PaO2 – Partial Pressure of Oxygen: Exam, Risks and Contraindications


Pa02, in a nutshell, is a measure of the actual oxygen content in the arterial blood; this is the partial pressure exerted by oxygen when it is incorporated into the arterial blood.

The doctor usually recommends the measurement of PaO2 to evaluate the movement of oxygen from the lungs to the bloodstream.

In other words, if a gas such as oxygen is present in an air space such as the lungs and dissolves in a liquid such as blood, and the air space and drink are in contact with each other, the two partial pressures are they will match.

The partial pressure of oxygen (PaO2) is one of the parameters measured during blood gases analysis.

This analysis allows specifically to study the amount of oxygen transported by the arterial blood to the organs.

When it comes to gases dissolved in liquids such as oxygen in the blood, the partial pressure is the pressure that the liquefied gas would have if the blood were allowed to equilibrate with a volume of gas in a container.


In general, this parameter is associated with other measures that ultimately allow a complete overview of the patient’s respiratory function.

The gases in the blood also evaluate the partial pressure of carbon dioxide (PaCO2), the oxygen saturation (SaO2), the pH, and the concentration of bicarbonate (HC03).

The study of oxygen and carbon dioxide concentration is indicated mainly in case of respiratory difficulties or heart failure.

How is the exam?

The measurement of PaO2 requires the performance of an arterial puncture.

As a general rule, health professionals prefer that the radial artery perform this type of sampling; the femoral artery is sometimes used.

In all cases, the fee must comply with all current asepsis rules.

Once the puncture is performed, a relatively strong compression point is practiced for approximately ten minutes. The arterial blood taken is brought to low temperatures rapidly in the analysis laboratory.

The conditions of preservation of the sample are, in fact, critical to avoid errors in the results. The measurement is reported in millimeters of mercury (mmHg).

It is considered normal when the results obtained are between 80 and 90 mmHg.

What are the risks and contraindications?

As with all arterial samples, the formation of a hematoma is one of the most frequent risks and, in general, the most benign.

However, you should know that a femoral puncture is more likely than a radial puncture. This is why the latter is always recommended as the first intention.

The femoral artery near the genital area increases the risk of infection.

In addition, the measurement of PaO2 and, more in general, blood gases is contraindicated in patients with coagulation disorders, significant immunodeficiency, arteriovenous fistula, or arterial thrombosis.

What is the difference between oxygen saturation and PaO2?

Hypoxaemia and hypoxia commonly occur among perioperative patients, so it is essential to recognize early signs of respiratory distress, such as tachycardia, tachypnea, cyanosis, agitation, and changes in mental status.

Pulse oximetry is a prominent monitoring tool to identify hypoxemia and hypoxia.

A common area of ​​confusion is related to understanding the critical distinction between arterial oxygen partial pressure (PaO2) and oxygen saturation (SaO2).

Multiple studies have identified this as a knowledge gap.

A study of pediatric nurses showed that, while 84% of physicians considered that they had received adequate training, only 40% correctly identified how a pulse oximeter worked. Only 15% correctly understood the pulse curve—oxyhemoglobin dissociation.

This is a critical concept that all health professionals should understand using this valuable monitoring tool.

Is it 100% normal oxygen saturation?

No, it is not. Let us take an example of a patient who breathes 50% FiO2 with a PaO2 of 100.

A simple formula to estimate the arterial oxygen concentration should be to multiply the oxygen concentration inspired by 5.

Someone who breathes ambient air with 21% oxygen should have a PaO2 of around 100.

Then, if the patient breathes 50%, we know that his PaO2 should be about 250. It is not; therefore, something is very wrong.

However, if you look at the oxygen-hemoglobin dissociation curve, a PaO2 of 100 and 250 has a SaO2 of 100% because both provide enough oxygen molecules to fill all the binding sites of Hgb.

So, in this case, the saturation of O2 does not help us much.