What is blood gas analysis?

Definition of blood gases

Blood gas analysis (blood gasometry) involves measuring acidity, oxygen, and carbon dioxide levels in arterial blood.

The examination evaluates lung exchange, including abnormal changes in oxygen and carbon dioxide concentrations in the arterial blood going to the tissues. Indeed, when blood passes through the lungs, it is enriched with oxygen and depleted in carbon dioxide.

Blood gas analysis also assesses a patient’s acid-base balance.

The parameters measured are as follows:

  • PaO2 (partial oxygen pressure): it evaluates how oxygen is able to move from the lungs to the blood. It, therefore, reflects the amount of oxygen carried by the blood and delivered to the organs.
  • PaCO2 (partial pressure of carbon dioxide): it corresponds to the residual amount of carbon dioxide in the arterial blood after the removal of excess carbon dioxide in the lungs.
  • pH: it reflects the concentration of hydrogen and measures the acidity of the blood.
  • HCO3-(bicarbonate): it “buffers” the pH of the blood and thus helps maintain an acid-base balance.
  • SaO2 (oxygen saturation): it is used to evaluate the amount of oxygen attached to hemoglobin.

Why do a blood gas analysis?

The doctor prescribes a blood gas test:

  • in case of breathing difficulties: shortness of breath, hypo or hyperventilation, asthma
  • to monitor the proper functioning of a lung disease treatment
  • in case of alteration of the state of consciousness
  • to ensure that a hospitalized person receives the right amount of oxygen
  • or to measure the acid-base level in patients suffering from heart failure, kidney failure, sleep disorders, infection, or after a drug overdose

What results can be expected from a blood gas analysis?

The examination consists of a blood test from an artery. In general, it is the radial (wrist), humeral (arm), or femoral (groin) artery. Once the sample has been taken, a gauze or cotton compress should be placed and the puncture site compressed firmly for 5-10 minutes.

Note that it is important to breathe normally during blood collection.

The normal values of blood gases are as follows:

  • PaO2: greater than 80 mm Hg (mm mercury)
  • PaCO2: between 35 and 45 mm Hg
  • pH: between 7.35 and 7.45
  • HCO3-(bicarbonates): between 22 and 28 mmol/l (millimoles per litre)
  • SaO2 (oxygen saturation): between 95 and 100%

Note that depending on the techniques used by the analysis laboratory, the results may differ. These can also vary depending on altitude.

The values of blood gases are not enough to make an accurate diagnosis but can be a sign of problems in the lungs, and kidneys, or a suspect of metabolic disease.

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