by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology
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Interactive Acid-Base Diagram: PCO2 vs. SBE

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(C) Alan W. Grogono 1999


  1. Drag the Mouse to Your Chosen pH and PCO2: the diagram provides the corresponding text report.
  2. Color change. The color of the spot changes: acid - red; alkaline - blue.
  3. Typical Zones: The typical zones appear green.
  4. Other Zones: The gaps between typical zones appear gray.
  5. Numerical Values: As you move round in the graph, the boxes below show the corresponding values for Standard Base Excess, pH, and the components of the Modified Henderson Equation.

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Development and Properties

Original Diagram

1. Original Diagram

This first In-Vivo Linear Acid Base Diagram was created specifically to teach medical students. It represented the behavior of the whole body during acid-base disturbances and the body's responses to therapeutic intervention.

The diagram was derived from the Siggaard-Andersen In-Vivo Nomogram . The difference was that the axes were respiratory (Y-axis) and metabolic (X-axis) and both scales were linear. This emphasized the two components that determine the pH and also eliminated the logarithmic scales.

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Modified Diagram

2. Original Diagram with Zones and Y-Axis Relabeled.

Grogono Diagram (Grogono et al 1976) This version shows the same axes as in Diagram 1 but the Y-axis is relabeled "Metabolic Acidosis" in place of Negative Base Excess. As before, acidity is greater as we move either upwards or to the right - a property to be expected in a well designed map or graph!

Characteristic Zones: Added in this version are the zones where typical syndromes occur. These zones were created after a review of the existing literature and represented a convenient approximation:

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New In-Vivo Diagram

3. New Diagram. Plots SBE vs. PCO2 with accurate zones

The Final Diagram (Schlichtig, Grogono & Severinghaus 1998) has the advantages of the previous teaching diagram but employs Standard Base Excess for the Y-Axis - data readily obtained from modern acid-base analyzers.

Accuracy The final version is derived from clinical data: the location of the typical zones corresponds to published data. The principal features are listed below:

To use the diagram: Enter the PCO2 along the x-axis; the intersection with the measured pH allows the level of SBE to be read from the left hand scale horizontally opposite the intersection. In addition, metabolic and respiratory changes are readily distinguished: each moves the patient along the appropriate axes without altering the other:

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Acid-Base Tutorial
Alan W. Grogono
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