Blood Oxygenation: Difference Between SpO2, SaO2, PaO2

The main difference between SpO2, SaO2, and PaO2 lies in what each represents. SpO2 is the oxygen saturation level measured by a pulse oximeter. SaO2 is the oxygen saturation level determined by a blood gas analysis. PaO2 is the partial pressure of oxygen flowing from the alveoli to the capillaries and the vessels. Here are the key differences:

SpO2

• SpO2 is oxygen saturation measured using a pulse oximeter.
• Pulse oximetry, used to measure SpO2, is a non-invasive procedure.
• SpO2 levels are important in determining whether a patient requires mechanical oxygen support.

SaO2

• SaO2 is the saturation of oxygen in arterial blood.
• The SaO2 levels are determined by arterial blood gas (ABG) analysis.
• The procedure is invasive and contraindicated in patients with blood disorders.

PaO2

• PaO2 is the pressure exerted by oxygen on the arterial wall.
• Dissociation of oxygen affects the PaO2 levels.

PaO2, SpO2, and SaO2 help your healthcare provider determine the amount of oxygen in the patient’s blood and assess its adequacy. Let’s explain further the role each plays in oxygen adequacy in the peripheral blood. 

Oxygen Saturation (SpO2) and Pulse Oximetry Explained

SpO2 represents the peripheral blood oxygen saturation levels. The circulating blood contains hemoglobin, which also carries oxygen, and SpO2 measures the oxygen-carrying capacity of the hemoglobin in the blood. Oxygen is carried and utilized by the body’s organs to maintain normal body function, so adequate oxygen delivery and saturation are essential.

Normal oxygen saturation levels range from 94 percent to 100 percent. The body maintains normal SpO2 by inhaling oxygen. The inhaled oxygen goes to the lungs, binds to the hemoglobin, and circulates throughout the body. Low SpO2 levels can result in hypoxemia, which can progress to hypoxia (low oxygen levels in the tissues).

A pulse oximeter measures SpO2 levels. The pulse oximeter is a small device a physician will place on your finger, and the oxygen saturation levels are displayed on the screen. If you take a pulse oximetry test yourself and your SpO2 levels are below 90 percent, you should see your healthcare provider.

Arterial Oxygen Saturation (SaO2) Explained

To determine the SaO2, the healthcare provider will order an arterial blood gas analysis. The ABG test aims to ascertain the percentage of oxygen in the hemoglobin molecules in the arterial blood, which cannot be accurately achieved through pulse oximetry. 

Standard SaO2 measurements range from 94 percent to 100 percent. Levels below 90 percent indicate hypoxia, which may be a result of underlying conditions such as the following:

• Acute or chronic kidney failure
• Anemia
• Asthma
• Chemical poisoning
• Drug overdose
• Heart failure
• Hemorrhage
• Inborn errors of metabolism
• Severe sepsis
• Shock
• Uncontrolled diabetes

Low SaO2 levels below 80 percent indicate the low oxygen-carrying capacity in the arteries. Low oxygen levels compromise the functioning of major organs, including the brain, kidneys, liver, and heart. If the oxygen levels drop further, the patient will get into shock with cardiac and respiratory arrest. Supplemental oxygen or other medical interventions may be necessary to improve oxygen saturation levels in such cases.

Unlike pulse oximetry, measuring SaO2 is an invasive procedure, as the healthcare provider must draw a blood sample from an artery. An arterial blood gas analysis provides information on blood oxygen levels, carbon dioxide, and pH.

What Is Measured in an ABG Test?

The blood drawn from an artery includes the following measurements:

• Oxygen content (O2CT): The oxygen levels in the blood.
• Hemoglobin (Hb): The amount of hemoglobin available to carry oxygen to the blood cells.
• SaO2: The arterial oxygen saturation level of your arterial blood, responsible for carrying oxygenated blood from the lungs to various cells and body organs.
• Partial pressure of oxygen (PaO2): The pressure at which the oxygen dissolves in your blood.
• pH: The measure of how acidic or alkaline the blood is. Normal pH ranges are 7.35 to 7.45. If it’s above that, it is considered alkaline or basic. If it is lower, it is acidic.
• Partial pressure of carbon dioxide (PaCO2): The level of carbon dioxide in the blood and how well the body can remove CO2.
• Bicarbonate (HCO3): The levels of bicarbonate, a basic compound that helps maintain pH balance.

Partial Pressure of Oxygen (PaO2) Explained

PaO2 measures the partial pressure of oxygen in arterial blood. It is one of the measurements made during an arterial blood gas test. 

Conditions that affect your breathing or cause breathing problems compromise the oxygen supply. The level of PaO2 assists doctors in determining whether a patient requires oxygen supplementation or mechanical breathing assistance.

PaO2 values help in the events of chronic medical conditions such as the following:

• Asthma
• Chronic obstructive pulmonary disease (COPD)
• Congestive heart failure
• Cystic fibrosis
• Heart attack
• Loss of consciousness
• Lung injury or chest trauma
• Sudden difficulty in breathing

Unlike other tests, such as pulse oximetry, the ABG test is invasive. It involves puncturing the arterial wall and drawing blood. This procedure poses a risk of bruising or excessive bleeding. The test is contraindicated in patients on blood thinners such as heparin and warfarin or those with bleeding disorders.

Interpreting PaO2 Results 

Normal PaO2 levels range from 75 to 100 mmHg (at sea level). The oxygen we inhale travels to the lungs and is stored in the alveoli. The deoxygenated blood from the body flows to the lungs through the pulmonary arteries. The oxygen pressure in the alveoli is higher than that in the adjacent capillaries. Hence, it flows to the capillaries. The blood leaving the lungs is well saturated with O2.

If O2 flow from the alveoli to the blood is compromised, your PaO2 levels will be lower. Several factors can cause low PaO2 levels:

• Low hemoglobin levels from iron-deficiency anemia
• Lung damage due to trauma or cancer
• Neurological conditions such as Guillain-Barre syndrome (GBS) or amyotrophic lateral sclerosis (ALS)
• Obesity
• Places way above sea level (e.g., mountains), where the atmospheric pressure and oxygen levels are lower, reducing the oxygen pressure in the lungs

The Oxygen-Hemoglobin Dissociation Curve

The oxygen-hemoglobin disassociation curve is a graph that shows the relationship between the partial pressure of oxygen (PaO2) in mmHg and the percentage of oxygen saturation. Body cells use the available O2, and the PaO2 begins to fall. For equilibrium, O2 should dissociate into the bloodstream at a speed equal to how the cells consume it.

In an average healthy adult, the oxygen saturation ranges from 94 to 100 percent with a PaO2 of 80 to 100 mmHg. The figures mean that the graph cannot be a straight line but a curve. Below is a typical image of the oxyhemoglobin dissociation curve.

Here are some important points to note on an oxyhemoglobin dissociation curve:

• Arterial Blood: Hemoglobin is usually saturated at 100 percent with an oxygen pressure of 100 mmHg.
• Venous Blood: Hemoglobin is saturated at approximately 75 percent with a PaO2 of around 40 mmHg.
• p50: This is the point on the curve where hemoglobin is 50 percent saturated, typically occurring at a PaO2 of about 26.8 mmHg.

Conditions That Shift the Curve

The speed at which oxygen is utilized by the cells influences the partial pressure of oxygen. The curve shifts to either the right or the left.

Right Shift

A curve deviation to the right indicates a decrease in oxygen affinity as hemoglobin holds the oxygen less tightly and delivers more oxygen to the cells at a given arterial oxygen pressure. More significant amounts of oxygen dissociate from hemoglobin, which improves tissue oxygenation. A shift to the right side means the hemoglobin is in a tense state (T state). The CO2 and metabolic acid from the tissues shift the curve rightward.

The causes of a rightward shift are the following:

• An increase in CO2
• High 2,3-BPG
• High temperatures
• Low pH or more acidic
• Low-affinity Hb variants

Left Shift

A leftward deviation indicates the cells have an increased affinity for oxygen, as the hemoglobin holds more tightly onto it and delivers less oxygen to the tissues. The left-sided shift means the hemoglobin is in a relaxed state (R state). The left-sided curve is crucial for a fetus with Hb F as it allows oxygen transfer from the maternal to fetal circulation.

The left-shifted curve is caused by the following:

• Carboxyhemoglobinemia
• Fetal Hb, especially Hb F
• High oxygen affinity Hb variants
• High pH or more basic
• Low 2,3-BPG
• Low temperatures
• Methemoglobinemia

Bottom Line

For the human cell to function normally, it requires a constant supply of oxygen. The oxygen from the air is inhaled into the lungs and stored by the alveoli. Low oxygen saturation leads to hypoxemia, which results in hypoxia and, if not corrected, can cause irreversible damage to vital organs.

SpO2, SaO2, and PaO2 may differ, but they all depend upon the concentration of oxygen present in the circulating blood. An imbalance in oxygen saturation causes cardiac and respiratory deficiencies. The SpO2, SaO2, and PaO2 levels should be within the normal range for the human cell to maintain homeostasis.