Blood gas analysis

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In medicine and physiology, blood gas analysis is "measurement of oxygen and carbon dioxide in the blood."[1] Capnography, in contrast, measures carbon dioxide in expired air. Pulse oximetry nonintrusively approximates oxygen levels based on the attenuation of multiple wavelengths of light by hemoglobin. Serum electrolyte panels complement the carbon dioxide level with the level of circulating bicarbonate ion.

While the analysis is most often performed in a laboratory, point of care analyzers are becoming available, and can give results at the bedside. In the critical care environment, continuous measurement can be provided by catheter-inserted sensors.

Venous blood

One study concluded "The mean difference between arterial and venous values of pH was 0.03 pH units."[2] If the pCO2 of of venous blood is less than 45 mm Hg, then the arterial pCO2 is very likely less than 50 mm hg.[3] Regarding pO2, venous pO2 is much lower.[4]

Measurement methods

There are two broad methods, surrogate and direct. A surrogate technique, such as pulse oximetry, approximates blood oxygen not by measuring the actual oxygen in blood, but by the optical attenuation of at least to infrared wavelengths through a fairly thin skin path, such as an earlobe or finger tip.

Direct methods work with liquid blood. Within this, there are two main categories: indwelling and real-time, and sampled with analysis in an instrument. Indwelling catheters carrying ion-selective electrodes are invasive but give real-time information.

Sampling usually involves blood taken with a syringe and then carried to an instrument. Increasingly, point of care instruments are being developed that minimize delay to getting information, as they are small portable equipment that can do the analysis at the bedside. Quick response, but a less invasive method, may be a good compromise outside the intensive care unit.

While syringe-based methods are far less intrusive than catheters, it should be noted that arterial, rather than venous blood, is usually preferred for blood gas analysis. Inserting a needle into an artery is more technically difficult for the operator, and potentially both riskier and more painful to the patient. While local anesthesia is rare for adult venepuncture, local anesthesia is wise for an arterial puncture; the sterile technique needs to be stringent.

Artifacts in measurement

Glass syringes may be better than plastic.[5]

Delay in analysis after collection of blood

Delays of 30 to 60 minutes may not matter if the specimens are stored on ice.[6]

Delay lowers the pO2 unless air bubbles or froth are present in which case delay raises the pO2.[7][8]

Exposure of blood to room air

Exposure to room air, either through not sealing the specimen or not removing air bubbles, can moves pO2 towards the pO2 of the ambient air (150 mm Hg at sea level).[9] Since the pO2 of blood is usually less than 150, the effect of air it to raise the pO2.

References

  1. Anonymous (2015), Blood gas analysis (English). Medical Subject Headings. U.S. National Library of Medicine.
  2. Middleton P, Kelly AM, Brown J, Robertson M (August 2006). "Agreement between arterial and central venous values for pH, bicarbonate, base excess, and lactate". Emerg Med J 23 (8): 622–4. DOI:10.1136/emj.2006.035915. PMID 16858095. Research Blogging.
  3. Kelly AM, Kerr D, Middleton P (May 2005). "Validation of venous pCO2 to screen for arterial hypercarbia in patients with chronic obstructive airways disease". J Emerg Med 28 (4): 377–9. DOI:10.1016/j.jemermed.2004.10.017. PMID 15837016. Research Blogging.
  4. Yildizdaş D, Yapicioğlu H, Yilmaz HL, Sertdemir Y (February 2004). "Correlation of simultaneously obtained capillary, venous, and arterial blood gases of patients in a paediatric intensive care unit". Arch. Dis. Child. 89 (2): 176–80. PMID 14736638. PMC 1719810[e]
  5. Mahoney JJ, Harvey JA, Wong RJ, Van Kessel AL (July 1991). "Changes in oxygen measurements when whole blood is stored in iced plastic or glass syringes". Clin. Chem. 37 (7): 1244–8. PMID 1823532[e]
  6. Woolley A, Hickling K (March 2003). "Errors in measuring blood gases in the intensive care unit: effect of delay in estimation". J Crit Care 18 (1): 31–7. DOI:10.1053/jcrc.2003.YJCRC7. PMID 12640611. Research Blogging.
  7. Biswas CK, Ramos JM, Agroyannis B, Kerr DN (March 1982). "Blood gas analysis: effect of air bubbles in syringe and delay in estimation". Br Med J (Clin Res Ed) 284 (6320): 923–7. PMID 6802352. PMC 1496510[e]
  8. Harsten A, Berg B, Inerot S, Muth L (July 1988). "Importance of correct handling of samples for the results of blood gas analysis". Acta Anaesthesiol Scand 32 (5): 365–8. PMID 3414345[e]
  9. Madiedo G, Sciacca R, Hause L (September 1980). "Air bubbles and temperature effect on blood gas analysis". J. Clin. Pathol. 33 (9): 864–7. PMID 7430400. PMC 1146247[e]

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