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Arterial blood gas measurement is a blood test that is performed to determine the concentration of oxygen, carbon dioxide and bicarbonate, as well as the pH, in the blood. Its main use is in pulmonology, as many lung diseases feature poor gas exchange, but it is also used in nephrology (kidney diseases) and electrolyte disturbances. As its name implies, the sample is taken from an artery, which is more uncomfortable and difficult than venipuncture.
Obtaining and processing the sample
Arterial blood is taken from any easily accessible artery (typically either radial, brachial or femoral) or out of an arterial line. The syringe is prepacked and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again.
Once the sample is obtained, care should be taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results. The sealed syringe is taken to a blood gas monitor. If the sample cannot be immediately analyzed it should be chilled in an ice bath to slow metabolic processes that may also cause inaccuracy. The machine aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide and the bicarbonate concentration, as well as the oxygen saturation of hemoglobin. Some machines can also measure glucose, lactate, hemoglobin and limited electrolytes (sodium and potassium).
The results are usually available within five minutes, and are now ready for interpretation.
Interpretation
The pH will indicate if a patient is acidotic (pH <7.35) or alkalotic (pH > 7.45), while the oxygen and carbon dioxide partial pressure and bicarbonate readings will indicate whether the cause is respiratory problems (increased/decreased CO2), or metabolic (such as ketoacidosis), and whether or not there has been any compensation by the body.
Other results can determine oxygen saturation, blood sugar status, and other metabolites in the blood. Various analyzers may also test hemoglobin, electrolyte, lactate and others metabolites (urea, creat, gluc) levels.
Contamination with room air will result in abnormally low carbon dioxide and (generally) high oxygen levels. Delays in analysis (without chilling) will result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.
Reference ranges
These are typical reference ranges, although various analysers and laboratories may employ different ranges.
| Analyte | Range |
|---|---|
| pH | 7.35 - 7.45 |
| pO2 | 10.0-13.0 kPa or 75-100 mmHg |
| pCO2 | 4.7-6.0 kPa or 35-45 mmHg |
| HCO3- | 22 - 30 mmol/l |
| Base excess | -2 to +2 mmol/l |
Lactate levels are often included on blood gas machines in neonatal wards; infants often have elevated lactic acid.
"Arterial blood gas"