Arterial blood gas
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/* External links */ obtaining samples
{{Refimprove|date=November 2007}}
{{mergefrom|Arterial blood gas sampling|date=November 2007}}
An '''arterial blood gas''' (also called "ABG'S") is a [[blood test]] that is performed specifically on arterial blood, to determine the concentrations of [[carbon dioxide]], [[oxygen]] and [[bicarbonate]], as well as the [[pH]] of the [[blood]].
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 disorder]]s such as [[acidosis]] and [[alkalosis]]. Combinations of disorders can be complex and difficult to interpret, so calculators <ref name="ABG interpreter">{{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</ref>, nomograms, and rules of thumb<ref>{{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}}</ref> are commonly used.
As its name implies, the sample is taken from an [[artery]], which is more uncomfortable and difficult than [[venipuncture]].
==Extraction and analysis==
Arterial blood for blood gas analysis is usually extracted by a [[phlebotomist]], [[nurse]], or [[respiratory therapist]].<ref>{{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 }}</ref> Blood may be taken from an easily accessible artery (typically the [[radial artery]], but during unusual or emergency situations the [[brachial artery|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.<ref>{{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 }}</ref>
The machine used for analysis aspirates this blood from the syringe and measures the [[pH]] and the [[partial pressure]]s 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]], [[lactic acid|lactate]], [[hemoglobins]], dys-haemoglobins, [[bilirubin]] and [[electrolyte]]s.
==Reference ranges and interpretation==
These are typical reference ranges, although various analysers and laboratories may employ different ranges.
{| class="wikitable"
! Analyte
! Range
! Interpretation
|-
| pH
| 7.35 - 7.45
| The [[pH]] or H<sup>+</sup> indicates if a patient is acidemic (pH < 7.35; H<sup>+</sup> >45) or alkalemic (pH > 7.45; H<sup>+</sup> < 35).
|-
|H<sup>+</sup>
| 35 - 45 n[[Mole (unit)|mol]]/[[litre|l]] (n[[Concentration#Molarity|M]])
| See above.
|-
| PO<sub>2</sub>
| 9.3-13.3 [[kPa]] or 80-100 [[mmHg]]
| Normal PO<sub>2</sub> is 70-100 mmHg (age-dependent). |A low O<sub>2</sub> indicates that the patient is not respiring properly, and is hypoxemic. At a PO<sub>2</sub> of less than 60 mm Hg, supplemental oxygen should be administered. At a PO<sub>2</sub> of less than 26 mm Hg, the patient is at risk of death and must be oxygenated immediately.
|-
| PCO<sub>2</sub>
| 4.7-6.0 kPa or 35-45 mmHg
| The carbon dioxide and partial pressure (P[[carbon dioxide|CO<sub>2</sub>]]) indicates a [[respiration (physiology)|respiratory]] problem: for a constant metabolic rate, the P[[carbon dioxide|CO<sub>2</sub>]] is determined entirely by ventilation.<ref name="02_calc">{{cite web | author=Baillie K, Simpson A | title=Altitude oxygen calculator | url=http://www.altitude.org/calculators/oxygencalculator/oxygencalculator.htm | publisher= Apex (Altitude Physiology Expeditions) | accessdate=2006-08-10}} - Online interactive oxygen delivery calculator</ref> A high P[[carbon dioxide|CO<sub>2</sub>]] ([[respiratory acidosis]]) indicates underventilation, a low P[[carbon dioxide|CO<sub>2</sub>]] ([[respiratory alkalosis]]) hyper- or overventilation. PCO<sub>2</sub> levels can also become abnormal when the respiratory system is working to compensate for a metabolic issue so as to normalize the blood pH.
|-
| HCO<sub>3</sub><sup>-</sup>
| 22–26 mmol/l
| The HCO<sub>3</sub><sup>-</sup> ion indicates whether a [[metabolism|metabolic]] problem is present (such as [[ketoacidosis]]). A low HCO<sub>3</sub><sup>-</sup> indicates [[metabolic acidosis]], a high HCO<sub>3</sub><sup>-</sup> indicates [[metabolic alkalosis]]. HCO<sub>3</sub><sup>-</sup> levels can also become abnormal when the kidneys are working to compensate for a respiratory issue so as to normalize the blood pH.
|-
| [[Standard bicarbonate concentration|SBC<sub>e</sub>]]
| 21 to 27 mmol/l
| the bicarbonate concentration in the blood at a [[carbon dioxide|CO<sub>2</sub>]] of 5.33 kPa, full oxygen saturation and 37 [[degrees Celsius]].<ref>[http://www.nda.ox.ac.uk/wfsa/html/u13/u1312_03.htm Acid Base Balance (page 3)]</ref>
|-
| [[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.
|-
| [[Phosphate|HPO<sub>4</sub><sup>2−</sup>]]
| 0.8 to 1.5 <ref name=boron849> {{cite book |author=Walter F., PhD. Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |location= |year= |pages= |isbn=1-4160-2328-3 |oclc= |doi=}} Page 849 </ref> mM
|
|-
| ''total CO<sub>2</sub> (tCO<sub>2</sub> (P)<sub>c</sub>)
| 25 to 30 mmol/l
| This is the total amount of CO<sub>2</sub>, and is the sum of HCO<sub>3</sub><sup>-</sup> and PCO<sub>2</sub> by the formula: <br> tCO<sub>2</sub> = <nowiki>[</nowiki>HCO<sub>3</sub><sup>-</sup><nowiki>]</nowiki> + α*PCO<sub>2</sub>, where α=0.226 mM/kPa, HCO<sub>3</sub><sup>-</sup> is expressed in [[Concentration#Molarity|molar concentration]] (M) (mol/l) and PCO<sub>2</sub> is expressed in kPa <ref>[http://www.labtestsonline.org/understanding/analytes/co2/test.html CO2: The Test]</ref>
|-
| ''total O<sub>2</sub> (tO<sub>2e</sub>)
|
| This is the sum of oxygen solved in plasma and chemically bound to [[hemoglobin]]. <ref>[http://www.meddean.luc.edu/lumen/MedEd/MEDICINE/pulmonar/physio/pf10.htm Hemoglobin and Oxygen Transport. Charles L. Webber, Jr., Ph.D.]</ref>
|}
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 [[neonate|neonatal]] wards, as infants often have elevated [[lactic acid]].
==References==
{{reflist}}
==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]
{{Respiratory physiology}}
{{Renal physiology}}
{{Mergefrom|Arterial blood gas sampling|date=October 2007}}
[[Category:Pulmonology]]
[[Category:Metabolism]]
[[Category:Chemical pathology]]
[[Category:Medical tests]]
[[Category:Respiratory therapy]]
[[de:Blutgasanalyse]]
[[es:Gasometría arterial]]
[[fr:Gaz du sang]]
[[nl:Bloedgas]]
[[ja:血液ガス分析]]
[[no:Blodgassanalyse]]
[[pl:Gazometria]]