Arterial blood gas interpretation

By Alex Yartsev - 26/01/2020
Last updated 03/06/2020 - 21:48
 Topic: Respiratory system

This chapter is at least loosely associated with Section F9(ii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "interpret normal and abnormal blood gases". It would be presumptuous for any homebaked online resource to be able to claim that, from this low platform, one might be able to bestow upon the reader that sort of high-level ability. However, the syllabus document insists that some ABG interpretation should be a part of First Part preparation. This stands in the face of the fact that there are no ABG interpretation questions in the First Part written papers.  Moreover, the interpretation of ABG data includes numerous non-respiratory things  (acid base balance, haemoglobin, electrolytes, etc), making it difficult to categorise this chapter into any specific area of revision study. 

So, in an effort to have one unified central point for blood gas interpretation, this chapter is offered as a directory and bibliography. Here is an ABG to be dissected (literally the first one the author came upon when he left his office to look for a picture of an ABG on the morning of writing this resource). The image is mapped so that a reader can click on the variables and be transported to a Deranged Physiology page which discusses the measurement and interpretation of that variable.

Le gas du jourMeasurement of PH using the glass electrode Measurement of PCO2 using the Severinghaus electrode Measurement of PO2 using the Clark electrode Calculated bicarrbonate value (from pH and PCO2) The actual base excess of the blood The standard base excess (of the extracellular fluid) Potassium measurement by the K-selective electrode Sodium measurement by the Na-selective electrode Ionised calcium measurement by the iCa-sensitive electrode Ionised calcium value corrected for pH Chloride measurement by the Cl-selective electrode Amperometric measurement of glucose Amperometric measurement of lactate Concentration of haemoglobin Oxygen saturation of TOTAL haemoglobin The calculated p50 value The alveolar/arterial ratio (one of the tension-based indices of oxygenation) Methaemoglobin concentration Carboxyhaemoglobin concentration The p50 value standardised for a set of "ideal" conditions An estimated shunt fraction, making ceratin assumptions about the mixed venous oxygen content and cardiac output Fraction of oxyegnated haemoglobin (a parameter which is quite separate from the oxygen saturation) A calculated haematocrit value

One might point out that this is a printout of a Radiometer ABL700 blood gas analyser, and therefore not representative of other blood gas analysers. Values presented here may have a different notation in other units, and even different Radiometer ABL700 installations may produce different variables depending on their users' selections. This resource, therefore, unfairly favours the trainees who are endemic in the authors' local environment.

Directory of local material related to blood gas interpretation

Blood Gas Values

Acid Base Status

Electrolyte values

Metabolyte values

Oxygen status

There are also a few ABG interpretation scenarios left over from an older version of this site, which may still be of some interest:

[Submit a comment or correction]