How to Read the PCO₂//pH Nomogram

The thick red bar moving from left to right across the Pco₂//pH nomogram represents the normal Pco₂ blood buffer line. This red bar is used to identify the pH and changes that occur immediately in response to an acute increase or decrease in Pco₂. The purple bar is used to identify the pH and changes that occur in response to acute metabolic acidosis and metabolic alkalosis conditions. The colored areas that surround the red and purple bars are used to identify (1) partial and complete renal compensation, (2) partial and complete respiratory compensation, and (3) combined metabolic and respiratory acid-base disturbances (see Figure 4-1).

For example, when the pH, Pco₂, and values all intersect in the light purple area—shown in the upper left hand corner of the Pco₂//pH nomogram—partial renal compensation has occurred in response to a chronically high Pco₂ level. When the increases enough to move the pH into the light-blue normal bar, complete renal compensation is confirmed. When the pH, Pco₂, and values all intersect in the in the green area—shown in the lower right hand corner of the Pco₂//pH nomogram—partial renal compensation has occurred in response to a chronically low Pco₂ level. When the decreases enough to move the pH into the light-blue normal bar, complete renal compensation is confirmed.

When the pH, Pco₂, and values all intersect in the in the orange area—shown immediately below the red bar on the left side of the Pco₂//pH nomogram—a combined respiratory and metabolic acidosis is confirmed. When the pH, Pco₂, and values all intersect in the in the blue area—shown immediately above the red bar on the right side of the Pco₂//pH nomogram—a combined respiratory and metabolic alkalosis is confirmed.

Finally, when the pH, Pco₂, and values all intersect in the yellow area—shown in the lower left corner of the Pco₂//pH nomogram—respiratory compensation has occurred in response to metabolic acidosis. When the pH, Pco₂, and values all intersect in the pink area—shown in the upper right corner of the Pco₂//pH nomogram—respiratory compensation has occurred in response to metabolic alkalosis.

Although it is beyond the scope of this textbook to fully explain how each of the acid-base disturbances listed in Box 4-1 can be identified on the Pco₂//pH nomogram, a basic understanding of the following two most commonly encountered Pco₂//pH relationships is important: (1) an acute Pco₂ increase and its effects on the pH and values, and (2) an acute Pco₂ decrease and its effects on the pH and values.*

How Acute PCO₂ Increases Affect the pH and Values

As mentioned previously, the red normal Pco₂ blood buffer bar shown on the Pco₂//pH nomogram is used to identify the pH and values that will result immediately in response to a sudden increase in Pco₂. For example, if the patient’s Paco₂ were to suddenly increase to, say, 60 mm Hg, the pH would immediately fall to about 7.28 and the level would increase to about 26 mEq/L. Furthermore, the Pco₂//pH nomogram shows that these ABG values represent acute ventilatory failure (acute respiratory acidosis). This is because (1) all of the ABG values (i.e., Pco₂, , and pH) intersect within the red normal Pco₂ blood buffer bar, and (2) the pH and readings are precisely what is expected for an acute increase in the Pco₂ of 60 mm Hg (Figure 4-2).

How Acute PCO₂ Decreases Affect the pH and Values

On the other hand, the red normal Pco₂ blood buffer bar shown on the Pco₂//pH nomogram is also used to identify the pH and values that will result immediately in response to a sudden decrease in Pco₂. For example, if the patient’s Paco₂ were suddenly to decrease to, say, 25 mm Hg, the pH would immediately increase to about 7.55 and the level would decrease to about 21 mEq/L. In addition, the Pco₂//pH nomogram shows that these ABG values represent acute alveolar hyperventilation (acute respiratory alkalosis). This is because (1) all of the ABG values (i.e., Pco₂, , and pH) intersect within the red normal Pco₂ blood buffer bar, and (2) the pH and readings are precisely what is expected for an acute increase in the Pco₂ of 25 mm Hg (Figure 4-3).

A Quick Clinical Calculation for the Effect of Acute PaCO₂ Changes on pH and

In addition to using the graphic Pco₂//pH nomogram (see Figure 4-1), the following simple calculations can also be used to estimate the expected pH and value changes that will immediately occur in response to a sudden increase or decrease in Paco₂.