Artifactual Effects on Gas Tension Measurement

Handling of arterial blood samples for measurement in a laboratory removed from the patient is important to maintain stability of gas tensions. Gas bubbles in the syringe allow diffusion of O₂ and CO₂ between the blood sample and the bubbles, usually lowering the values in the blood. This is particularly true for high PO₂ values, because in this region, O₂ is less soluble in blood. Small bubbles in the syringe tend to have less influence on PCO₂ . Removal of all air bubbles before capping the syringe and placing it in ice water can maintain stability of blood gas samples for several hours. Erythrocytes do not contain mitochondria and therefore do not consume O₂ , but leukocytes and platelets do, and in the presence of extreme leukocytosis or thrombocytosis, significant O₂ consumption by the blood sample may occur.^[43] This may be suspected when PO₂ values are inexplicably low in the presence of high leukocyte or platelet counts. Inhibition of cellular O₂ consumption may be accomplished by adding to the sample sodium fluoride^[44] (available in evacuated tubes used for collection of blood for glucose measurement) or cyanide. Dilution of blood samples with saline (as may occur when drawing an insufficient volume of dead space from an indwelling arterial catheter before obtaining a sample) tends to lower PCO₂ , with less effect on PO₂ .