How do I interpret a Davenport diagram?

Uncertain about Davenport diagrams? Check out this article on the relationship between carbon dioxide, bicarbonate, and pH.
Last update4th Dec 2020

The relationship among the partial pressure of carbon dioxide in the arteries, bicarbonate concentration, and pH may be depicted graphically in a Davenport diagram.

In the Davenport diagram shown, plasma bicarbonate concentration is plotted against pH at a variety of levels of arterial carbon dioxide tension (isobars). Point A depicts the normal values for arterial pH, arterial carbon dioxide tension, and bicarbonate concentration. Point B represents the relationships when carbonic acid is added to blood (e.g., by increasing arterial carbon dioxide tension). Point C represents the relationships when carbonic acid is removed from blood (e.g., by decreasing arterial carbon dioxide tension). The line connecting Points A, B, and C defines the buffer line for arterial blood.

Figure 1. The Davenport diagram shows that shifts in pH at various levels of arterial carbon dioxide tension (PaCO2) with, A, normal bicarbonate concentrations, B, increased carbonic acid levels, C, decreased carbonic acid levels in the blood. The line connecting the points is the buffer line for arterial blood.

Changes in arterial carbon dioxide tension

Alkalemia, or an arterial pH > 7.44, arises when, in the setting of a constant plasma bicarbonate concentration, arterial carbon dioxide tension is reduced, as represented by a shift in arterial carbon dioxide tension from Point A to an arterial carbon dioxide tension isobar to the right of Point A.

Acidemia, or an arterial pH < 7.36, arises when, in the setting of a constant arterial bicarbonate concentration, arterial carbon dioxide tension is increased, reflected in a shift from Point A to an arterial carbon dioxide tension isobar to the left of Point A.

Figure 2. The Davenport diagram shows how changes in arterial carbon dioxide tension (PaCO2) with constant plasma bicarbonate (HCO3-) concentrations alter arterial pH. Reduced PaCO2 is associated with increased pH (alkalemia), while increased PaCO2 is associated with reduced pH (acidemia).

Changes in serum bicarbonate

Alkalemia or acidemia can also be produced by increases or decreases in the serum bicarbonate concentration, respectively. This is reflected in changes in pH with changes in bicarbonate concentration along a single carbon dioxide tension isobar (i.e., at a constant arterial carbon dioxide tension).

Figure 3. The Davenport diagram shows how changes in plasma bicarbonate concentrations (HCO3-) with constant carbon dioxide tension (PaCO2) alter arterial pH. Reduced HCO3- is associated with reduced pH (acidemia), while increased HCO3- is associated with increased pH (alkalemia).

Changes in bicarbonate concentration or arterial carbon dioxide tension can be mitigated by concomitant changes in the other variable. An increase in serum bicarbonate has less of an effect on pH if there is a concomitant change in arterial carbon dioxide tension (increase).

Figure 4. The Davenport diagram shows how increases in both, 1, plasma bicarbonate concentrations (HCO3-) and, 2, carbon dioxide tension (PaCO2) maintain a constant arterial pH.

Become a great clinician with our video courses and workshops

The same is true for the effect of a decrease in bicarbonate accompanied by a concomitant decrease in arterial carbon dioxide tension.

Figure 5. The Davenport diagram shows how decreases in both, 1, plasma bicarbonate concentrations (HCO3-) and, 2, carbon dioxide tension (PaCO2) maintain a constant arterial pH.

The effect on arterial pH of changes in arterial carbon dioxide tension can be mitigated by concomitant changes in serum bicarbonate, with a rise in arterial carbon dioxide tension offset by a rise in bicarbonate.

Figure 6. The Davenport diagram shows how, 1, decreases in carbon dioxide tension (PaCO2), resulting in decreases in pH, can be mitigated by, 2, increases in serum bicarbonate levels (HCO3-) to maintain a constant arterial pH.

Or a decrease in arterial carbon dioxide tension offset by a decline in serum bicarbonate.

Figure 7. The Davenport diagram shows how, 1, decreases in carbon dioxide tension (PaCO2), resulting in increases in pH, can be mitigated by, 2, decreases in plasma bicarbonate levels (HCO3-) to maintain constant arterial pH.

That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.

Recommended reading

  • Grippi, MA. 1995. “Gas exchange in the lung”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 137–149)
  • Grippi, MA. 1995. “Clinical presentations: gas exchange and transport”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 171–176)
  • Grippi, MA and Tino, G. 2015. “Pulmonary function testing”. In: Fishman's Pulmonary Diseases and Disorders, edited by MA, Grippi (editor-in-chief), JA, Elias, JA, Fishman, RM, Kotloff, AI, Pack, RM, Senior (editors). 5th edition. New York: McGraw-Hill Education. (Grippi and Tino 2015, 502–536)
  • Tino, G and Grippi, MA. 1995. “Gas transport to and from peripheral tissues”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Tino and Grippi 1995, 151–170)
  • Wagner, PD. 2015. The physiologic basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases. Eur Respir J45: 227–243. PMID: 25323225

About the author

Michael A. Grippi, MD
Vice Chairman, Department of Medicine | Pulmonary, Allergy, and Critical Care Division, Perelman School of Medicine, University of Pennsylvania, USA.
Author Profile