Health workers must understand the working principle and limitations of pulse oximetry tool to avoid errors in treating patients and diagnosing. Measurement of oxygen saturation becomes initial screening in patients with respiratory disorders and continuous monitoring in patients with critical conditions. The working principle of the pulse oximetry tool is based on the ability to distinguish the light absorbance by oxyhemoglobin (O2Hb) from Deoxihemoglobin (HHb).

The Working Principles and Limitations of Pulse Oximetry
Fingertip Pulse Oximetry
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The average difference between the reading results of pulse oximetry tools with reference standards Saturation (SaO2) is less than 2%, with a standard deviation of less than 3%. Therefore, the use of pulse oximetry tool can be trusted if saturations above 90%. However, there are several conditions where pulse oximetry cannot be relied on, namely in patients with saturation below 70%, severe anemia, and excessive movement of the patient.

The working principles on pulse oximetry

Pulse oximetry can identify the difference in absorbance of red light (R) and near-infrared (IR) of hemoglobin. Oxyhemoglobin (O2Hb) absorbs more IR light than deoxyhemoglobin (HHb). This is matched with the macroscopic appearance of arterial blood; high levels of O2Hb will appear bright red because not much red light is absorbed while venous blood does not look too red, because higher HHb levels cause much absorbing red light.

The pulse oximetry design has two side probes that can flank the tissue. One side of the probe is a light-emitting diode (emitter) that can emit two different wavelengths, namely 660 nm red waves and 940 nm near-infrared. On the other side, there is a light sensor (photodiode) that detects light that has passed through body tissue. Due to differences in the absorbance ability of light from O2Hb and HHb, pulse oximetry can determine the proportion of Hb bound to oxygen.

In theory, pulse oximetry measures 2 absorbance components, namely direct current (DC) and alternating current (AC). DC represents light passing through tissues, veins, and capillaries, which tend to be static and are not influenced by other factors. AC represents the light that passes through the arteries and fluctuates according to the cardiac cycle. Changes in the cardiac cycle influence the amount of arterial blood volume, so the proportion of absorbance of R and IR light changes.

Pulse oximetry uses amplitude absorbance to calculate the R: IR modulation ratio of the two components, AC and DC, so that the R-value is obtained. In conditions where oxygen saturation is low, the HHb level increases, and there is an increase in the absorbance of light R resulting in a high R-value. Whereas when oxygen saturation is high, O2Hb levels increase, causing an increase in IR light absorbance and decreasing R-value.

Inside the pulse oximetry, there is a microprocessor that can process measured ratios from several series of pulses. This tool determines SpO2 levels based on calibration curves, which are produced empirically through measurement of R values ​​in volunteers with a saturation range of 100% to about 70%. Therefore, Pulse oximetry cannot be relied on quantitatively to evaluate a patient's condition if the measurement result is below 70%.

Limitations of Pulse Oximetry

The use of pulse oximetry to measure oxygen saturation in arterial blood has several limitations. several errors can occur:
  • failure to read SpO2 or intermittent dropouts, 
  • SpO2 Falsely Normal, falsely Elevated or falsely low, 
  • and Falsely low of the fraction of O2Hb (FO2Hb).

Causes of intermittent drop-outs or inability to read SpO2

Pulse oximetry cannot read oxygen saturation in patients with poor perfusion. Poor peripheral perfusion causes low wave amplitude, so the accuracy of the SpO2 reading decreases. In other words, essential components for producing accurate pulse oximetry readings are adequate blood volume and arterial pulse. When pulse oximetry tries to measure oxygen saturation in patients experiencing perfusion problems, the results tend to be inaccurate and even unreadable.

Some medical conditions that can cause poor peripheral perfusion are vasoconstriction and hypotension. These conditions are caused by hypovolemic shock, hypothermia, vasoconstrictor drugs, and decreased cardiac output due to heart failure or dysrhythmias. Also, the imposition of tension cuffs and blockade of peripheral arteries in the upper part of the hand, which is installed pulse oximetry, can result in low wave amplitude.

Causes of Falsely Normal or Elevated SpO2

Pulse oximetry shows oxygen saturation results, which are false normal and falsely increased in patients with carbon monoxide poisoning, and vaso-occlusive crisis in sickle cell anemia.

a. Carbon Monoxide Poisoning
The affinity of carbon monoxide for hemoglobin is 240x greater than the affinity of O2 with hemoglobin. Carbon monoxide and hemoglobin form COHb bonds that cause O2Hb levels to decrease. The COHb character is similar to O2Hb in absorbing red light (660 nm), but COHb absorbs less IR (940 nm) compared to O2Hb. Therefore, standard pulse oximetry tools that only emit red and near-IR light cannot distinguish between COHb and O2Hb.
b. The Vasocclusive Crisis of Sickle Cell Anemia
Measurement of oxygen saturation using the pulse oximetry tool in patients with sickle cell anemia is often imprecise, especially in vaso-occlusive crisis conditions. Many studies show that pulse oximetry tests in these patients show false normal or false increased results. This is due to the condition of dyshemoglobinemia, which causes the SpO2 reading results to tend to be higher than the actual O2Hb (FO2Hb) condition.

Causes falsely Low SPO2

The pulse oximetry tool shows the results of falsely decreased oxygen saturation in venous pulsation conditions, excessive movement, use of intravenous biological dyes, applying nail polish, hereditary hemoglobin abnormalities, and anemia Gravis with hypoxic conditions.
a. Vein Pulsations
Some clinical conditions can cause changes in venous volume and affect the results of pulse oximetry readings. A significant increase in venous volumes causes venous saturation to be measured by pulse oximetry, thus showing low oxygen saturation.
b. Excessive Movement
Excessive movements such as tremors or seizures can change the R-value generated by pulse oximetry because there is a change in the absorbance component of the veins and static tissue to be dynamic due to the tremor. So saturation readings tend to be lower.
c. Intravenous Pigmented Dyes
The use of Intravenous Pigmented Dyes such as methylene blue for some clinical procedures can change the color of the blood so that it resembles HHb and has a higher R-value.
d. Finger Polish
Recent studies have shown a <2% reduction in saturation reading results in patients applying black and brown nail polish.
e. Hereditary Hemoglobin Abnormality
Hereditary hemoglobin abnormalities, such as Hb Lansing, Hb Bonn, Hb Koln, Hb Hammersmith, and Hb Cheverly, can alter the ability of erythrocytes to absorb R and IR light without affecting the actual oxygen affinity. Therefore, it often shows saturation readings that are lower than the actual condition.
f. Severe Anemia with Hypoxic Conditions
The results of saturation readings in patients with anemia Gravis (severe anemia) without oxygenation disorders are not affected. However, the results of saturation readings (SpO2), which are lower than the saturation conditions (SaO2 / oxygen saturation of arterial blood), are found in patients with anemia Gravis with hypoxia. This is because the decreasing amount of erythrocytes can cause a decrease in the spread of light and produce an R-value that is not following the calibration curve. The difference between SpO2 and SaO2 can be more significant in more severe hypoxic conditions.

Causes of Decreased Level of Accuracy in SpO2 Readings

In addition to false declining results, several clinical conditions can reduce the level of accuracy of SpO2 readings, thereby affecting the clinical decisions that must be taken for patients.
a. Dishemoglobinemia
In dyshemoglobinemia conditions such as methemoglobinemia and sulfhemoglobinemia, the results of pulse oximetry saturation readings tend to be inaccurate. This inaccuracy is caused by the two types of hemoglobin, which have almost the same absorbance IR and R, resulting in an R-value close to 1 and a saturation reading of 85%. Of course, this results in a decrease in the accuracy of the saturation reading results by using pulse oximetry. Readings can be higher or lower than the actual clinical condition.
b. Poor probe positioning
If the probe position is not suitable, it can reduce the absorption of R and IR light so that the R-value approaches 1 and the saturation reading is 85%. The saturation reading will not be following clinical conditions.
c. Sepsis and Septic Shock
In the condition of sepsis, the results of pulse oximetry accuracy tend to decrease. SpO2 that reads pulse oximetry can be higher or lower than the actual patient's SaO2. This inaccuracy is due to the course of the disease, concomitant diseases, as well as the management of fluid resuscitation in sepsis patients, which is very varied, making it difficult to predict the direction of SpO2 bias.

Causes FO2Hb Falsely Low

Measurement of O2Hb fraction (FO2Hb) is a measurement of saturation using a co-oximeter. The FO2Hb reading results obtained from the O2Hb concentration divided by the total concentration of other Hb species such as HHb, MetHb, and COHb. Although considered more accurate than the SpO2 examination, some conditions can affect FO2Hb reading results.
a. Severe hyperbilirubinemia
Severe hyperbilirubinemia (> 30 mg / dL) can be found in patients with increased heme metabolism (hemolysis). This condition causes an increase in MetHb and COHb levels as a result of metabolism, which affects the accuracy of FO2Hb reading results.
b. Hb Fetal (HbF)
Fetal Hb (HbF) is a type of hemoglobin commonly found in neonates. Although there is no difference in nature between HbF and HbA, HbF can be identified as COHb by the co-oximeter, thereby reducing FO2Hb reading results.

Pulse oximetry works with the basic principle of differentiating the absorbance of red (R) and near-infrared (IR) light on hemoglobin. Through this differentiation, pulse oximetry can detect oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb) levels.

Because the working principle is based on differences in the absorbance of light, changes in blood composition, smooth flow of blood, and other factors that interfere with the process of measuring light absorbance can affect the pulse oximetry reading results.

Some limitations of pulse oximetry are that it cannot be applied in patients with saturation below 70%, and there can be a condition of failure to read SpO2 or intermittent dropouts, spo2 falsely normal, falsely increase, falsely decrease, and FO2Hb falsely decrease false. Also, the level of accuracy of SpO2 readings can decrease in the condition of dyshemoglobinemia, the position of the probe that is not right, or in sepsis patients.

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