Arterial blood sample
If frequent samples need to be taken, an indwelling cannula enables blood to be taken painlessly. For one-off samples, the most commonly used arterial sampling sites are the radial, brachial, and femoral arteries. Of these three sites the radial artery (lying in the wrist area beside the thumb) is the preferred site because: ✅ It is easy to access ✅ It is a superficial artery that is easy to palpate, stabilize, and puncture ✅ It has a collateral blood supply from the ulnar artery. A modified Allen’s test should be performed prior to taking the sample. This is a test to check the integrity of the radial and ulnar arteries at the wrist. • The examiner compresses the patient’s radial and ulnar arteries at the wrist • The patient is then asked to open and close the hand rapidly until the palm appears white • The examiner then releases either the radial or the ulnar artery and looks for return of pink colour and circulation to the hand • The test is then repeated releasing the other artery • The hand should return to its pink colour within six seconds if circulation through that artery is adequate
Taking the sample
• An arterial stab is performed with a 21 gauge needle attached to a syringe that is pre-filled with heparin (0.1–0.2ml of 1000iu heparin/ml) • The heparin prevents the sample from clotting • The needle is inserted at an angle of 45° and the amount of blood drawn ranges from 0.5ml (children) to 3ml, depending on local policy and the blood gas analyser available • Pressure needs to be applied to the puncture site for 5 mins, longer if the patient is on anticoagulant therapy or has a bleeding disorder • Once the sample is obtained, care should be taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results • The sealed syringe should be taken to a blood gas analyser immediately • If the sample cannot be analysed immediately it should be kept on ice to slow down metabolic processes that may cause inaccuracy • The machine aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen, carbon dioxide and the bicarbonate concentration, as well as the oxygen saturations of haemaglobin • The results are usually available within 5 mins, and are then ready for interpretation • It is important to record whether the patient is breathing air or receiving oxygen when the sample is taken. If the patient is on oxygen, the percentage of oxygen inspired should be recorded.