All the tests listed in Table 12.3 are usually measured from a blood sample from a vein (venous). By definition, an arterial blood sample is taken from an artery. Blood pressure in the arteries is much higher than in the veins so bleeding occurs from the puncture site more profusely and for much longer and so firm pressure for an extended time is required after the sample is taken. The ABG is used to:
• evaluate derangements of acid-base balance;
• establish levels of blood oxygenation and CO2 elimination;
• assess whether the underlying problem is respiratory or metabolic.
Normal metabolism creates a slight excess of acidic compounds, which are normally excreted by the kidneys and also the process of respiration. Cells of the body require a degree of acidosis/ alkalosis that is normal or close to normal (Driscoll et al. 1997).
During a cardiac arrest an acidaemia is likely to develop due to:
• build-up of lactic acid as the tissues are starved of oxygen (metabolic);
• build-up of CO2 as it cannot be removed by the lungs (respiratory).
The post-arrest patient is highly likely to be suffering from an acidaemia that is both metabolic and respiratory in origin.
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