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 [1], nomograms, and rules of thumb[2] are commonly used.
[edit] Extraction and analysis
Arterial blood for blood gas analysis is usually extracted by a phlebotomist, nurse, or respiratory therapist.[3] 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.[4]
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.
[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 acidemic (pH < 7.35; H+ >45) or alkalemic (pH > 7.45; H+ < 35). |
| H+ |
35 - 45 nmol/l (nM) |
See above. |
| PO2 |
9.3-13.3 kPa or 80-100 mmHg |
A low O2 indicates that the patient is not respiring properly, and is hypoxemic. At a PO2 of less than 60 mm Hg, supplemental oxygen should be administered. At a PO2 of less than 26 mm Hg, the patient is at risk of death and must be oxygenated immediately. |
| 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.[5] A high PCO2 (respiratory acidosis) indicates underventilation, a low PCO2 (respiratory alkalosis) hyper- or overventilation. PCO2 levels can also become abnormal when the respiratory system is working to compensate for a metabolic issue so as to normalize the blood pH. |
| HCO3- |
22–26 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. HCO3- levels can also become abnormal when the kidneys are working to compensate for a respiratory issue so as to normalize the blood pH. |
| SBCe |
21 to 27 mmol/l |
the bicarbonate concentration in the blood at a CO2 of 5.33 kPa, full oxygen saturation and 37 degrees Celsius.[6] |
| Base excess |
-3 to +3 mmol/l |
The base excess is used for the assessment of the metabolic component of acid-base disorders, and indicates whether the patient has metabolic acidosis or metabolic alkalosis. A negative base excess indicates that the patient has metabolic acidosis (primary or secondary to respiratory alkalosis). A positive base excess indicates that the patient has metabolic alkalosis (primary or secondary to respiratory acidosis).[7] |
| HPO42− |
0.8 to 1.5 [8] mM |
|
| total CO2 (tCO2 (P)c) |
25 to 30 mmol/l |
This is the total amount of CO2, and is the sum of HCO3- and PCO2 by the formula:
tCO2 = [HCO3-] + α*PCO2, where α=0.226 mM/kPa, HCO3- is expressed in molar concentration (M) (mol/l) and PCO2 is expressed in kPa [9] |
| total O2 (tO2e) |
|
This is the sum of oxygen solved in plasma and chemically bound to hemoglobin. [10] |
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.
[edit] References
- ^ Baillie K. "Arterial Blood Gas Interpreter". Apex (Altitude Physiology Expeditions). Retrieved on 2007-07-05. - Online arterial blood gas analysis
- ^ Baillie JK (2008). "Simple, easily memorised "rules of thumb" for the rapid assessment of physiological compensation for acid-base disorders". Thorax 63 (3): 289–90. doi:10.1136/thx.2007.091223. PMID 18308967.
- ^ Aaron SD, Vandemheen KL, Naftel SA, Lewis MJ, Rodger MA (2003). "Topical tetracaine prior to arterial puncture: a randomized, placebo-controlled clinical trial". Respir Med. 97 (11): 1195–1199. doi:10.1016/S0954-6111(03)00226-9. PMID 14635973.
- ^ Mahoney JJ, Harvey JA, Wong RL, Van Kessel AL (1991). "Changes in oxygen measurements when whole blood is stored in iced plastic or glass syringes". Clin Chem. 37 (7): 1244–1248. PMID 1823532.
- ^ Baillie K, Simpson A. "Altitude oxygen calculator". Apex (Altitude Physiology Expeditions). Retrieved on 2006-08-10. - Online interactive oxygen delivery calculator
- ^ Acid Base Balance (page 3)
- ^ RCPA Manual: Base Excess (arterial blood)
- ^ Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 849
- ^ CO2: The Test
- ^ Hemoglobin and Oxygen Transport. Charles L. Webber, Jr., Ph.D.
[edit] External links
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