What Is the Partial Pressure of Oxygen (PaO2) Test?

What to expect when undergoing this test

Oxygen therapy
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The partial pressure of oxygen, also known as PaO2, is a measurement of oxygen pressure in arterial blood. It reflects how well oxygen is able to move from the lungs to the blood, and it is often altered by severe illnesses.

The PaO2 is one of the components measured in an arterial blood gas (ABG) test—which also reports oxygen (O2) saturation, bicarbonate (HCO3), the partial pressure of carbon dioxide (CO2), and the pH level in red blood cells.

Purpose of Test

The PaO2 test can be used to assess the effects of breathing problems on oxygen supply, especially in a hospital setting or during an episode of severe respiratory distress.

The results are often used to determine whether emergency treatment—such as oxygen supplementation or mechanical breathing support—are needed. And PaO2 values can also be used (along with other tests) to help in the diagnosis of a number of chronic medical conditions.

Some conditions for which a PaO2 test may be needed include:

In some situations, such as when oxygen therapy or ventilation (mechanical breathing support) have been started, a repeat PaO2 test is used to assess whether a respiratory condition is improving or worsening, and whether the treatment needs to be adjusted.

Risks and Contraindications

There are very few risks associated with a PaO2 test. Because the test uses blood obtained with an arterial puncture instead of blood drawn from a vein, there is a slightly higher risk of bleeding or bruising.

If you are taking a blood thinner or you have a bleeding disorder, your medical team may take special precautions when you have this test, such as monitoring for bleeding from your puncture site for a longer period of time or avoiding the test unless absolutely necessary.

Before the Test

If you're having a PaO2 test as part of an outpatient diagnostic evaluation, you will make an appointment to do so.

A PaO2 test may need to be done urgently if you are having a respiratory emergency. When you are hospitalized and having this test to monitor your treatment or to follow the progression of your illness, it may be scheduled to be done at a certain time in relation to your treatment (such as when your oxygen support is being adjusted).

Timing

Drawing blood for this test takes about five minutes, but you should allot about an hour if you are having the test done in an outpatient setting. This will give you time for registration, waiting for your turn, and assurance that your puncture site is not bleeding.

Location

Outpatient PaO2 tests are done at a doctor's office, or a clinic or lab where blood draws are done.

If you have this test in the hospital or during an emergency situation, you will be able to remain in your hospital bed during the arterial blood draw.

What to Wear

You can dress comfortably to have your PaO2 test. Often, blood is drawn from the radial artery, which runs along the inner surface of the wrist, so it is a good idea to wear either a short-sleeve shirt or a shirt with sleeves that are easy to pull up.

Food and Drink

You can eat and drink whatever you want prior to this test because your diet will not affect the results.

Cost and Health Insurance

When possible, it is a good idea to check with your doctor or health insurer about whether the cost of the test will be covered by your health plan and if you have a co-pay.

The full out-of-pocket cost for a PaO2 can range between $12 and nearly $150. When paying for the test yourself, it is worthwhile to ask about the cost ahead of time and to consider shopping around for price if you have the time to do so.

What to Bring

When you go for your PaO2 test, bring your health insurance card, a form of identification, and a form of payment. It is also a good idea to have someone with you who can drive you home. You may experience mild wrist soreness for a day or so after the test, which can interfere with driving.

During the Test

Whether you are in a hospital setting or having your PaO2 measured as an outpatient, your procedure will be performed by a doctor or a specialized nurse or technician.

Pre-Test

First, you will have your arterial pulse checked. While blood is usually drawn from the radial artery in your wrist, that's not always ideal. If your pulse is very weak due to severe illness or blood loss, the brachial artery in your arm or the femoral artery in your groin may be used instead, as they are larger.

Sometimes a catheter (tube) is placed in an artery during surgery or during a prolonged illness. In this case, your PaO2 can be checked without a needle puncture.

Throughout the Test

The skin around the intended puncture site will be cleaned, usually with an alcohol pad. A small needle, which is attached to a tube, will then be inserted into your artery. The pressure or pain is a bit uncomfortable—more uncomfortable than the pain felt when you have blood drawn from a vein. However, the test is tolerable for most people.

Typically, blood pumps rapidly through arteries. Because of this, the volume of blood needed for your test can be collected quickly. Once enough blood is collected, the needle is removed and cotton or gauze is placed over the puncture site.

Your practitioner will place pressure over the puncture site to help stop the bleeding.

Post-Test

After your test, you will have a bandage placed over your puncture wound. You might also need to have gauze wrapped around your wrist for a few hours.

After the Test

Your wrist may feel a bit sore for several days after your test. You may need some assistance with eating or with other tasks.

Many people are able to resume normal activities after having a PaO2 test. Still, it is best to avoid heavy lifting with the arm used for the blood draw until a few days after your procedure.

Managing Side Effects

If you feel sore, you can gently place an ice pack over your wrist. Ask your doctor if you can use mild pain relievers for soreness and pain. Be careful with respect to medications that are also blood thinners (like aspirin or ibuprofen), as they can trigger additional bleeding.

When to Call Your Doctor

If you experience any of the following, be sure to let your doctor know immediately:

  • Severe wrist, hand, or arm pain
  • Persistent bleeding
  • Pale or bluish color of your fingers or hand
  • Numbness, tingling, or diminished sensation of your hand or fingers
  • Swelling of your fingers, hand, or arm
  • Weakness of your fingers, hand, or arm

If you have you arterial blood collected from an artery other than your radial artery, be sure to tell your doctor if you experience these symptoms near the area of your puncture site.

Interpreting Results

Each breath delivers air to the alveoli in your lungs. There, oxygen and carbon dioxide are transferred between the lungs and the blood. Because oxygen pressure is higher in the alveoli than it is in the adjacent capillaries (tiny blood vessels), it flows into the capillaries.

When the body is functioning normally, PaO2 is between 75 and 100 mmHg (at sea level). A result in this range means a sufficient amount of oxygen flowing from the alveoli to the blood.

If your PaO2 is below the normal range, that is not the case. Lung diseases and breathing problems can increase the risk of developing low PaO2.

Factors Affecting PaO2 Levels

A number of factors can lower your PaO2 levels, including:

  • The partial pressure of oxygen in the air you inhale: At higher altitudes (such as in mountainous areas), the decrease in atmospheric pressure reduces oxygen availability—and oxygen pressure in your lungs.
  • Obstructions in your lungs caused by conditions like emphysema or pulmonary fibrosis
  • Damage to the lungs due to trauma or cancer
  • A reduction in the concentration of hemoglobin in your blood cells: Having iron deficiency anemia means that your red blood cells are not able to effectively carry oxygen molecules.
  • Heart disease

All of these conditions diminish the oxygen pressure in your lungs, and this is reflected in the arterial oxygen pressure as measured by PaO2.

Follow-Up

You may need to have a follow up PaO2 test, especially if your symptoms do not improve. You may also need a follow-up test when it is time to change your oxygen supply or ventilation assistance.

Often, when respiratory support is decreased due to improvement, PaO2 is rechecked to confirm that breathing and oxygenation remain at optimal levels even with less support.

A Word From Verywell

Your oxygen pressure is an important reflection of your heart, lung, and blood function. While this test is fast, it can be mildly uncomfortable. Try not worry if you need to have a blood sample taken for measurement of a PaO2. The results will guide your treatment, and resolution of your breathing problem should make you more comfortable.

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