Arterial blood gas

An arterial blood gas (ABG) is a blood test that is performed specifically on blood from an artery. It involves puncturing an artery with a thin needle and syringe and drawing a small volume of blood. The most common puncture site is the radial artery at the wrist, but sometimes the femoral artery in the groin or other sites are used. It is usually somewhat more uncomfortable and difficult to perform compared to the regular technique of drawing blood from a vein (venipuncture). The blood can also be drawn from a previously placed arterial catheter.

The test is used to determine the concentrations of gases like carbon dioxide and oxygen, bicarbonate, as well as the pH of the blood. Many blood gas analyzers will simultaneously determine the concentration of lactate, hemoglobin, several electrolytes, oxyhemoglobin, carboxyhemoglobin and methemoglobin. Thus, it provides more information than pulse oximetry. Its main use is in pulmonology, to determine gas exchange levels in the blood related to lung function, but it is also used in nephrology, and used to evaluate metabolic disorders such as acidosis and alkalosis.

Combinations of disorders can be complex and difficult to interpret, so calculators cite web | author=Baillie K| title=Arterial Blood Gas Interpreter | url=http://www.altitude.org/calculators/ABGcalculator.htm | publisher= Apex (Altitude Physiology Expeditions) | accessdate=2007-07-05 - Online arterial blood gas analysis] , nomograms, and rules of thumb [cite journal |author=Baillie JK |title=Simple, easily memorised "rules of thumb" for the rapid assessment of physiological compensation for acid-base disorders |journal=Thorax |volume=63 |issue=3 |pages=289–90 |year=2008 |pmid=18308967 |doi=10.1136/thx.2007.091223] are commonly used.

Extraction and analysis

Arterial blood for blood gas analysis is usually extracted by a phlebotomist, nurse, or respiratory therapist. [cite journal |author=Aaron SD, Vandemheen KL, Naftel SA, Lewis MJ, Rodger MA |title=Topical tetracaine prior to arterial puncture: a randomized, placebo-controlled clinical trial |journal=Respir Med. |volume=97 |issue=11 |pages=1195–1199 |year=2003 |pmid=14635973 |doi=10.1016/S0954-6111(03)00226-9 ] Blood may be taken from an easily accessible artery (typically the radial artery, but during unusual or emergency situations the brachial or femoral artery may be used), or out of an arterial line.

The syringe is pre-packaged 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 is 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 is chilled in an ice bath in a glass syringe to slow metabolic processes which can cause inaccuracy. Samples drawn in plastic syringes should not be iced and should always be analyzed within 30 minutes. [cite journal |author=Mahoney JJ, Harvey JA, Wong RL, Van Kessel AL |title=Changes in oxygen measurements when whole blood is stored in iced plastic or glass syringes |journal=Clin Chem. |volume=37 |issue=7 |pages=1244–1248 |year=1991 |pmid=1823532 ]

The machine used for analysis aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide. The bicarbonate concentration is also calculated. These results are usually available for interpretation within five minutes.

Standard blood tests can also be performed on arterial blood, such as measuring glucose, lactate, hemoglobins, dys-haemoglobins, bilirubin and electrolytes.

Reference ranges and interpretation

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

Contamination with room air will result in abnormally low carbon dioxide and (generally) normal oxygen levels. Delays in analysis (without chilling) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.

Lactate level analysis is often featured on blood gas machines in neonatal wards, as infants often have elevated lactic acid.

References

External links

* [http://www.acid-base.com/ Alan Grogono's comprehensive guide to acid-base physiology]
* [http://www.altitude.org/calculators/oxygencalculator/oxygencalculator.htm An online model of arterial blood gas changes with respiration]
* [http://www.bloodgas.org Bloodgas.org - a knowledge site about blood gas and critical care testing]
* [http://www.lakesidepress.com/ABGinterpretation.ppt Dr. Martin's powerpoint presentation on blood gas interpretation]
* [http://eleapcourses.com/arterial-blood-gas-interpretation-866.html Arterial Blood Gas Interpretation Course]
* [http://www.altitude.org/calculators/ABGcalculator.htm Online ABG calculator]
* [http://www.vectors.cx/med/apps/abg.cgi Interactive ABG quiz]
* [http://www.rtcorner.net RT Corner (Educational Site for RT's and Nurses)] at rtcorner.net
* [http://www.qub.ac.uk/cskills/ABG/ABG.htm Graphical demonstration of obtaining ABG samples]