What is the Difference Between SAO2 and SPO2

The main difference between SaO₂ and SpO₂ is that SaO₂, or the arterial oxygen saturation, is the percentage of hemoglobin molecules in the arterial blood saturated with oxygen, but SpO₂, or the peripheral oxygen saturation, is the percentage of hemoglobin molecules in the peripheral blood saturated with oxygen. Furthermore, SaO₂ can be measured by blood gas analysis, while SpO₂ refers to the SaO₂ measured by the pulse oximeter. Therefore, the values of SaO₂ and SpO₂ may differ as SaO₂ measures the oxygen saturation of both functional and non-functional hemoglobin, but SpO₂ measures the oxygen saturation of only functional hemoglobin. 

In short, both SaO₂ and SpO₂ are two measurements of oxygen saturation in the arterial blood, indicating the percentage of hemoglobin molecules saturated with oxygen. Generally, these measurements are important in treating anemic conditions, injuries, surgical procedures, etc. 

Key Areas Covered 

1. What is SaO2
     – Definition, Measurement, Importance
2. What is SpO2
     – Definition, Measurement, Importance
3. What are the Similarities Between SaO2 and SpO2
     – Outline of Common Features
4. What is the Difference Between SaO2 and SpO2
      – Comparison of Key Differences

Key Terms 

Functional Hemoglobin, Oxygen Saturation, Pulse Oximeter, SaO₂, SpO₂

What is SaO2 

SaO₂, or the oxygen saturation of the arterial blood, is the percentage of hemoglobin molecules in the arterial blood saturated with oxygen. In general, this reading may vary from 0 to 100%. However, the normal reading rages from 94% to 100% for a healthy adult. Typically, the levels below 90% indicate hypoxic conditions, which may occur as a result of anemia. Also, arterial oxygen saturation below 80% can compromise the function of organs including the brain and the heart. Consequently, it may result in cardiac or respiratory arrest. Hence, the human body regulates a specific balance of oxygen saturation in a precise way. 

Figure 1: Oxyhemoglobin Saturation Curve

Measurement

Concerning the measurement, blood gas analysis is the method, which permits the indirect measurement of SaO₂. Basically, it is an invasive method, measuring the amount of oxygen and carbon dioxide the arterial blood as well as its pH. Also, it is performed for patients with certain medical conditions such as kidney failure, heart failure, uncontrolled diabetes, hemorrhage, chemical poisoning, drug overdose, shock, etc. Besides, this method measures the partial pressure of oxygen in the blood, which in turn permits to determine the oxygen saturation according to the oxyhemoglobin dissociation curve. 

What is SpO2 

SpO₂ or the peripheral oxygen saturation is the oxygen saturation in the arterial blood as measured by the pulse oximeter. Generally, pulse oximetry is a noninvasive as well as a safe, convenient, and inexpensive method for measuring oxygen saturation for clinical use. 

Figure 2: Pulse Oximeter

Measurement

When it comes to measurement of SpO₂, transmissive pulse oxymetry is the commonest method in which a device passes two wavelengths of light to a photodetector through a body part, usually fingertip or earlobe in adults. Here, red (660 nm) and infrared (940 nm) are the two wavelengths. Generally, oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. In contrast, deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light. Ultimately, the ratio of the transmitted red light measurement to the infrared light measurement gives the SpO₂ value. The following formula helps to calculate the SpO2 value.  

SpO₂ = HbO₂ / (HbO2 + Hb_Deoxygenated) 

Moreover, the SpO₂ value serves as an unreliable surrogate marker for SaO₂ in critically ill patients. However, the validity of the relationship between SpO₂ and SaO₂ is dependent on a number of factors, including the adequacy of peripheral perfusion. Also, besides its non-invasive nature, pulse oximetry provides attempts to the continuous measurement of oxygen saturation, which is useful in several clinical situations. These include surgery and post-anesthetic care units, neonatal care and NICU, emergency care, noninvasive transcutaneous pacing, etc.

Figure 3: Absorption Spectra of Oxygenated Hemoglobin (HbO2) and Deoxygenated Hemoglobin (Hb) for Red and Infrared Wavelengths

Moreover, a significant feature of SpO₂ measurement is that it only measures the oxygen saturation of the functional hemoglobin, which is more accurate. Here, the functional hemoglobin refers to the hemoglobin capable of carrying oxygen. On the other hand, non-functional hemoglobins include carboxyhemoglobin (HbCO) and methemoglobin (METHb), which are incapable of carrying oxygen. Thus, the formula for the ideal blood saturation measurement is: 

SpO₂ = HbO₂ / (Hb + HbO₂ + HbCO + METHb + other non-functioning hemoglobins)  

Similarities Between SaO2 and SpO2  

• SaO2  and SpO2 are two measurements of oxygen saturation in the arterial blood.  
• Both are measured in the peripheral blood.  
• They indicate the percentage of hemoglobin molecules that are saturated with oxygen.    
• Both are important in treating anemic conditions, injuries, surgical procedures, etc. 

Difference Between SaO2 and SpO2 

Definition 

SaO₂ refers to the measurement of oxygen saturation in arterial blood, but SpO₂ refers to the oxygen saturation in the arterial blood as measured by a pulse oximeter. 

Measuring 

SaO₂ can be measured by blood gas analysis, while SpO₂ refers to the SaO₂ measured by the pulse oximetry.  

Method 

SaO₂ is measured in the peripheral blood taken from an artery through puncturing. But, SpO₂ is a noninvasive method, which commonly uses a transmissive pulse oximeter. 

Types of Hemoglobins 

SaO₂ measures the oxygen saturation of both functional and non-functional hemoglobin, but SpO₂ measures the oxygen saturation of only functional hemoglobin. 

Importance 

SaO₂ is important to determine anemic conditions, while SpO₂ is important in surgery and post-anesthetic care units, neonatal care and NICU, emergency care, noninvasive transcutaneous pacing, etc. 

Conclusion 

SaO₂ is the oxygen saturation in the arterial blood. It measures the amount of hemoglobin saturated with oxygen in the peripheral blood, including both functional and non-functional hemoglobin. Also, blood gas analysis is the method for measuring SaO₂, and it is an invasive method. But, on the other hand, SpO₂ is the oxygen saturation of the arterial blood according to the measurement of the pulse oximeter. It is a noninvasive method of measuring oxygen saturation. Also, it only measures the oxygen saturation of the functional hemoglobin. Hence, the main difference between SaO₂ and SpO₂ is the method and the type of measurement. 

References:

1. “Understanding Pulse Oximetry SpO2 Concepts.” SpO2 Monitoring, 1 January 2013. Available Here.
2. Luo, Elaine K. “ Blood Gas Test.” Healthline, Healthline Media, 1 July 2019. Available Here.

Image Courtesy:

1. “Oxyhaemoglobin dissociation curve” By Ratznium at English Wikipedia (Public Domain) via Commons Wikimedia  
2. “Wrist-oximeter” By UusiAjaja – Own work (CC0) via Commons Wikimedia   
3. “Oxy and Deoxy Hemoglobin Near-Infrared absorption spectra” By Adrian Curtin – Own work (CC BY-SA 3.0) via Commons Wikimedia