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Acid-Base Balance

by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology

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Sea Level Acid-Base Diagram.

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Sea Level IconI think of the ocean as the blue heart of the planet. Sylvia Earle.

Introduction: This diagram shows the relationship between the blood's acidity (pH), respiration component (PCO2), and metabolism component (Base Excess). At lower altitudes (below 1500 meters or 4920 feet.) choose this Sea Level Diagram that uses a normal PCO2 of 40 mmHg (5.33 pPa). At higher altitudes choose the High Altitude Diagram that uses a "normal" PCO2 appropriate for the altitude (See Notes Below). Select the Units that you normally use for PCO2:

Click to Select:


Siggaard-Andersen Acid-Base Chart
Siggaard-Andersen Acid-Base Chart

Siggaard-Andersen Acid-Base Chart
  1. Acid-Base Diagrams: Many diagrams have been devised to represent the relationship between arterial PCO2 and pH. The best known is probably the Siggaard-Andersen Acid-Base Diagram1 (S-A Diagram). However, the axes are logarithmic scales for the PCO2 and [H+] concentrations, the typical zones are irregular and curvilinear, and the Lines for Base Excess are oblique suggesting that they are dependent on the pH and PCO2.
  2. Derivation: Data from the S-A Diagram provided the basis for the new diagram. The axes are linear scales for the respiratory and metabolic components, the pH lines (the dependent variable) are oblique, and the typical zones are readily-visualized, straight, linear bands. These features make the new diagram easier to understand and use.
  3. Typical Zones Location: The Zones shown on the diagram represent values at sea level and are based on a review of over twenty papers carried out by Dr. Robert Schlichtig2. At higher altitudes a more appropriate value for Base Excess is provided by the High Altitude Version.
  4. Acid-Base Balance Diagram: All the Typical Zones
    Acid-Base Balance Diagram: All the Typical Zones

    Acid-Base Balance Diagram: All the Typical Zones
  5. Calculating Metabolic Treatment: To treat metabolic acidosis using bicarbonate it is customary to multiply the Standard Base Excess value by the patient's treatable volume (SBE x 0.3 x Wt in Kg) and initially only administer half this dose, see Metabolic Treatment.


  1. 1 Siggaard Andersen O and Engel K. "A New Acid-Base Nomogram. An Improved Method for the Calculation of the Relevant Blood Acid-Base Data." Scandinav. J. Clin. & Investigation 177-186, 12, 1960.
  2. 2 Schlichtig R, Grogono AW, Severinghaus JW. "Human PaCO2 and standard base excess compensation for acid-base imbalance." Critical Care Medicine. 26:1173-1179. 1998.
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Acid-Base Tutorial
Alan W. Grogono
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