Arterial blood gases: what it is, reference values, results


What is arterial blood gas analysis?

“Gaso” refers to gas and “ metria ” to measure. Thus, in an objective way, gasometry performs the measurement of gases in the blood.

In general, the test is indicated in very specific cases, such as patients in severe condition, in the ICU or hospitalized with suspicion of serious infections, such as kidney. In cases where oxygen-based treatments are being carried out, the test also helps to verify the evolution of the condition and the therapeutic efficiency.

As it is an exam indicated mainly in severe cases, its request is not frequent outside the hospital environment.

Used mainly to assess lung conditions, the test allows to assess whether the oxygen supplied to the tissues and the collection of carbon dioxide are at ideal levels. The levels of pH (acidity), CO2 (carbon dioxide), O2 saturation (oxygen) and normal oxygenation are measured. The results indicate the correct functioning of ventilation and the acid-base balance in the blood.

Arterial blood gases collect a blood sample from the artery, as venous blood is low in O2 and becomes inefficient to perform the dosage. The most recurrent artery is the radial (on the wrist), however the femoral (on the groin) and brachial (on the arm) can be used.

The procedure is quick, lasting less than 1 minute, and allows the diagnosis and monitoring of pulmonary, metabolic and renal diseases.

The blood

The circulatory system, also called the cardiovascular system, is composed of the heart, blood, arteries, veins and capillaries.

Blood is classified as a connective tissue of liquid consistency. Its composition includes several cell groups and its main function is to transport nutrients and oxygen, which are essential to maintain the body’s functions.

Approximately 45% of the blood is composed of red blood cells (erythrocytes), white blood cells (leukocytes) and platelets. The remainder is formed by plasma, which is the liquid part of the tissue.

Plasma has a slightly yellowish color, consisting mostly of water (90%) and the remainder of mineral salts and proteins . It is there that white and red blood cells are suspended and can be transported.

Platelets, however, are not cells, but cellular fragments, also called thrombocytes. They act to control bleeding, as they perform blood clotting.

Red blood cells, or red blood cells, are the cells most present in the human body. Men have an average of 4.5 million per mm³ and women have an average of 5.5 per mm³. The production occurs in the bone marrow and, inside, is present hemoglobin, which is the protein capable of transporting oxygen.

Leukocytes, known as white blood cells, are also produced in the bone marrow. The main function is the defense of the organism, making up the immune system.

The structure of leukocytes is greater than that of red blood cells, however, the amount in the body is much less. When the organism is attacked by bacteria, viruses, toxic substances or foreign agents, the volume of leukocytes increases significantly to protect the organism.

The arteries

The arteries carry oxygenated blood and nutrients to the body’s tissues. The transport has high pressure, because it is necessary that the liquid travels throughout the body, reaching the cells.

The circulatory system has pulmonary arteries (carrying blood to the lungs), systemic arteries (carrying blood to capillaries and blood vessels), aorta (leaving the left ventricle and branching into other arteries) and arterioles (carrying blood to capillaries).

Acid-base balance

There are 3 ways that the body regulates hydrogen and maintains the acid-base balance:

The buffer system , which occurs naturally and automatically in the body to avoid sudden changes in pH. The action takes place through chemical releases that combine elements and avoid acidity or alkalinity (changes in blood pH).

Lung function occurs with the release of carbon dioxide through exhalation, in which the blood transports CO2 gas to the lungs and is then released.

And finally, kidney function , in which the kidneys act as blood filters and excrete excess acids and bases. The regulatory process is slower than the others, so the return of blood balance by the renal system can take days.

The pH

The pH, or Hydrogenionic potential, indicates the blood acid-base balance.

The more acidic the blood is, the lower the pH value will be, indicating a deficiency in oxygen levels. The more alkaline (or basic), the higher the oxygen concentration and, consequently, the higher the pH value.

In general measurements, a pH can vary between 0 and 14, where 7 is considered as a neutral value. Below that, you have an acidic substance and above 7, you have an alkaline substance.

The blood pH should be between 7.35 and 7.45. When there are acidic or alkaline changes, disorders, pathologies or anomalies of the organism are indicated.

Food, emotional stress and immune reactions are some factors that can interfere with values.

Alkalosis (high pH) does not always show symptoms. Usually the complaints are due to the disease that is causing the pH change. But the change can cause:

  • Muscle spasms;
  • Weakness;
  • Fatigue;
  • Headache;
  • Dizziness and seizures.

The symptoms of acidosis (low pH) are more noticeable and can cause:

  • Shortness of breath or difficulty breathing;
  • Somnolence;
  • Nausea and vomiting;
  • Mental confusion.

Gas transport

Oxygen can reach cells in two ways: dissolved in plasma or combined with hemoglobin, through red blood cells.

The combination occurs in a reversible (non-permanent) manner, forming oxyhemoglobin. When combined with the O2 molecule, the blood starts to be oxygenated and called arterial blood .

When oxygen-rich blood reaches the blood capillaries (small vessels), the O2 molecules (oxygen) detach themselves from hemoglobin and diffuse into the cells of organs and tissues.

The transport of CO2 (carbon dioxide) is more complex than that of oxygen. A small part dissolves in the plasma (about 8%), another binds to hemoglobin (about 11%), while the rest is transported in the form of bicarbonate ions that dissolve in the plasma.

Venous blood (rich in CO2) is sent to the lungs and, upon reaching the organs, carbon dioxide is eliminated by the pulmonary alveoli.

In summary, from the left ventricle arterial (oxygenated) blood flows to the tissues of the body, distributing O2 and absorbing CO2.

The venous (deoxygenated) blood reaches the right atrium of the heart through the vena cava, being sent from the right ventricle to the lungs, where it undergoes the oxygenation process and returns to the left atrium.

What is blood gas used for?

Arterial blood gases make it possible to verify and monitor metabolic and respiratory changes, accurately assessing the oxygen and carbon dioxide levels in the blood, identifying problems or organic malfunctions.

When there are symptoms such as nausea, difficulty breathing and mental confusion, which can be indicative of inadequate concentrations of gases in the blood, the test can indicate the presence of lung, kidney, metabolic diseases or, still, injuries that affect the respiratory tract.

Other cases in which the exam is required may involve:

  • Presence of obstructive pulmonary diseases;
  • Cystic fibrosis;
  • Severe diabetes decompensation;
  • Sleep disorders;
  • Serious infections;
  • Serious lung problems, such as cystic fibrosis;
  • Drug overdose;
  • Heart or kidney failure.

In addition to the diagnosis, arterial blood gases are performed to check the organism’s response and evolution to treatment. Therefore, in hospitalized patients, the test may be requested more than once.

How is it done?

The examination consists of the removal of blood (puncture) performed in an artery, preferably in the radial one.

The collection region needs to have blood flow preserved (verified by the Allen test) and not have lesions or bruises in the tissue, as they can alter the results due to the concentration of white blood cells.

The places to perform the puncture are:

  • Radial artery: located in the region of the radial bone, close to the wrist, and running parallel to the bone, being easily palpable;
  • Femoral artery: It is close to the groin, between the middle and medial thirds of the inguinal ligament (close to the pubis). It is an alternative to blood gas analysis, but it tends to be less palpable than the radial artery;
  • Brachial artery: located above the elbow, inside the arm.

The syringe must contain a small amount of heparin (anticoagulant substance), about 0.1 ml, to react with the blood sample and the collection follows the common procedure, with the patient seated (except hospitalized patients who need to lie down).

Identifying the artery, the needle falls on the skin at an angle of 30º (the angulation in the brachial and femoral punctures may vary), and aspirates approximately 2.0 mL of blood.

After removing the needle, the puncture site is pressed to stop the bleeding.

Air bubbles can appear in the sample and change the results, so it is necessary for the nurse to remove them and move the blood tube, homogenizing the blood.

In some cases, anesthetics are administered to reduce recurrent pain and discomfort in arterial collections. In this situation, before the puncture, an adrenaline-free anesthesia is applied subcutaneously (a small syringe deposits the substance just under the skin), reducing anxiety and hyperventilation (accelerated breathing), which can alter the results of the exam.

Allen test

Before starting the collection, it is necessary to perform the Allen test, a procedure that ensures the proper flow of blood in the artery. The process is quick and checks the conditions to perform the puncture.

Arterial blood flow must be compressed by pressing on the pulse in the ulnar and radial region (region close to the pulse). The patient should open and close the hand approximately 5 times or keep it tightly closed until the skin pales. Then the blood flow is released and the staining returns (perfusion) in a few seconds.

If the return is fast, the test is positive and the artery is ready to be punctured. In cases where there is a delay in blood return, the test indicates non-preserved collateral flow and the puncture must be performed in another artery.

When should it be done?

Arterial blood gases should be performed when there are suspicions of metabolic, pulmonary or renal diseases that alter the acid-base balance or cause respiratory problems, in addition to being used to monitor the condition.

Patients hospitalized with the use of oxygen therapy (use of oxygen as treatment) undergo the exam in order to verify the functionality and progress of the therapy. The evaluation is also used to check the pulmonary and renal conditions in hospitalized patients who are going to undergo surgery, especially in cardiopulmonary or brain surgeries.

The examination can also be ordered after accidents with the presence of head or neck injuries, as soon as the traumatic condition can affect breathing. In hospitalized patients in severe condition, arterial blood gases should be performed when there is:

  • Chronic obstructive pulmonary diseases;
  • Cystic fibrosis;
  • Uncontrolled diabetes;
  • Serious infections;
  • Heart or kidney failure;
  • Drug overdose.

In newborns, gas analysis is performed on the umbilical cord, which may indicate respiratory problems, helping to prevent and prompt intervention.


The test is contraindicated in patients diagnosed with severe peripheral arterial disease (when blood flow is reduced due to blockages in the arteries, causing a decrease in O2 in the blood).

The use of anticoagulants, blood thinners or when there are bleeding problems are limiting factors for the performance of blood gas analysis.

Pre- and post-examination care

Not much care is needed before the exam. In general, it is indicated that the patient is at rest for at least 10 minutes, in order to avoid respiratory changes due to physical effort.

Some diagnostic centers require a 3-hour fast, however, it is not a mandatory procedure. One should consult and follow the guidelines provided by the laboratory.

In those hospitalized with oxygen, it is recommended to turn off the O2 supply and wait about 30 minutes. When suspension is not possible, oxygen therapy must be taken into account when interpreting the result, as there may be interference with the values ​​presented.

After the exam, it is indicated to press the collection site and avoid physical efforts and weight lifting for at least 1 hour.

Results of

In the exam, in general, the following values ​​are measured:

  • pH: evaluates the hydrogen ions (H +) in the blood;
  • PaCO (partial pressure of carbon dioxide): measures the amount of carbon dioxide that dissolves in the blood and the intensity with which it reaches the alveoli;
  • PaO (partial pressure of oxygen): measures the pressure of oxygen and how is the movement of O2 molecules between the alveoli (lung structure) and capillaries (blood vessels);
  • HCO (Bicarbonate): measures the levels of bicarbonate in the blood. The substance acts in the regulation of the pH, preventing the acid-base imbalance;
  • SaO  (Oxygen saturation): Measures the amount of hemoglobin that is bound to oxygen molecules.

The reference values ​​must follow those adopted by the laboratory. However, you usually have:

  • pH: 7.35 – 7.45;
  • PCO2  (partial pressure of carbon dioxide) : 35mmHg – 45 mmHg;
  • PaO 2 (partial pressure of oxygen): 80mmHg – 100 mmHg;
  • HCO3 (Bicarbonate): 22 – 26 mEq / L;
  • SaO 2 (Oxygen saturation): greater than 95%.

The results should not be interpreted in isolation, it is important to consider the particularities of each patient. In addition, a change must be analyzed together with the other values.

Variations are classified according to the type (acidosis or alkalosis) and according to the primary cause (metabolic or respiratory).

In summary, you can have:

  • Metabolic acidosis : the pH is between 7.35 – 7.45. Bicarbonate (HCO3) is low . The carbon dioxide pressure (PaCO2) is low . Symptoms are: diabetic ketoacidosis, shock, renal failure;
  • Metabolic alkalosis : the pH is between 7.35 – 7.45. Bicarbonate (HCO3) is high . The carbon dioxide pressure (PaCO2) is high . Symptoms are: low potassium concentration (hypokalemia), chronic vomiting;
  • Respiratory acidosis : the pH is between 7.35 – 7.45. Bicarbonate (HCO3) is high . The carbon dioxide pressure (PaCO2) is high . Symptoms are: Pulmonary diseases (COPD, pneumonia);
  • Respiratory alkalosis : the pH is between 7.35 – 7.45. Bicarbonate (HCO3) is low . The carbon dioxide pressure (PaCO2) is low . Symptoms are: Hyperventilation, pain, anxiety, stress.

Thus, there are 4 possible conditions: respiratory acidosis and respiratory alkalosis (caused mainly by pulmonary or respiratory disorders that interfere with the release of carbon dioxide), and metabolic acidosis and metabolic alkalosis (arising from the imbalance in the production of acids or bases and in the excretory function of the kidneys).

Respiratory acidosis

It is characterized by low pH and high PaCO2, resulting from breathing difficulties. In this case, little oxygen is absorbed and little carbon dioxide is eliminated.

Among the main causes are pneumonia , chronic obstructive pulmonary disease and excessive sedation are the most frequent.

Respiratory alkalosis

High pH and low PaCO2 levels indicate respiratory alkalosis, caused by hyperventilation. Usually, respiratory changes of this type are caused by emotional disturbances (anxiety and phobia), or pain.

Metabolic acidosis

When there is a decrease in pH and bicarbonate, there is metabolic acidosis, mainly resulting from decompensated diabetes ( diabetic ketoacidosis ), renal failure and shock.

Metabolic alkalosis

In the presence of high pH and bicarbonate, there is metabolic alkalosis. The condition can be caused by hypokalemia (low levels of potassium in the blood), chronic vomiting that causes the loss of gastric acid and excessive intake of bicarbonate.

What can change the exam?

Some factors can momentarily influence the results and must be reported so that the diagnosis is not mistaken:

  • Severe anemia;
  • Fever and hyperthermia;
  • Oxygen use before the exam;
  • Smoking or inhaling smoke (can alter blood oxygen concentration);
  • Have practiced intense physical activity a few minutes before the exam.


The test, in general, does not pose any health risks. Cases that present complications usually involve the use of blood thinners or difficulty in clotting, which can make it difficult for bleeding to stop after puncture.

Edemas at the collection site, dizziness and fainting can occur, but without future complications.


The arterial blood gas examination, on average, costs from R $ 10.00 and can reach R $ 60.00, depending on the laboratory.

As there is no need for special equipment, which can be performed in most laboratories, arterial blood gases are normally within the coverage of health plans.

Complementary exams

If changes in arterial blood gases are found, other tests of bacteriology, hematology and biochemistry may be requested to verify the causes.

Generally, tests are requested to help determine the causes of changes in pH and oxygenation. Among those requested may be:

  • Dosage of electrolytes;
  • Glucose and glycated hemoglobin;
  • Urea and creatinine;
  • Blood culture;
  • Blood count;
  • Ions.

Common questions

Does it hurt to take the exam?

Blood collection from the artery is usually painful .

The intensity of the pain is relative and depends on the sensitivity of each patient, but in general, the laboratories suggest or enable the use of local anesthetics to ease the discomfort.

What is the difference between arterial and venous blood gases?

Gasometry can be performed with the collection of arterial or venous blood.

The main difference is that the arterial blood allows a better assessment of the supply and transport of oxygen to the tissues, while the venous blood will give results referring only to metabolic functions.

Therefore, venous blood gas analysis is not efficient when the objective is pulmonary analysis.

Can pregnant women get tested?

-Yeah . The exam is not contraindicated for pregnant or lactating women. The blood gas analysis performs a blood collection, without altering the organic functions.

Arterial blood gas analysis is a quick exam and in general quite simple. Through a blood sample, collected from the artery, it is possible to identify changes in gas and blood pH, as well as the correlation with other metabolic, pulmonary or renal dysfunctions.

The results allow the doctor to make a correct diagnosis and evaluate the clinical conditions of the patients, the evolution of the condition and the effectiveness of the treatment.