Blood acid-base alignment nomogram. Scales for pH, pCO2 base excess of whole blood of different hemoglobin concentrations, plasma bicarbonate, and plasma total-CO2.

A description of the acid-base status of arterial blood includes three values : pH, carbon dioxide tension (pC0,) and base excess (BE). pH, normally about 7.40 (7.36 to 7.42), changes by changing pC0, as well as by accumulation of non-volatile acid or base. The p H value is practically always determined directly by means of the glass electrode. pC02, normally about 40 mm Hg (33 to 47), is inversely proportional with the alveolar ventilation. The pC0, value can be determined directly by means of the pC0, electrode (Severinghaus & Bradley 195S), or indirectly by calculation from the pH and the total-CO, of plasma, or from the p H and the C0,-equilibration curve (Astrup 1956, Siggaard-Andersen, Engel, Jargensen & Astrup 1960). Base excess, normally about zero meq/l (fZ.3 to -2.3), indicates the accumulation of non-volatile acid or base in the blood. A positive value indicates a base excess (= non-volatile acid deficit), a negative value indicates a base deficit (= non-volatile acid excess). The base excess value can theoretically he determined directly by titrating with strong acid or base to a p H of 7.40 at a pC0, of 40 mm Hg at 38" C. However, it is difficult to carry out this titration without hemolysis, dilution of the sample, or change in ionic strength. Therefore B E is determined indirectly by calculation from the pH, pC0, and hemoglobin concentration. In the clinical routine other values are often used as indicators of the base excess, i. e. buffer base, standard bicarbonate or eventually total-CO, or actual bicarbonate of the plasma. A discussion of the base excess value as compared to these indicators is given elsewhere (Siggaard-Andersen 1963). It appears from this introduction that the pC0, and the base excess values must often be determined indirectly by calculation. A blood acid-base nomogram for these calculations has previously been published (Siggaard-Andersen & Engel 1960, revised by Siggaard-Andersen 1962 a, see Fig. 1). The purpose of this paper is to present a modified nomogram facilitating certain calculations, see Fig. 3.