When dealing with patients who have respiratory problems or are critically ill, monitoring their oxygen levels is crucial. So if you work in the ICU or emergency room, the terms PO2 and SpO2 will be as prevalent as the respiratory rate. The terms can be confusing but are vital to understanding, especially if you are new. So what is the difference between PO2 and SpO2?

In a nutshell, PO2, which stands for partial pressure of oxygen, measures the pressure of oxygen in arteries. On the other hand, SpO2 (oxygen saturation) refers to the amount of oxygen bound to hemoglobin. PO2 value is determined by an ABG test, while pulse oximeters measure SpO2. The PO2 test is invasive, while pulse oximetry is not invasive.

What Is Partial Pressure of Oxygen (PO2)?

PO2 is a measure of the blood’s dissolved oxygen. It determines how well the lungs can pull oxygen from the atmosphere into the bloodstream. A normal PO2 is around 95 mmHg, which means that there is 95 mmHg of oxygen in a person’s blood.

To measure PO2, an arterial blood gas (ABG) test is done. Mainly, PO2 measurement is conducted in cases where the emergency department is needed, especially if the patient is having difficulty breathing.

What Is Oxygen Saturation (SpO2)?

SpO2 (oxygen saturation) measures oxygen-carrying oxygen over total hemoglobin (oxygenated and deoxygenated). It is often expressed as a percentage.

Hemoglobin (Hb or Hgb) is a crucial protein in the red blood cells (RBC) and is responsible for carrying O2 from the lungs to other body organs and tissues through gas exchange. It has four subunits, each of which has an iron molecule to which oxygen binds. Each Hb molecule can carry four oxygen molecules.

When all four heme subunits are bound, Hb will be fully saturated, meaning 100 percent oxygen saturation. One gram of Hgb can carry 1.34 milliliters of oxygen.

What Is the Difference between SpO2 and PO2?

PO2 is a measure of oxygen dissolved in plasma. On the other hand, SpO2 determines the amount of Hb saturated with oxygen. The two measures are very closely related, so some people may use these terms interchangeably since they don’t understand the difference. The relationship between PO2 and SpO2 is evident in the oxyhemoglobin dissociation curve.

PO2 is measured in millimeters of mercury (mmHg), whereas SpO2 is expressed as a percentage (%). The result for SpO2 is more instant (less than one minute), while PO2 can take roughly fifteen minutes.

Measuring PO2 and SpO2

One major thing that differentiates PO2 and SpO2 is the method used for measurement. While one is invasive and expensive, the other is noninvasive and more affordable.

Arterial Blood Gas Test

To measure PO2, you must conduct an arterial blood gas (ABG) test. The PO2 test assesses a patient’s lung functionality. It can show whether they are getting enough oxygen or why a patient may have trouble breathing.

An ABG test is conducted by drawing blood from the radial artery since there is more oxygen there than in the vein. The sample can be drawn from a newborn baby’s heel or umbilical cord. This test is one of the most common in the intensive care unit (ICU) and provides more accurate results that can help determine the correct treatment for patients.

An average PO2 value is between 80 and 100 mmHg. A reading below 80 mmHg may indicate the patient is not getting enough oxygen.

The ABG test not only reveals the PO2 values in the bloodstream but also includes other crucial information:

pH. Measures the acidity.
Partial pressure of carbon dioxide (PaCO2). Determines CO2 levels in the blood and how well it moves out of your body.
Oxygen saturation (SaO2). Checks the amount of Hb saturated with oxygen.
Oxygen content (O2CT). Measures how much oxygen is in your bloodstream.
Bicarbonate (HCO3). Bicarbonate is a form of CO2. An HCO3 test checks the levels of bicarbonate.
Hemoglobin. Determines the Hb in the bloodstream.

Low PaO2 can cause low O2 levels, leading to hypoxemia. A PO2 of 60 to 79 mmHg indicates mild hypoxemia; 40 to 59 mmHg, moderate hypoxia; and less than 40 mmHg, severe hypoxemia.

Pulse Oximetry

There are two ways to determine oxygen saturation levels: ABG testing and pulse oximetry. Pulse oximetry is a noninvasive way of measuring oxygen saturation. A small clip-like device known as a pulse oximeter is placed on a thin body part, usually a finger, an earlobe, or a toe.

The device passes two different wavelengths of light through the body part to a sensor on the other side. This leads to the absorption of light by the blood. The oximeter then calculates the oxygen saturation based on the absorbed light and displays the results on a monitor.

The arterial blood oxygen analysis measurement is known as SaO2. A regular SpO2 reading is typically between 95 percent and 100 percent. Another method of measuring Spo2 is ABG testing.

Skin pigmentation, poor perfusion, temperature, movement, and fingernail polish can easily affect SpO2 readings. While pulse oximetry is easy, cheaper, and painless, we cannot say the same about ABG. However, SaO2 is more accurate.

In addition, there is important information that a pulse oximeter cannot tell you. For example, it cannot show you the hemoglobin level or identify hemoglobin that is not functioning.

The good thing about pulse oximeters is that you can use them at home. This makes it a convenient way to monitor your health, especially if you have chronic illnesses.

The oxyhemoglobin dissociation curve (OHDC) visually represents the relationship between SaO2 and PaO2.

Image source: NIH

Understanding the OHDC is handy for anyone working in emergency departments to handle critically ill patients.

If you’re unfamiliar with PO2 and SpO2, it is easier to believe that 90 percent oxygen saturation translates directly to 90 mmHg. However, this is not the case. As you can see from the graph, the OHDC is not a straight line but has an S-shape, known as a sigmoid shape.

A significant increase in PaO2 results in a slight increase in SpO2, making the upper part of the curve relatively flat. When SpO2 levels are at 90 percent, PO2 drops to 60 mmHg. SpO2 levels should not go below 90 percent. Once the level goes below 90 percent, PO2 falls drastically, which can lead to reduced O2 delivery to body tissues and cause severe organ damage.

It’s good to note that OHDC is not constant since it can shift right or left. Factors that affect the position of the curve include blood pH, body temperature, 2,3-DPG, and PaCO2.

A shift to the right. Indicates a decreased affinity for hemoglobin's oxygen. This means it is difficult for the hemoglobin to bind to oxygen, but it releases oxygen more quickly to the tissues. A shift to the right in OHDC shows higher demand for O2 in the body than usual. It can be caused by decreased pH, increased body temperature, or partial pressure of CO2 levels.
A shift to the left. Shows an increased affinity for hemoglobin’s oxygen, meaning O2 binds more easily to hemoglobin but unbinds it slowly. An increased pH or reduced PaCO2 levels can cause a shift to the left.

What Causes Decreased SpO2 and PO2 Levels?

There are many possible causes of decreased PO2 levels. Some of the most common include the following:

Hypoventilation. Refers to shallow or slow breathing, which reduces the oxygen that reaches the lungs during gas exchange.
Anemia. A decrease in the blood’s oxygen-carrying capacity—meaning you have a low RBC count.
Congestive heart failure. A health condition in which the heart cannot pump blood efficiently, resulting in a fluid buildup in the lungs (pulmonary edema). This can make it difficult for oxygen to reach the bloodstream.
COPD (chronic obstructive pulmonary disease). It’s a group of lung diseases, including emphysema, which makes breathing difficult. COPD has no cure.
Strong medications. Medications such as narcotics and anesthetics can slow breathing, leading to less oxygen intake.

Conditions That Can Lead to Hypoxemia

Hypoxemia is a condition in which there is not enough oxygen in the blood. Conditions that may lead to hypoxemia may include the following:

Pneumonia
Sleep apnea
Pulmonary fibrosis
Pneumothorax
Anemia
COPD (chronic obstructive pulmonary disease)
ARDS (acute respiratory distress syndrome)
Asthma
COVID-19
Congenital heart disease
COPD (chronic obstructive pulmonary disease)
Pulmonary edema
Pulmonary embolism

Generally, any lung disease can lead to low PO2 and SpO2 levels since they affect your normal lungs. Symptoms of less oxygen in the body include shortness of breath, restlessness, rapid breathing, headaches, and confusion.

The Importance of Maintaining Healthy PO2 and SpO2 Levels

There may be a difference between PO2 and SpO2, but they both relate to blood oxygen, a vital element in humans and animals. Oxygen is important for the cells in our body to function correctly, and having healthy PO2 and SpO2 levels ensures our cells get the oxygen they need.

Maintaining healthy PO2 and SpO2 levels eliminates conditions such as hypoxemia. There are several ways to maintain healthy PO2 and SpO2 levels. Some of these include regular exercise, breathing properly, and staying hydrated.

If you have a condition that may cause your SpO2 levels to drop, ensure you have a pulse oximeter in your home so you can easily check your pulse oximetry levels. Seek the doctor’s advice if the levels are below what your health provider has advised.

Let’s Talk Blood Oxygen: Difference between PO2 vs. SpO2