Arterial blood gas

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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.

1 Obtaining and processing the sample
2 Reference ranges and interpretation=
- 2.1 References
- 2.2 External links

[edit] 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 analyzer. 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. The bicarbonate concentration is calculated. Some blood gas analyzers can also measure glucose, lactate, hemoglobins, dys-hemoglobins, oxygen saturation, bilirubin and electrolytes (sodium, potassium, calcium and chloride).

The results are usually available within five minutes, and are now ready for interpretation.

[edit] Reference ranges and interpretation=

These are typical reference ranges, although various analysers and laboratories may employ different ranges.

Analyte	Range	Interpretation
pH	7.35 - 7.45	The pH or H⁺ indicates if a patient is acidotic (pH < 7.35; H⁺ >45) or alkalotic (pH > 7.45; H⁺ < 35).
H⁺	35 - 45 nmol/l	See above.
pO₂	10.0-13.0 kPa or 75-100 mmHg	Normal pO₂ is 70-100. Values below 60 may require immediate action and possibly mechanical ventillation.
pCO₂	4.7-6.0 kPa or 35-45 mmHg	The carbon dioxide partial pressure (PCO₂) indicates a respiratory problem: for a constant metabolic rate, the PCO₂ is determined entirely by ventilation.^[1] A high PCO₂ (respiratory acidosis) indicates underventilation, a low PCO₂ (respiratory alkalosis) hyper- or overventilation.
HCO₃^-	22 - 30 mmol/l	The HCO₃^- ion or base excess indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO₃^- or negative base excess indicates metabolic acidosis, a high HCO₃^- or high positive base excess, metabolic alkalosis.
Base excess	-2 to +2 mmol/l	See above.

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.

Lactate levels are often included on blood gas machines in neonatal wards; infants often have elevated lactic acid.

[edit] References

^ Kenneth Baillie and Alistair Simpson. Altitude oxygen calculator. Apex (Altitude Physiology EXpeditions). Retrieved on August 10, 2006. - Online interactive oxygen delivery calculator