Interpreting arterial blood gas results
BMJ 2013; 346 doi: https://doi.org/10.1136/bmj.f16 (Published 16 January 2013) Cite this as: BMJ 2013;346:f16
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I am grateful to Drs. Cowley & Owen, Prof. Bion and the Editors of the BMJ for printing the useful paper: Interpreting Arterial Blood Gas Results. BMJ 2013;346:f16.
Assessing acid-base balance is not simple at first sight.
A difficulty with acid-base balance is that three variables are concomitantly involved (paCO2, pH and HCO3-) whereas we usually deal with one or two variables at a time.
In interpreting acid-base physiology and disorders I find graphs which include all three variables on an x-y graph with a superimposed third variable quite useful. The result of a blood gas sample is a point on such graphs, i.e. values for paCO2, pH and HCO3-.
The position of this point on the graph corresponds to normality or to a disorder: acidosis, alkalosis, acute, chronic, respiratory, metabolic or mixed.
As a result acid-base disorders are assessed at a glance on these graphs (there exist various such graphs).
An acid-base graph (nomogram) is shown below.(1)
I recommend assessing acid-base disorders on such graphs, aware that our body (lungs, kidneys, blood buffers) strives to normalize blood pH (compensation).
Reference
1. Levinsky NG. Acidosis and Alkalosis, p. 231. In: Petersdorf RG, Adams RD, Braunwald E, Isselbacher KJ, Martin JB, Wilson JD, (eds). Harrison's Principles of Internal Medicine. Mc Graw-Hill Book Company, New York, 1983. The graph is from: G.S. Arbus. An in vivo acid-base nomogram for clinical use. Can Med Assoc J 1973; 109:291-93.
Competing interests: No competing interests
Re: Interpreting arterial blood gas results
Dear Editor,
I am grateful to Dr Cowley and his colleagues for publishing an interesting article on the "Interpretation of arterial blood gas results" and to medical student Camilli Simini for encouraging the use of acid base nomograms as an aid to interpretation in her rapid response.
I would like to point out a potential patient saftey issue in failing to appreciate the difference bewteen Standard and Actual HCO3-. Dr Cowley and coauthors emphasise the use of standard bicarbonate (sHCO3-) whereas the inpretation of most acid base nomograms requires the actual bicarbonate.
Standard refers to standard conditions and is the concentration in whole blood that has been equilibrated at 37 degrees C with a PCO2 of 40 mmHg (5.3 kPa) with oxygen to give full saturation of haemoglobin.
Actual bicarbonate is claculated by the gas analyser from the Henderson Hasselbach equation using pH and PCO2 [1].
[1] Lawrrie, A. and Golda, B.P. Actual or Standard Bicarbonate. Lancet 1979 Jul 28;2(8135):201-2.
Competing interests: No competing interests