Principle: detects change in absorption of light by OxyHb and DeoxyHb in red (660 nm) and infrared (940 nm) regions of 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: +/- 4% at SpO2 >70% (the error is higher 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%)
Advantages of Pulse Oximetry
Decreases 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 Initial Purchase): easily and rapidly performed in critically ill patients
Limitations of Pulse Oximetry
Pulse Oximetry Does Not Measure pCO2 or pH and Gives No Information About Ventilation
During acute hypoventilation, the pCO2 can rise 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 oximeters are designed to render accutate oxygen saturations between 70-100% (below 70%, they are less reliable)
Pulse Oximetry Accuracy as a Reflection of Arterial pO2 is Impacted by the Sigmoidal Shape of the Oxyhemoglobin Dissociation Curve (see Hypoxemia, [[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 Close to 90% or Lower: 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 or left can significantly influence the relationship between SpO2 and arterial pO2
Pulse Oximetry May Be Affected by Severe Anemia
Severe Anemia: in severe anemia (with Hb <5 g/dL) with SaO <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, [[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 presence of 20% MetHb and a SpO2 of 90%: 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, [[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 Oximtery 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 Discoloration
Skin Pigmentation (African Americans, etc): variable effect on SpO2
Depending on the brand of oximeter
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
Pulse Oximetry May Be Affected by 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, [[Methylene Blue]]): falsely decreases SpO2
Used to treat methemoglobinemia (see Methemoglobinemia, [[Methemoglobinemia]])
Absorbs significantly at 670 nm
Indocyanine Green Dye: 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 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
The accuracy of pulse oximeters: a comparative clinical evaluation of five pulse oximeters. Anaesthesia 198;43:229-32
Comparison of recorded values from six pulseoximeters. Crit Care Med 1989;17:678-81
Pulse oximetry. Uses and abuses. Chest. 1990 Nov;98(5):1244-50 [MEDLINE]
Do changes in pulse oximeter oxygen saturation predict equivalent changes in arterial oxygen saturation? Crit Care. 2003 Aug;7(4):R67. Epub 2003 Jun 11 [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]
