A re-appraisal of the tri-axial chart for monitoring arterial acid-base values.

OBJECTIVE

To demonstrate the practicability of a tri-axial chart for the graphical and quantitative monitoring of arterial pH, arterial carbon dioxide partial pressure (PaCO2) and actual arterial bicarbonate-ion concentration (a[HCO3-]) in intensive care patients.

DESIGN

Case report.

SETTING

A general intensive care unit (ICU).

METHODS

Using a standard mathematical transformation, a data set of pH, log PaCO2 and log a[HCO3-] values can be transformed in such a way that a graphical display of all three variables is possible while being faithful to their linear relationship. Remarkably, the graphical display closely resembles the tri-axial chart that Hastings and Steinhaus described in 1931 for studying displacements of the acid-base balance. Two new monitoring parameters based on the chart and the transformation are described. One monitors the abnormality of the acid-base status while the other monitors the rate of acid-base changes.

CONCLUSIONS

With the tri-axial acid-base chart, the complete acid-base status can be faithfully monitored. Moreover, the proposed monitoring parameters provide extra information about the arterial acid-base status that, otherwise, would remain hidden.