Arterial Blood Gas Analysis Part 1 – The Basics

Arterial blood gas (ABG) interpretation helps us with the assessment of a patient’s respiratory gas exchange and acid-base balance. It forms a key part of the management of sick patients and provides invaluable information, particularly when combined with a thorough clinical assessment.

 Normal arterial blood gas values

The normal values on an ABG can vary a little between analysers, but broadly speaking are as follows:

The normal values on an ABG can vary a little between analysers, but broadly speaking are as follows:
pH -------7.35-7.45
PaO2----10-14kPa
PaCO2----4.5-6kPa
HCO3----22-26mmol/L
Base excess---2-2mmol/l

 pHIn order to properly interpret an ABG an understanding of each of the key components of an arterial blood gas is necessary.

The pH of a solution is a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are acidic, and higher values are alkaline. The scale is inversely proportional to the concentration of hydrogen ions in the solution.The body’s pH is maintained within a range of 7.35 to 7.45 under normal circumstances and this homeostasis is achieved through the buffering and excretion of acids. Important buffers include plasma proteins and bicarbonate extracellularly, and proteins, phosphate, and haemoglobin intracellularly.

This buffering system provides only a temporary solution to the production of acids, and once the buffers are consumed acids will begin to accumulate. A system of acid excretion therefore exists, which allows time for the regeneration of buffers. This system occurs via the kidneys and the lungs, with hydrogen ions being excreted via the kidney and carbon dioxide being excreted via the lungs.

If both the buffering and renal excretion systems are overwhelmed then the pH will fall and a metabolic acidosis will occur. If the ability of the lungs to excrete carbon dioxide is compromised then a respiratory acidosis will occur.

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Arterial Blood Gas Analysis Part 1 – The Basics