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Byline: M. Hekking (1), H. J. L. M. Ulenkate (2), B. Speelberg (3), M. J. E. Van Puyenbroek (3), H. M. J. Goldschmidt (2), E. S. Gelsema (1) Keywords: Key words Monitoring; Acid-base chart; pH; PaCO2; HCO3; Intensive care unit Abstract: Objective: To demonstrate the practicability of a tri-axial chart for the graphical and quantitative monitoring of arterial pH, arterial carbon dioxide partial pressure (PaCO.sub.2) and actual arterial bicarbonate-ion concentration (a[HCO.sub.3 .sup.--]) 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 PaCO.sub.2 and log a[HCO.sub.3 .sup.--] 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. Author Affiliation: (1) Department of Medical Informatics, Erasmus University Rotterdam, P. O. Box 1738, 3000 DR, Rotterdam, The Netherlands Tel.: + 31 10 4 08 81 16 Fax: + 31 10 4 36 28 82 email: hekking@mi.fgg.eur.nl, NL (2) Department of Clinical Chemistry and Hematology, St. Elisabeth Hospital, Hilvarenbeekseweg 60, 5022 GC, Tilburg, The Netherlands, NL (3) Intensive Care Unit, St. Elisabeth Hospital, Hilvarenbeekseweg 60, 5022 GC, Tilburg, The Netherlands, NL Article note: Received: 28 October 1997 Accepted: 4 June 1998