Diagnostic test that consists of drawing of a blood sample for later analysis and determination of the concentration of different gases: oxygen or carbon dioxide, to assess the suitability of ventilation and oxygenation treatments, as well as the pH balance in blood. The sample can arterial or venous blood, so the method is different depending on whether a vein or an artery is to be punctured. If a vein is used, we will proceed as for any standard blood sample. If an artery is required, we need special equipment, and puncture is more painful because the wall of an artery is thicker than that of a vein. It is usually performed on the internal side of the wrist or elbow flexure.