) mismatch, atelectasis or right-to-left intracardiac shunting as seen with Patent Ductus Arteriosus.
Reliability and Limitations
When validating an SpO2 reading, the plethysmograph should have a strong waveform which mimics an arterial blood pressure trace and it should also match the heart rate. The upward wave represents systole (the heart contracting) and the downwards wave represents diastole (when the heart is filling before the next beat). A dicrotic notch can be seen on the downwards wave when the aortic valve closes and the distended aorta contracts, creating a brief change in pressure into the arterial circulation. Both the waveform and dicrotic notch can be interpreted alongside other parameters to understand the patient's cardiovascular status:
Pulse oximetry provides a reading based on the red blood cells present as they pass the probe sensors. A limitation in SpO2 readings occurs in anaemic animals where there are fewer red blood cells but they are fully saturated giving a normal SpO2 , but there is still a reduced oxygen carrying capacity overall because there is less haemoglobin. Haemoglobin is estimated as ⅓ of the packed red blood cell volume (haematocrit) and is normally 12.1-20.3g/dL in dogs, and 9.3-15.9g/dL in cats (Klaassen, 1999).
Even if both the SpO2 and PaO2 are normal, it does not always indicate that the Oxygen Content of Arterial Blood (CaO2) is normal. To fully assess the oxygen carrying ability of the patient, the haemoglobin levels must be measured. A normal CaO2 in dogs has been reported between 14.2-17.8mL/dL (Haskins et al., 2005) and can be calculated as follows:
CaO2 (mL/dL) = (1.36 × [Hb] × % saturation) + (PaO2 × 0.003)
To estimate the PaO2 , the FiO2 is multiplied by five; e.g. room air is 21% oxygen and when multiplied by five, the expected PaO2 is 80-100mmHg. In a patient under general anaesthesia breathing 100% oxygen, the PaO2 should be 400-500mmHg. A limitation of pulse oximetry when the patient is breathing an oxygen rich concentration (high levels of PaO2 are achieved), is that the SpO2 will read 99-100%. Therefore, the PaO2 must fall to a dangerously low level for there to be noticeable changes in the SpO2 reading in this instance. A small change in the SpO2 percentage actually reflects a huge change in PaO2. Desaturation (SpO2 <90%) can occur within 60 seconds in dogs breathing room air, compared to 300 seconds in those that were preoxygenated for 3 minutes prior to anaesthesia induction (McNally, Robertson and Pablo, 2009) despite a PaO2 that would have been rapidly decreasing.
Two other forms of haemoglobin exist but they do not carry oxygen;
- Carboxyhemoglobin - occurs when a patient is exposed to carbon monoxide (e.g. house fire) which provides falsely high readings.
- Methemoglobin - occurs in paracetamol toxicity which lowers the reading towards 80-85%.
As mentioned previously, pulse oximetry provides information on oxygen transport, not oxygen delivery to the tissue. Oxygen delivery is determined by cardiac output and haematocrit (available haemoglobin). Additionally, it does not provide information on ventilation, and a hypoventilating patient under anaesthesia breathing 100% oxygen can have a normal SpO2 but a dangerously high level of CO2.
Troubleshooting abnormal readings
Although there are few limitations with pulse oximetry use, inaccurate readings can be given. Readings can be erroneously provided, and they are usually artificially low rather than artificially high (Haskins, 2015).
- Alpha-2 agonists (e.g. medetomidine) cause peripheral vasoconstriction resulting in limited pulsatile blood flow available to obtain a reading. It usually occurs in the early stages of premedication and will eventually wane.
- Vasoconstriction can also occur in hypothermic patients, and actively warming the patient can improve blood flow to the periphery.
- Hypotension can reduce blood flow to the capillary beds and readings may be difficult to obtain. Measures to correct the blood pressure should be taken.
- Patient movement makes it difficult for the pulse oximetry probe to sense arterial blood flow, and a reading may not be provided at all.
- Ambient room light, especially fluorescent can reduce the ability of the two probes to differentiate oxyhaemoglobin from deoxyhemoglobin. Covering the probe with a drape or blanket helps to block out the room light.
- The pulse oximetry probe must make contact with the tissue, and the use of a wet swab placed between the probe and the tissue keeps it moist and encourages contact. Some probes may also have a tight clip and compress the blood from the capillary bed, providing an underestimation of true oxygen saturation - these may need to be wedged open (a clear syringe cap helps).
Using Pulse Oximetry in the Pre and Post Anaesthetic Period
Pulse oximetry is widely used in the peri-anaesthetic period, but it can also be used in the pre- and post-anaesthesia period.
A pre-anaesthesia reading can help guide the expected SpO2 reading in the recovery period. For example, brachycephalic breeds are often chronically hypoxic and a “normal” reading prior to anaesthesia can manage expectations post operatively.
In the anaesthesia recovery period, hypoventilation still occurs due to residual sedative effects and when coupled with breathing room air (21% oxygen), can cause hypoxemia. If the patient is experiencing a delayed recovery, consider taking an SpO2 reading and providing supplemental oxygen.
Conclusion
While pulse oximetry monitoring is very prevalent and useful in practice, interpretation against its limitations is important.
Courtney is originally from New Zealand where she trained and qualified, and has been working as a Veterinary Nurse since 2007. After working in a number of small animal clinics there, an anaesthesia passion took her to a large referral hospital in Australia in 2015. In 2016 she came to the UK and is now an Anaesthesia RVN at the Royal Veterinary College.
Courtney completed her Nurses Certificate in Anaesthesia in 2017 and throughout her studies, she started Veterinary Anursethesia on various social media platforms to share anaesthesia tips. She has written a number of articles for journals and speaks at congresses for Student Veterinary Nurses and Registered Veterinary Nurses on anaesthesia.
Originally published: Thursday, 9th January 2020
References
Brodbelt, D., Blissitt, K., Hammond, R., Neath, P., Young, L., Pfeiffer, D. and Wood, J. (2008). The risk of death: the Confidential Enquiry into Perioperative Small Animal Fatalities. Veterinary Anaesthesia and Analgesia, 35(5), pp.365-373.
Haskins, S., Pascoe, P., Ilkiw, J., Fudge, J., Hopper, K. and Aldrich, J. (2005). Reference cardiopulmonary values in normal dogs. Comparative Medicine, 55(2)
Haskins, S. (2015). Monitoring Anesthetized Patients. In: K. Grimm, L. Lamont, W. Tranquilli, S. Greene and S. Robertson, ed., Veterinary Anesthesia and Analgesia, 5th ed. John Wiley & Sons, p.104.
Klaassen, J. (1999). Reference Values in Veterinary Medicine. Laboratory Medicine , 30(3), pp.194-197.
McNally, E., Robertson, S. and Pablo, L. (2009). Comparison of time to desaturation between preoxygenated and nonpreoxygenated dogs following sedation with acepromazine maleate and morphine and induction of anesthesia with propofol. American Journal of Veterinary Research, 70(11), pp.1333-1338
Schauvliege, S. (2016). Patient Monitoring and Monitoring Equipment. In: T. Duke-Novakovski, M. de Vries and C. Seymour, ed., BSAVA Manual of Canine and Feline Anaesthesia and Analgesia, 3rd ed. Gloucester: John Wiley & Sons, p.83
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Sevoflurane requirement in dogs premedicated with medetomidine and butorphanol
Little information is available about the effect that different doses of medetomidine and butorphanol may have when using sevoflurane for maintenance of anaesthesia in dogs. This recent study evaluates heart rate and median sevoflurane concentration required at different dose rates.
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Capnography II - What happened to the elephants? A summary of abnormal traces
In this second article of the capnography series, James provides a guide to a few of the most common traces that you will encounter during surgery. Scroll to the end of the article to download a printable capnography cheatsheet.
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Pain, what a Pain! (Part 2) – Practical Tips On How To Perform Dental Nerve Blocks In Companion Animal Practice
In this second article of the Pain, what a Pain! series, Dan takes us through the LRA techniques associated with dental and oral surgery. In this article, you will find practical tips and pictures on common dental nerve blocks as well as safety concerns to consider.
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Peri-anaesthetic mortality and nonfatal gastrointestinal complications in pet rabbits
This recent retrospective study looks at the cases of 185 pet rabbits admitted for sedation or general anaesthetic and evaluates the incidence and risk factors contributing to peri-anaesthetic mortality and gastrointestinal complications.
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Pain, what a Pain! How Locoregional Anaesthesia can Improve the Outcome and Welfare of Veterinary Patients (Part 1)
In this first article out of a series of two, Dan takes us through an introduction and practical tips for appropriate local anaesthesia delivery. Find out why these anaesthesia techniques, that are well recognised in human medicine, have seen an increase in popularity in veterinary medicine over the recent years
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Perspectives on Premeds – Opioids
Perspectives on Premeds is a series of articles touching on different pharmacological, physiological and clinical aspects of pre-anaesthetic medication. This second article aims to provide a refresher on opioids.
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Effects of Dexmedetomidine with Different Opioid Combinations in Dogs
Read the highlights of a recently published research paper that evaluates cardiorespiratory, sedative and antinociceptive effects of dexmedetomidine alone and in combination with morphine, methadone, meperidine, butorphanol, nalbuphine and tramadol.
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Preoxygenation Study Highlights
This study evaluates the effectiveness of two methods of preoxygenation in healthy yet sedated dogs and the impact of these methods on time taken to reach a predetermined haemoglobin desaturation point (haemoglobin saturation (SpO2) of 90%) during an experimentally induced period of apnoea.
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Capnography – Not Just a Load of Hot Air
Capnography is the measurement of inhaled and exhaled carbon dioxide (CO2) concentration. The graphical illustration of CO2 within respired gases versus times is known as the capnogram.
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Perspectives on Premeds – Alpha-2 Agonists
Perspectives on Premeds is a series of articles touching on different pharmacological, physiological and clinical aspects of pre-anaesthetic medication. This first article aims to provide a refresher on α2 agonists.
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Alfaxan - now licensed for use in pet rabbits
Jurox Animal Health is delighted to announce that Alfaxan is now licensed for cats, dogs and pet rabbits. This is an exciting advance and could change the way rabbits are anaesthetised in the U.K.
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