What is the difference between SAO2 and SPO2?

The main difference between SaO 2 and SpO 2 is that SaO 2 or arterial oxygen saturation is the percentage of hemoglobin molecules saturated with oxygen in arterial blood , but SpO 2 or peripheral oxygen saturation is the percentage of hemoglobin molecules in oxygenated peripheral blood. Furthermore, SaO 2 can be measured by blood gas analysis, while SpO 2 refers to the SaO 2 measured by the pulse oximeter. Therefore, the values ​​of SaO 2 and SpO 2 may differ because SaO 2 measures the oxygen saturation of both functional and non-functional hemoglobin, but SpO 2 only measures the oxygen saturation of functional hemoglobin.  

In short, both SaO 2 and SpO 2 are two measurements of oxygen saturation in arterial blood that indicate the percentage of hemoglobin molecules that are saturated with oxygen. In general, these measurements are important in treating anemic conditions, injuries, surgery, etc.  

Key areas covered  

1. What is SaO2 - definition, measurement, meaning 2. What is SpO2 - definition, measurement, meaning 3. What are the similarities between SaO2 and SpO2 - overview of the common characteristics 4. What is the difference between SaO2 and SpO2 - comparison of main differences

key terms  

Functional hemoglobin, oxygen saturation, pulse oximeter, SaO 2 , SpO 2

Difference Between SAO2 and SPO2 - Summary of Comparison

What is SaO2  

SaO 2, or arterial blood oxygen saturation, is the percentage of hemoglobin molecules in arterial blood that is saturated with oxygen. In general, this reading can vary from 0 to 100%. However, the normal reading is between 94% and 100% in a healthy adult. Typically, values ​​below 90% indicate hypoxic conditions that can occur as a result of anemia. In addition, arterial oxygen saturation below 80% can impair the function of organs including the brain and heart. As a result, cardiac arrest or respiratory failure can occur. Thus, the human body precisely regulates a certain balance of oxygen saturation.

SAO2 vs. SPO2

Figure 1: Oxyhemoglobin saturation curve

Measurement

Regarding the measurement, the blood gas analysis is the method that enables the indirect measurement of SaO 2 . Basically, it is an invasive method in which the amount of oxygen and carbon dioxide in the arterial blood as well as the pH value are measured . It is also used in patients with certain medical conditions such as kidney failure, heart failure, uncontrolled diabetes, bleeding, chemical poisoning, drug overdose, shock, etc. In addition, this method measures the oxygen partial pressure in the blood, which in turn enables the oxygen saturation to be determined according to the oxyhemoglobin dissociation curve .  

What is SpO2  

SpO 2 or peripheral oxygen saturation is the oxygen saturation in arterial blood measured with the pulse oximeter. In general, pulse oximetry is both a non-invasive and safe, convenient, and inexpensive method of measuring oxygen saturation for clinical use.

Difference between SAO2 and SPO2

Figure 2: Pulse oximeter

Measurement

When it comes to measuring SpO 2, the most common method is transmissive pulse oximetry , in which a device sends two wavelengths of light through a part of the body, typically the fingertip or earlobe in adults, to a photodetector . Here red (660 nm) and infrared (940 nm) are the two wavelengths. In general, 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 SpO 2 value. The following formula helps to calculate the SpO2 value.  

SpO 2 = HbO 2 / (HbO2 + Hb oxygen-free )

In addition, the SpO 2 value serves as an unreliable surrogate marker for SaO 2 in critically ill patients . H the validity of the relationship between SpO 2 and SaO 2 owever, depending on a number of factors , including the adequacy of the peripheral perfusion. In addition to its non-invasive nature, pulse oximetry also offers experiments in continuous measurement of oxygen saturation, which is useful in several clinical situations. These include surgical and post-anesthesia wards, neonatal care and NICU, emergency care, non-invasive transcutaneous stimulation, etc.

Key difference - SAO2 vs. SPO2

Figure 3: Absorption spectra of oxygenated hemoglobin (HbO2) and oxygen-free hemoglobin (Hb) for red and infrared wavelengths

Another key feature of SpO 2 measurement is that it only measures the oxygen saturation of the functional hemoglobin, which is more accurate. Here, functional hemoglobin refers to the hemoglobin that is able to carry oxygen. On the other hand, non-functional hemoglobins contain carboxyhemoglobin (HbCO) and methemoglobin (MetHb), which is incapable of carrying oxygen. So the formula for the ideal blood saturation measurement is:  

SpO 2 = HbO 2 / (Hb + HbO 2 + HbCO + METHb + other non-functioning hemoglobins)

Similarities Between SaO2 and SpO2   

  • SaO2 and SpO2 are two measurements of oxygen saturation in 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, surgery, etc.  

Difference between SaO2 and SpO2  

definition  

SaO 2 refers to the measurement of oxygen saturation in arterial blood, but SpO 2 refers to oxygen saturation in arterial blood as measured by a pulse oximeter.  

Measurement  

SaO 2 can be measured by blood gas analysis, while SpO 2 refers to the SaO 2, which is measured by pulse oximetry.  

method  

SaO 2 is measured in peripheral blood drawn from an artery by puncture. However, SpO 2 is a non-invasive method that commonly uses a transmissive pulse oximeter.  

Types of hemoglobins  

SaO 2 measures the oxygen saturation of both functional and non-functional hemoglobin, but SpO 2 measures the oxygen saturation of functional hemoglobin only.  

meaning  

SaO2 is important to determine anemic states, while SpO in surgical and postanesthetic care units, neonatal care and NICU, emergency care, non-invasive transcutaneous stimulation is so important. 2  

diploma  

SaO 2 is the oxygen saturation in arterial blood. It measures the amount of oxygenated hemoglobin in peripheral blood, including both functional and non-functional hemoglobin. In addition, blood gas analysis is the method of measuring SaO 2 and it is an invasive method. On the other hand, however, SpO 2 is the oxygen saturation of the arterial blood as measured by the pulse oximeter. It is a non-invasive way of measuring oxygen saturation. In addition, it only measures the oxygen saturation of the functional hemoglobin. Therefore, the main difference between SaO 2 and SpO 2 is the method and type of measurement.  

References:

1. “Understanding the Concepts of Pulse Oximetry SpO2.” SpO2 Monitoring, Jan 1, 2013. Available here .2. Luo, Elaine K. "Blood Gas Test". Healthline, Healthline Media, July 1st, 2019. Available here .

Image courtesy:

1. “Oxyhemoglobin dissociation curve” By Ratznium in the English Wikipedia (Public Domain) via Commons Wikimedia 2. “Wrist Oximeter” By UusiAjaja - Own work (CC0) via Commons Wikimedia 3. “Oxy and Desoxy Hemoglobin Near-Infrared Absorption Spectra “By Adrian Curtin - Own work ( CC BY-SA 3.0 ) via Commons Wikimedia

About the author: Lakna

Lakna, a graduate in molecular biology and biochemistry, is a molecular biologist and has a broad and strong interest in discovering things related to nature