Healthcare professionals must understand the principles of pulse oximetry and its limitations to ensure accurate diagnosis and patient management. Oxygen saturation measurement can serve as an initial screening tool for patients with respiratory disorders and continuous monitoring for critically ill patients. Pulse oximetry operates based on its ability to differentiate the absorbance of light by Oxyhemoglobin (O2Hb) and Deoxyhemoglobin (HHb).[1-3]
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In patients with saturations above 90%, the use of pulse oximetry is reliable, as the average difference between pulse oximetry readings and the standard reference saturation (SaO2) is less than 2%, with a standard deviation of less than 3%. However, there are certain conditions where pulse oximetry may not be dependable, such as in patients with saturations below 70%, severe anemia, or excessive patient movement.[1-3]
Principles of Pulse Oximetry
Pulse oximetry distinguishes between the absorbance of red (R) and near-infrared (IR) light by hemoglobin. Oxyhemoglobin (O2Hb) absorbs more IR light than deoxyhemoglobin (HHb). This corresponds to the macroscopic appearance of arterial blood, where high levels of O2Hb appear bright red due to less red light absorption.[1,2]
For this purpose, pulse oximetry is designed with dual-sided probes that can be applied to tissue. One probe side contains a light emitter diode that emits two different wavelengths: red light at 660 nm and near-infrared light at 940 nm. On the other side, there is a light sensor (photodiode) that detects the light transmitted through the body tissue. Because of the differing absorbance capabilities of O2Hb and HHb, pulse oximetry can determine the proportion of hemoglobin bound to oxygen.[1,2]
In theory, pulse oximetry measures two components of absorbance: direct current (DC) and alternating current (AC). DC represents the light passing through tissues, veins, and capillaries, which is relatively static and unaffected by other factors. AC represents the light passing through arteries and fluctuates with the cardiac cycle. Cardiac cycle changes affect arterial blood volume, thus altering the proportions of R and IR light absorption.[2,3]
Pulse oximetry uses the amplitude of absorbance to calculate the R:IR light modulation ratio from both AC and DC components, resulting in the R value. When oxygen saturation is low, HHb levels rise, causing increased R light absorption and a higher R value. Conversely, when oxygen saturation is high, O2Hb levels increase, leading to greater IR light absorption and a lower R value.[2,3]
Within pulse oximetry, there is a microprocessor that processes the measured ratio from several pulse beats. The device determines SpO2 levels based on a calibration curve empirically derived from measuring R values in volunteers with a saturation range of 100% to approximately 70%. Therefore, if measurements fall below 70%, pulse oximetry cannot be relied upon quantitatively to evaluate a patient’s condition.[2,4]
Limitations of Pulse Oximetry
Using pulse oximetry to measure arterial blood oxygen saturation has several limitations. Various errors can occur, including SpO2 reading failure or intermittent dropouts, false-normal SpO2 readings, falsely elevated readings, falsely decreased readings, and falsely decreased FO2Hb levels.[1-3]
Causes of SpO2 Reading Failure or Intermittent Dropouts
Pulse oximetry devices may fail to read oxygen saturation in patients with poor perfusion. Poor peripheral perfusion leads to low waveform amplitudes, reducing the accuracy of SpO2 readings. In other words, adequate blood volume and arterial pulse are crucial for accurate pulse oximetry readings. When attempting to measure oxygen saturation in patients with perfusion issues, the results are often inaccurate or unreadable.[1-3]
Medical conditions that can cause poor peripheral perfusion include vasoconstriction and/or hypotension, which may result from conditions such as hypovolemic shock, hypothermia, the use of vasoconstrictor drugs, and reduced cardiac output due to heart failure or arrhythmias. Additionally, using a blood pressure cuff or peripheral artery disorders in the same arm where the pulse oximetry device is placed can lead to low waveform amplitudes.[1,2]
Causes of False-Normal or Falsely Elevated SpO2
Pulse oximetry devices may yield false-normal or falsely elevated SpO2 readings in patients with carbon monoxide (CO) poisoning or vasocclusive crises in sickle cell anemia.[2]
Carbon Monoxide Poisoning
CO gas binds to hemoglobin more strongly than oxygen, approximately 240 times stronger, forming carboxyhemoglobin (COHb), reducing O2Hb levels. COHb has similar properties to O2Hb in terms of red light absorption (660 nm), but it absorbs less IR light (940 nm) compared to O2Hb. Standard pulse oximetry devices that only emit red and near-IR light cannot distinguish between COHb and O2Hb.[2-4]
Vasocclusive Crises in Sickle Cell Anemia
Oxygen saturation measurements with pulse oximetry in sickle cell anemia patients are often inaccurate, especially during vasocclusive crises. Many studies have shown that pulse oximetry readings in these patients tend to be normal or falsely elevated. This is due to dishemoglobinemia, causing SpO2 readings to be higher than the actual Fraction of O2Hb (FO2Hb).[2,5]
Causes of Falsely Decreased SpO2
Pulse oximetry devices may yield falsely decreased oxygen saturation readings in conditions involving venous pulsation, excessive movement, intravenous biological dyes, nail polish, hereditary hemoglobin abnormalities, and severe anemia with hypoxia.[2]
Venous Pulsation
Certain clinical conditions can alter venous blood volume and affect pulse oximetry readings. A significant increase in venous blood volume, as seen in tricuspid regurgitation, can lead to the saturation of venous blood being detected by pulse oximetry, resulting in falsely low oxygen saturation readings.[2]
Excessive Movement
Excessive movements, such as tremors or seizures, can alter the R value produced by pulse oximetry due to changes in absorbance components in veins and dynamic tissue caused by tremors. As a result, saturation readings tend to be lower.[6]
Intravenous Biological Dyes
The use of intravenous biological dyes, such as methylene blue for certain clinical procedures, can change the color of blood to resemble HHb and produce a higher R value.[2,5]
Nail Polish
Recent studies have shown a less than 2% decrease in saturation readings in patients using black and brown nail polish.[8]
Hereditary Hemoglobin Abnormalities
Hereditary hemoglobin abnormalities, such as Hb Lansing, Hb Bonn, Hb Koln, Hb Hammersmith, and Hb Cheverly, can alter the absorbance capabilities of erythrocytes to R and IR light without affecting actual oxygen affinity. As a result, they often show lower saturation readings than actual conditions.[2,7]
Severe Anemia with Hypoxia
Saturation readings in patients with severe anemia without oxygenation disturbances remain unaffected. However, lower saturation readings (SpO2) than actual saturation (SaO2) can be found in patients with severe anemia and hypoxia. This is because reduced erythrocyte count can lead to decreased light scattering, resulting in an R value that does not align with the calibration curve. The difference between SpO2 and SaO2 can be more significant in severe hypoxia conditions.[2,5]
Causes of Reduced SpO2 Reading Accuracy
In addition to falsely decreased readings, several clinical conditions can reduce the accuracy of SpO2 readings, affecting clinical decisions for patients.
Dishemoglobinemia
In conditions of dishemoglobinemia, such as methemoglobinemia and sulfhemoglobinemia, pulse oximetry readings tend to be inaccurate. This is because these hemoglobin types have almost identical IR and R light absorbance, resulting in an R value close to 1 and a saturation reading of 85%. This leads to decreased accuracy in saturation readings using pulse oximetry, with readings potentially being higher or lower than the actual clinical condition.[2-4]
Improper Probe Placement
If the probe is not correctly positioned, it can lead to reduced R and IR light absorption, resulting in an R value close to 1 and a saturation reading of 85%. The saturation readings will not align with the clinical condition.[2]
Sepsis and Septic Shock
Accuracy of pulse oximetry readings tends to decrease during sepsis. SpO2 readings obtained from pulse oximetry can be either higher or lower than the patient’s actual SaO2. This is due to the variable course of the disease, comorbid conditions, and fluid resuscitation management in sepsis patients, making it difficult to predict SpO2 bias direction.[9]
Causes of Falsely Decreased FO2Hb Levels
Measurement of Fraction of O2Hb (FO2Hb) is a saturation measurement using a co-oximeter. FO2Hb readings are obtained from O2Hb concentration divided by the total concentration of other Hb species, such as HHb, MetHb, and COHb. While considered more accurate than SpO2 measurement, there are some conditions that can affect FO2Hb readings.[2]
Severe Hyperbilirubinemia
Severe hyperbilirubinemia (>30 mg/dL) can be found in patients with increased heme metabolism (hemolysis). This condition leads to an increase in MetHb and COHb levels as a result of metabolism, affecting the accuracy of FO2Hb readings.[2,3]
Fetal Hemoglobin (HbF)
Fetal hemoglobin (HbF) is commonly found in neonates. Although there are no differences in properties between HbF and HbA, HbF can be identified as COHb by a co-oximeter, lowering FO2Hb readings.[2-4]
Conclusion
Pulse oximetry operates based on the fundamental principle of differentiating the absorbance of red (R) and near-infrared (IR) light by hemoglobin. Due to this principle, pulse oximetry can detect the levels of oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb) in a patient’s blood. Because its operation relies on light absorbance differences, changes in blood composition, smooth blood flow, and other factors that interfere with light absorbance can affect pulse oximetry readings.
Several limitations of pulse oximetry include its inability to be used on patients with saturations below 70%, the potential for SpO2 reading failures or intermittent dropouts, falsely normal SpO2 readings, falsely elevated readings, falsely decreased readings, and falsely decreased FO2Hb levels. Additionally, the accuracy of SpO2 readings can decrease in conditions of dishemoglobinemia, improper probe placement, and in sepsis patients.
