Skewed Sats: Limitations of Pulse Oximetry

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Steve Haywood
Former Respiratory Therapist working to de-mystify the ventilator through hands-on workshops and online education. Additional interests include HFNC in CHF and COPD, Medical Education, and Tupperware.

The Pre-brief

Pulse oximetry has proven to be a cheap, non-invasive, continuous way to monitor a patient’s oxygenation status. However, if the pulse ox is not an accurate reflection of the patient’s oxygenation status, the tool becomes worthless or even harmful. Here are some instances when your pulse ox reading may not be a true reflection of the patient’s oxygenation status.

Dark Skin

Historically, pulse oximetry has been widely studied in white patients and that data has been applied to black patients. For decades no one asked if darker skin pigmentations could contribute to inaccurate pulse oximetry values. When researchers finally studied the effect of darker skin pigmentations on pulse oximetry, it was discovered that black patients were 3 times as likely to suffer from occult hypoxemia when compared to white patients.

Hemoglobinopathies

Abnormal hemoglobin can also give falsely elevated readings on a pulse oximeter. Always consider toxicologic exposures that may have altered the hemoglobin. A low threshold should be held to check a carboxyhemoglobin level in patients with any history even slightly suspicious for CO poisoning. Some common CO exposures include running automobiles in confined spaces such as garages or tunnels. During winter months, heaters or generators inappropriately used indoors are another common source of CO poisoning. Obviously, fire or smoke exposure should warrant a COOX. 

Methemoglobinemia occurs when the ionic charge of the iron molecule in hemoglobin changes making it unable to bind oxygen. This change can be caused by nitrates, inhaled anesthetics, certain antibiotics, and some drugs of abuse. Patients will present with a pulse oximeter reading around 85% +-3%. 

There are some pulse oximeters in some facilities that utilize additional wavelengths of light to calculate carboxyhemoglobin and methemoglobin. These oximeters are expensive compared to traditional pulse oximeters therefore they are not widely available.

Tissue Hypoperfusion

A pulse-oximeter only measures the amount of oxygenated hemoglobin in arterial blood. It accomplishes this incredible feat by measuring the arterial pulsation and using this pulsation to identify arterial feedback versus venous and capillary feedback. (Physics nerds read more HERE).

In a patient with hypoperfusion, the arterial pulsations are significantly less profound in the fingers. With less arterial pulsation, there is a greater chance that the oximeter will be less accurate in identifying arterial blood and may be measuring a mixed sample.

An inaccurate pulse oximeter can also be an early indicator that your patient has hypoperfusion of their distal extremities. This finding may indicate that your patient is decompensating.

Confirming good pulsations on the pulse oximeter plethysmograph will increase the likelihood of an accurate reading.

Profound Hypoxemia

When looking at the oxyhemoglobin dissociation curve (Image below), there is a broad range of PaO2 levels associated with hemoglobin saturations of 80% and greater (blue shaded area on the curve). However, at saturation levels below 80% , there is a drastic drop in hemoglobin saturations with small drops in the PaO2 (Orange shaded area on the curve). Because of the volatility of hemoglobin saturation when saturation levels are below 80%, the accuracy of noninvasive pulse oximetry becomes less reliable. Interestingly, it seems that pulse oximeters under estimate hemoglobin saturation levels in the setting of profound hypoxemia.

The Debrief

  • Pulse oximetry allows us to monitor oxygenation levels without repeated blood gas sampling. However, we must know when we can and cannot trust a pulse oximeter. 
  • Patients with dark skin are more likely to have occult hypoxemia when monitored by pulse oximetry.
  • Patients with hemoglobinopathies such as carbon monoxide poisoning or methemoglobinemia will have an inaccurate pulse oximeter reading. 
  • When distal extremities are hypoperfused, the pulse oximeter may be incorrect. Always confirm a good waveform on your monitor. 
  • At hemoglobin saturations below 80%, the pulse oximeter may be underestimating the SaO2. 
  • When in doubt, check an ABG!

References

  1. Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial Bias in Pulse Oximetry Measurement. N Engl J Med. 2020 Dec 17;383(25):2477-2478. doi: 10.1056/NEJMc2029240. PMID: 33326721; PMCID: PMC7808260.
  2. Urner M, Calfee CS, Fan E. Titrating Oxygen Therapy in Critically Ill Patients. JAMA. 2021 Aug 31. doi: 10.1001/jama.2021.9843. Epub ahead of print. PMID: 34463699.
  3. Chan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respir Med. 2013 Jun;107(6):789-99. doi: 10.1016/j.rmed.2013.02.004. Epub 2013 Mar 13. PMID: 23490227.
  4.  Severinghaus JW, Naifeh KH, Koh SO. Errors in 14 pulse oximeters during profound hypoxia. J Clin Monit. 1989 Apr;5(2):72-81. doi: 10.1007/BF01617877. PMID: 2723709.

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