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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.

Contents

Obtaining and processing the sample

Reference ranges and interpretation

References

External links

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 for interpretation within five minutes.

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 acidemic (pH < 7.35; H+ >45) or alkalemic (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 and 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 indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO3- indicates metabolic acidosis, a high HCO3- indicates metabolic alkalosis.
Base excess	-2 to +2 mmol/l	The base excess indicates whether the patient is acidotic or alkalotic. A negative base excess indicates that the patient is acidotic. A high positive base excess indicates that the patient is alkalotic.

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.

References

1. Altitude oxygen calculator Kenneth Baillie and Alistair Simpson - Online interactive oxygen delivery calculator

External links

★ Alan Grogono's comprehensive guide to acid-base physiology

★ An online model of arterial blood gas changes with respiration

★ Bloodgas.org - a knowledge site about blood gas and critical care testing

★ Dr. Martin's powerpoint presentation on blood gas interpretation

★ Arterial Blood Gas Interpretation Course

★ Online ABG calculator.