Blood gas analysis
In medicine and physiology, blood gas analysis is "measurement of oxygen and carbon dioxide in the blood." 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.
One study concluded "The mean difference between arterial and venous values of pH was 0.03 pH units." 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. Regarding pO2, venous pO2 is much lower.
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.
Delay in analysis after collection of blood
Delays of 30 to 60 minutes may not matter if the specimens are stored on ice.
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). Since the pO2 of blood is usually less than 150, the effect of air it to raise the pO2.
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