Arterial blood gas interpretation

This chapter is at least loosely associated with Section J2(i) from the 2023 CICM Primary Syllabus, which expects the exam candidates to be able to "interpret normal and abnormal arterial blood gases". The author has left it here, because the old 2017 version of the syllabus had this item duplicated in the respiratory section, whereas in the 2023 edition it was removed. 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 electrodeMeasurement of PCO2 using the Severinghaus electrodeMeasurement of PO2 using the Clark electrodeCalculated bicarrbonate value (from pH and PCO2)The actual base excess of the bloodThe standard base excess (of the extracellular fluid)Potassium measurement by the K-selective electrodeSodium measurement by the Na-selective electrodeIonised calcium measurement by the iCa-sensitive electrodeIonised calcium value corrected for pHChloride measurement by the Cl-selective electrodeAmperometric measurement of glucoseAmperometric measurement of lactateConcentration of haemoglobinOxygen saturation of TOTAL haemoglobinThe calculated p50 valueThe alveolar/arterial ratio (one of the tension-based indices of oxygenation)Methaemoglobin concentrationCarboxyhaemoglobin concentrationThe p50 value standardised for a set of "ideal" conditionsAn estimated shunt fraction, making ceratin assumptions about the mixed venous oxygen content and cardiac outputFraction 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: