How do I interpret a Davenport diagram?

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.

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.

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).

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).

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

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.

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

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