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One of the main factors determining oxygen delivery to cells is the oxygen content of the blood. Blood gas tensions are measured by direct blood sampling or transcutaneous diffusion and oxygen saturation of haemoglobin from pulse oximetry. Arterial blood gas analysis is widely available in hospitals and the direct measurements (pH, PaO2, paCO2) are among the most precise in medicine. The value of such measurements, however, depends on the ability of doctors to interpret the results properly. Taking arterial blood sample Arterial puncture may result in spasm, intraluminal clotting, or bleeding and haematoma formation, as well as a transient obstruction of blood flow. These factors may decrease the arterial flow in distal tissues unless adequate collateral arterial vessels are available. The brachial and femoral arteries do not have adequate collateral supplies. The radial artery at the wrist is the best site for obtaining an arterial sample because it is near the surface, relatively easy to palpate and stabilise, and usually has good collateral supply from the ulnar arteries. This can be confirmed by a modified Allen's test. ### Problems of taking arterial blood samples Allen's test. The radial and ulnar arteries are occluded by firm pressure while the fist is clenched. The hand is opened and the arteries released one at a time to check their ability to return blood flow to the hand It is kinder to patients to use local anaesthesia over the radial artery before puncture. Analgesic patches can be used for children. Use a 20 or 21 gauge needle with a preheparinised syringe. Express the heparin from the syringe before taking the sample; adequate heparin will remain in the 0.2 ml dead space of the barrel and needle. At least 3 ml of blood is required to avoid a dilution effect from the heparin. Any sample with more than fine …