An Ode to the Pulse Oximeter

Dr. Fissekis discusses the trials and tribulations of the pulse oximeter! She completed her DVM degree at Cornell University College of Veterinary Medicine in 2011 and passed her ACVAA board exam in 2017. Before her anesthesiology residency at Kansas State University, Dr. Fissekis practiced as a primary care veterinarian and emergency veterinarian in Massachusetts.

Stephanie Fissekis DVM DACVAA

Oh, pulse oximeter, you marvelous, misunderstood piece of equipment! How I love thee, let me count the ways! You are the only piece of equipment that has been documented to reduce the likelihood of death in cats and dogs.(1)

You can be used anywhere there is a pulse on any species of patient. Awake, sedate, obtunded, or anesthetized, we can use you. You are non-invasive, easily applied, provide continuous measurements, and provide an audible signal of pulse. Most models of you are even cheap! You can alert us to potentially fatal issues with oxygenation in our anesthetized patients and even give us information about perfusion and cardiac rhythms.

Yet, you are cursed and maligned; we hate you and ignore you at times. Why, gentle pulse oximeter? Your function relies on pulsatile flow, which sadly, can be finicky. With vasoconstriction, pigment, motion, and ambient light interference, getting a reliable and consistent reading can be frustrating.

A Masimo pulse oximeter. This brand also has a reflectance probe which works quite well in conscious animals if placed on the shaved tail base.

And dare we start commenting on using you in cats? The alpha-2 agonists, the large surface area-to-volume ratio, the tiny arteries, the rapid heart rates… cats conspire to give you a bad name. Lest we forget, so many of your probes have springs in the hinges, compressing tissues and dampening the pulsatile flow over time. Conspiracy! You may try to lie to us, but we know that if we palpate a pulse and confirm that it matches your displayed plethysmograph pulse rate, you are likely producing a number that is true (even if we don’t like it). If the numbers don’t match, the saturation you display is likely inaccurate (even if we do like it). At least the perfusion index helps tell us the strength of the pulse you are detecting. Vexing and infuriating, nonetheless.

We don’t know if a patient is at risk if we don’t look. For any patient with altered consciousness – chemically or otherwise – we can (and should!) use you. The oxygen-hemoglobin saturation curve shows us that as oxygen saturation slides below 95%, arterial oxygen partial pressure is decreasing rapidly and risk of hypoxemia increases.

If a patient can saturate >95% consistently and consistently on room air, that patient is likely to live. If a patient’s saturation is less than 95%, we can provide supplemental oxygen. So easy! So effective! So underappreciated! There’s math to prove it! (Dear readers, repeat after me: “The arterial oxygen content and alveolar gas equations are my friends. Math and physiology inform appropriate medical practices. I will be a good anesthetist!!”)

The oxygen-hemoglobin dissociation curve. Note that on 100% O2, the arterial O2 content should be >500 mmHg & the pulse oximeter should read close to 100%. When a patient is breathing room air, the arterial O2 content is much lower (~100 mmHg) but the pulse oximeter still should read close to 100%. If pulse oximetry values below 95% are obtained, immediately check your patient before dismissing the values. A pulse oximeter is a late but serious indicator of a major respiratory issue.

What’s more, oh pulse oximeter, you can be used to anticipate problems. The coughing patient? The one with exercise intolerance? The one that has exposure to a smoking environment – either acutely or chronically? Increased respiratory rate? Increased respiratory effort? Prolonged expiratory time? We are free to include you in our physical exam for each of these patients. You give us an objective reason that we can provide to recalcitrant owners for why we would like to take thoracic radiographs or perform a complete blood count or arterial blood gas. You even help us adjust our goals when recovering a patient with reduced oxygen saturation as a “normal” part of the patient’s life. So many applications! So little effort needed!!

Pulse oximeter, you are wonderful. I could wax poetically all day. Though you may be temperamental, and we may be impatient, irascible, and ungrateful, know you are important and valued. You save lives. We know what makes you cranky, and we promise to assess for and correct factors that impair your ability to work well. You should be in every room – imaging, induction, surgery, and treatment.

All the patients, all the time! Pulse oximeters for everyone!!

1 Brodbelt DC, Blissitt KJ, Hammond RA et al. The risk of death: the confidential enquiry into perioperative small animal fatalities. Veterinary Anaesthesia and Analgesia 2008; 35: 365-373

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