Pulse Oximetry Detects Change in Absorption of Light by Oxyhemoglobin (OxyHb) and Deoxyhemoglobin (DeoxyHb) in Red (660 nm) and Infrared (940 nm) Regions of the Spectrum
Beer-Lambert Law
States that the absorption of light of a given wavelength passing through a non-absorbing solvent (which contains an absorbing solute) is proportional to the product of the solute concentration, the length of the light path, and an extinction coefficient
The Percentage of OxyHb Can Be Determined by the Passage of Light at a Known Wavelength Through Arterial Blood
95% Confidence Limit for Pulse Oximetry
95% Confidence Limit is +/- 4% at SpO2 >70% (US FDA Communication February 19, 2021) [MEDLINE]
Confidence Limit is Wider at SpO2 <70%
Measured SpO2 of 95%: reflects a pO2 of 60-160 mm Hg (SpO2 of 91-99%)
Measured SpO2 of 85%: reflects a pO2 of 46-56 mm Hg (SpO2 of 81-89%)
Minimum Mean Accuracy (US FDA Communication February 19, 2021) [MEDLINE]
FDA Only Reviews the Accuracy of Prescription Use Oximeters, Not Over-the-Counter Oximeters Meant for General wellness or Sporting/Aviation Purposes
Testing Compares the Pulse Oximeter Saturation Readings to Arterial Blood Gas Saturation Readings for Values Between 70-100%
The Typical Accuracy (Reported as Accuracy Root Mean Square or Arms) of Recently FDA-Cleared Pulse Oximeters is within 2-3% of Arterial Blood Gas Values
This Means that About 66% of SpO2 Values were within 2-3% of Arterial Blood Gas Values and About 95% of SpO2 values were within 4-6% of Arterial Blood Gas Values, Respectively
Advantages of Pulse Oximetry
Decreases the Need for Arterial Blood Gas (ABG) Testing
May be especially useful during procedures that might affect oxygenation (bronchoscopy, intubation, dialysis, suctioning, Swan-Ganz placement, etc.) or during titration of O2 in ventilated patients
Inexpensive (After the Initial Purchase of the Device)
Easily and rapidly performed in critically ill patients
An accurate SpO2 value is critical in order to optimally titrate oxygen delivery to patients and to follow oxygenation guidelines
Limited prospective data exist on real world performance of pulse oximeters in critically ill patients
*The objective of this study was to assess accuracy and bias of the SpO2 values measured by several oximeters in hospitalized patients.We included stable adults in the intensive care unit with an arterial catheter in place
Main exclusion criteria were poor SpO2 signal, and SpO2 >96%
In each subject, we simultaneously evaluated four oximeters: Nonin (Plymouth, MN) embedded in the FreeO2 device (Oxynov, QC, Canada), Masimo (Radical 7, Irvine, CA), Philips (FAST, Eindhoven, Netherlands), and Nellcor (N600, Pleasanton, CA)
Arterial blood gases were drawn and simultaneously, each oximeters’ SpO2 values were collected
SpO2 values were compared to the reference (SaO2 value) to determine bias and accuracy
The ability for oximeters to detect hypoxemia and the impact of oximeters on oxygen titration were evaluated
We included 193 subjects (153 men, mean age 66.3 years) in whom 211 sets of measurements were performed
The skin pigmentation evaluated by Fitzpatrick scale showed 96.2% of subjects were light skin (types 1 and 2)
One oximeter overestimated SaO2 (Philips, +0·9%) while the three others underestimated SaO2 (Nonin -3·1%, Nellcor -0·3%, Masimo -0·2%)
SaO2 was underestimated with Nonin oximeter in 91·3% of the cases while it was overestimated in 55·2% of the cases with Philips oximeter
Moderate hypoxemia (SaO2 86-90% or PaO2 55-60 mmHg) was detected in 92%, 33%, 42% and 11% of the cases with Nonin, Nellcor, Masimo and Philips respectively
We found significant bias and moderate accuracy between the tested oximeters and the arterial blood gases, in the studied population
These discrepancies may have important clinical impact on the detection of hypoxemia and management of oxygen therapy
Pulse Oximetry Does Not Measure pCO2 or pH and Gives No Information About Ventilation
During Acute Hypoventilation, the pCO2 Can Increase Significantly Before Desaturation Occurs on the SpO2
This is especially true if the patient is receiving supplemental oxygen (in this case, the pCO2 can continue to rise while the SpO2 is artificially “supported” by the supplemental oxygen)
Pulse Oximetry Measurements are Signal-Averaged Over Several Seconds
Therefore, the pulse oximeter may not detect a hypoxemic event until several seconds after it has occurred
This may be particularly important when the pulse oximeter is being used to monitor SpO2 during intubation
Pulse Oximetry Accuracy is Limited to SpO2 >70%
Pulse Oximeter Accuracy is Highest with SpO2 90-100%, Intermediate with SpO2 80-90%, and Lowest with SpO2 <80% (US FDA Communication February 19, 2021) [MEDLINE]
Pulse Oximetry Accuracy as a Reflection of Arterial pO2 is Impacted by the Sigmoidal Shape of the Oxyhemoglobin Dissociation Curve (see Hypoxemia)
At High Levels of Oxygenation, Pulse Oximetry is Insensitive at Detecting Significant Changes in pO2: since these are being measured in the flat part of the oxyhemoglobin dissociation curve
Example: the pO2 could drop from 150 to 70 mmHg without an appreciable change in the SpO2, as this change occurs over the flat part of the oxyhemoglobin dissociation curve
Example: once hemoglobin is 100% saturated, no further increase in the pO2 will be reflected in the SpO2 -> this makes SpO2 poor at quantifying the degree of hyperoxemia
Pulse Oximetry Prediction of pO2 is Less Accurate When SpO2 is ≤90%
This is Due to the Error Rate of +/-4% of the Pulse Oximeter and the Steep Slope of the Curve in this Region
Example: a large change in SpO2 (for example, from 90-86%) could reflect a relatively small change in pO2, as this change occurs over the steep part of the curve
Pulse Oximetry Does Not Account for Shifts in the Oxyhemoglobin Dissociation Curve
Shifts in the Curve to Right (for Example, by Acidosis or Hypercapnia) or to the Left Can Significantly Influence the Relationship Between SpO2 and Arterial pO2
In Severe Anemia (with Hb <5 g/dL) with SpO2 <80%, SpO2 underestimates the SaO2 (this may be due to increased signal/noise ratio from the surrounding tissue) [MEDLINE]
Pulse Oximetry Does Not Measure Methemoglobin (MetHb) (see Methemoglobinemia)
Arterial Blood Gas Co-Oximetry Oxygen Saturation (SaO2): normal (the difference between the SpO2 and the SaO2 in methemoglobinemia has been termed the “saturation gap”)
Arterial Blood Gas Co-Oximetry: reveals methemoglobinemia
pO2: normal
Impact of Methemoglobinemia on SpO2 Obtained by Pulse Oximeter
When MetHb is <30%: pulse oximetry will overestimate the percentage of OxyHb in presence of MetHb by an amount roughly equal to 50% of the amount of MetHb present
In the Presence of 20% MetHb and a SpO2 of 90%, the Percentage of OxyHb Will Be 80%
When MetHb is >30%: pulse oximetry will plateau at about 85%
Pulse Oximetry Does Not Measure Carboxyhemoglobin (COHb) (see Carboxyhemoglobinemia)
Summary of Expected Findings in Carboxyhemoglobinemia
Pulse Oximeter Oxygen Saturation (SpO2): normal
Arterial Blood Gas Co-Oximetry Oxygen Saturation (SaO2): decreased
Arterial Blood Gas Co-Oximetry: reveals carboxyhemoglobinemia
pO2: normal
Impact of Caroboxyhemoglobinemia on SpO2 Obtained by Pulse Oximeter
Pulse oximetry may produce erroneous measurements in the presence of carboxyhemoglobinemia (since COHb will absorb with similar characteristics to OxyHb)
Pulse oximetry will overestimate the percentage of OxyHb in presence of COHb by an amount roughly equal to the amount of COHb present (ie: in presence of 30% COHb and a SpO2 of 90%, the percentage of OxyHb will be 60%)
Pulse Oximetry May be Affected by Other Abnormal Hemoglobins
Glycosylated Hemoglobin: glycohemoglobin A1c levels >7% in type 2 diabetics with poor glucose control have been shown to result in overestimation of oxygen saturation as assessed by pulse oximetry [MEDLINE]: mechanism probably is due to increased glycohemoglobin A1c affinity for oxygen
Sickle Cell Disease: while sickle hemoglobin usually produces normal pulse oximeter readings, cases of falsely elevated and falsely decreased readings have been reported
Note: fetal hemoglobin gives identical readings to adult hemoglobin
Pulse Oximetry May Be Affected by the State of Perfusion
Hypotension/Hypoperfusion/Vasoconstriction: these have variable effects on the SpO2
Accuracy of pulse oximeters decreases significantly with SBP <80 mmHg, usually giving a falsely low SpO2 reading
Warming the extremity or topical vasodilators (nitropaste or oil of wintergreen) may be useful to increase the signal
Ear or forehead oximetery probes may alleviate this problem
Hypothermia (see Hypothermia, [[Hypothermia]]): hypothermia may interfere with pulse oximetry due to vasoconstriction (especially in the digits)
Ear or forehead oximetery probes may alleviate this problem
Venous Congestion (Due to due to tricuspid regurgitation or congestive heart failure): may yield falsely low SpO2 readings (due to the presence of venous pulsations causing the oximeter to register these as arterial pulsations)
Pulse Oximetry May Provide Erroneous Measurement in Presence of Skin/Nail Discoloration
Skin Pigmentation (African Americans, etc) Has a Variable Effect on SpO2
Difference is typically small at saturations above 80% and greater when saturations are less than 80%
Study of Pulse Oximeters in Ventilator-Dependent Patients (Chest, 1990) [MEDLINE]
Authors Concluded that to maintain satisfactory oxygen saturation as measured by arterial blood gases, target saturations of 92% were adequate in white patients, yet target saturations of 95% were required in black patients
Study of Effects of Skin Pigmentation on Pulse Oximeter Accuracy (Anesthesiology, 2005) [MEDLINE]
Paired oxygen saturations as measured by pulse oximetry and arterial blood gases, in healthy volunteers with hypoxamia created in controlled conditions, varied from 2.3% to 4.3% higher than the true value in black individuals compared to white individuals
Retrospective Study of the Effects of Skin Pigmentation on Pulse Oximetry (NEJM, 2020) [MEDLINE]
Black patients had nearly three times the frequency of occult hypoxemia (low oxygen levels in the blood) as detected by blood gas measurements but not detected by pulse oximetry, when compared to White patients
Study May Have Been Limited by the Fact that it Relied on Previously Collected Health Record Data from Hospital Inpatient Stays and Could Not Statistically Correct for All Potentially Important Confounding Factors
FDA recommends that every clinical study have participants with a range of skin pigmentations, including at least Two Darkly pigmented participants or 15% of the participant pool, whichever is larger (US FDA Communication February 19, 2021) [MEDLINE]
Prospective Study of Pulse Oximetry in Various Ethnic Groups in the Setting of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) (see Severe Acute Respiratory Syndrome Coronavirus-2) (Eur Respir J, 2022)[MEDLINE]
Pulse Oximetry overestimated the oxygen saturations compared to blood gas measurement across all ethnicity groups when arterial blood gas oximetry measured saturations were below 90% (two way ANOVA excluding unknown ethnicity p<0.0001)
Pulse Oximetry underestimated these when arterial blood gas oximetry measured saturations were above 95% (two way ANOVA excluding unknown ethnicity p<0.0001)
These mean differences were particularly marked in the clinically important range when the arterial blood gas demonstrated a true oxygen saturation of 85 to 89%
Individuals with a Black ethnicity had a mean pulse oximetry reading that was +3.9% higher (95% CI: -8.0 to +15.9), those with an Asian ethnicity 5.8% higher (95% CI: -1.6 to -13.2) and individuals with a White ethnicity +2.4% (95% CI: -14.2 to +19.0) higher, when compared to arterial blood gas oxygenation
Similarly, in a mixed effects linear model, individuals with Black, Asian or mixed ethnicity had a higher reading for oxygen saturation as measured by pulse oximetry than blood gas compared to individuals with a White ethnicity (excluding patients without ethnicity recorded) and adjusting for arterial blood gas oxygen saturation (Black +1.8%; 95% CI: +0.2 to +3.4, p = 0.04; Asian +1.9%; 95% CI: +0.6 to + 3.2, p = 0.005; Mixed +3.2%; 95% CI: -0.1 to +6.6, p = 0.06)
A final mixed effects model of the size of the difference between oxygen saturation measured by pulse oximetry compared to arterial blood gases demonstrated that pulse oximetry over-estimated arterial oxygen saturation by a mean +1.4% (95% CI: +0.5 to +2.3, p = 0.003) in patients with Black, Asian or Mixed ethnicity compared to patients with White ethnicity, after adjustment for sex, age and arterial blood oxygen level
Cohort Study of Racial/Ethic Disparities Using Pulse Oximetry to Determine Treatment Eligibility for Severe Acute Respiratory Syndrome Coronavirus-2 (COVID) (JAMA Intern Med, 2022) [MEDLINE]: n = 7,126
Racial/Ethnic Biases in Pulse Oximetry Accuracy were Associated with Greater Occult Hypoxemia in Asian, Black, and Non-Black Hispanic Patients with COVID-19
These Biases in Accuracy were Associated with Significantly Delayed or Unrecognized Eligibility for COVID-19 Therapies Among Black and Hispanic Patients
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Black patients have higher rates of occult hypoxemia compared with White patients, including after adjustment for underlying differences in arterial blood gas (ABG) distributions, according to a study in Critical Care
Impact of racial differences in hypoxia distribution on the measured prevalence of occult hypoxemia by pulse oximeters. Crit Care. Published online July 30, 2023. doi: https://doi.org/10.1016/j.chstcc.2023.100011 [MEDLINE]
Nail Polish: black, green, and blue nail polish appear to falsely decrease SpO2 (although red nail polish does not appear to have an effect)
Later generation pulse oximeters appear to be less affected
Artificial Arcrylic Nails
It is recommended to remove the acrylic nail before using pulse oximetry
Pulse Oximetry May Be Affected by Dyes/Pigments
Bilurubin (Jaundice): falsely decreases SpO2
Predominantly a problem with older ear oximeters
Methylene Blue (see Methylene Blue): falsely decreases SpO2
Used in ophthalmic angiography, cardiac output determination, and hepatic function studies
Fluorescein Dye: falsely decreases SpO2
Used in ophthalmic angiography
Indigo Carmine Dye: falsely decreases SpO2
Used to localize ureteral orifices
Isosulfan Blue Dye: falsely decreases SpO2
Used intraoperatively to mark breast cancers and melanomas
Pulse Oximetry May Be Affected by Magnetic Resonance Imaging (MRI) Scanners and Other Radiofrequency-Emitting Devices
Radiofrequency emissions from MRI scanners (and from cell phones and electrocautery devices) may interfere with pulse oximetry
In addition, 2nd/3rd-degree burns beneath pulse oximeter probes have been reported in patients undergoing MRI scans: due to generation of electrical skin currents beneath the looped pulse oximeter cables, which act as an antenna
Pulse Oximetry May Be Affected by External Light Sources
Intense Daylight/Fluorescent Light/Incandescent Light/Infrared Light/Xenon Light: all have been reported to falsely decrease SpO2
Some Clinical Scenarios Where Discordance May Occur Between Pulse Oximetry Saturation (SpO2) and pO2 from Arterial Blood Gas
Pulse Oximeter Equipment Malfunction: in this case, SpO2 may vary widely from the pO2
Venous Blood Sample Inadvertently Drawn (Instead of Arterial Blood Sample): in this case, pO2 would be lower than one would expect from an arterial blood sample
Arterial Blood Sample Drawn from Ischemic Body Site (Such as an Ischemic Limb): in this case, the arterial pO2 would be lower than the SpO2 would predict (and would be lower than the arterial pO2 obtained at another body site)
Pseudohypoxemia: in cases with very high WBC count, in vitro consumption of oxygen can occur in the arterial blood gas sample during transit (prior to processing) on the blood gas machine -> results in artifactually low pO2, as compared to the SpO2
Methemoglobinemia (see Methemoglobinemia, [[Methemoglobinemia]]): in this case, SpO2 will give an abnormally low reading with a normal pO2
References
Principles
Pulse oximetry. Intensive Care Med. 2004 Nov;30(11):2017-20 [MEDLINE]
Limitations
Pulse oximetry. Uses and abuses. Chest. 1990 Nov;98(5):1244-50 [MEDLINE]
The accuracy of pulse oximeters: a comparative clinical evaluation of five pulse oximeters. Anaesthesia 1988 Mar;43(3):229-32. doi: 10.1111/j.1365-2044.1988.tb05549.x [MEDLINE]
Comparison of recorded values from six pulse oximeters. Crit Care Med 1989;17:678-81 [MEDLINE]
Reliability of Pulse Oximetry in Titrating Supplemental Oxygen Therapy in Ventilator-Dependent Patients. Chest 1990, 97(6):1420-1425 [MEDLINE]
Do changes in pulse oximeter oxygen saturation predict equivalent changes in arterial oxygen saturation? Crit Care. 2003 Aug;7(4):R67 [MEDLINE]
Effects of skin pigmentation on pulse oximeter accuracy at low saturation. Anesthesiology 2005, 102(4):715-719 [MEDLINE]
Increased blood glycohemoglobin A1c levels lead to overestimation of arterial oxygen saturation by pulse oximetry in patients with type 2 diabetes. Cardiovasc Diabetol. 2012 Sep 17;11:110. doi: 10.1186/1475-2840-11-110 [MEDLINE]
Racial Bias in Pulse Oximetry Measurement. N Engl J Med. 2020 Dec 17;383(25):2477-2478. doi: 10.1056/NEJMc2029240 [MEDLINE]
Pulse oximeters’ measurements vary across ethnic groups: An observational study in patients with Covid-19 infection. Eur Respir J. 2022 Jan 27;2103246. doi: 10.1183/13993003.03246-2021 [MEDLINE]
Racial and Ethnic Discrepancy in Pulse Oximetry and Delayed Identification of Treatment Eligibility Among Patients With COVID-19. JAMA Intern Med. 2022 May 31. doi: 10.1001/jamainternmed.2022.1906 [MEDLINE]
Accuracy of multiple pulse oximeter brands in stable critically ill patients – Oxygap study. Respir Care. 2023 Jan 3;respcare.10582. doi: 10.4187/respcare.10582 [MEDLINE]
