Chapter 77. Acid-Base Balance

• The blood [H+] and pH are determined by the strong ion difference (SID), the PCO2, and the total concentration of weak acids, mostly consisting of phosphate and albumin.
• Both acidemia and alkalemia have potentially harmful physiologic effects, and the presence of either is related to mortality.
• Most acid-base derangements do not benefit from specific correction of the abnormal pH; instead, the intensivist should focus on detecting and treating the underlying condition.
• Acid-base disorders are easily characterized using a stepwise approach.
• Lactic acidosis is the most important acid-base abnormality in ICU patients. Inadequate tissue oxygenation underlies the lactic acidosis in some patients (acute hemorrhage, critical hypoxemia, cardiogenic shock) but probably does not in others (such as the resuscitated septic patient).

Acid-base balance and acid-base disorders are imperfect terms for the determining factors and disease processes that lead to a particular hydrogen ion concentration [H+] in the blood. The methodology used routinely to determine an acid-base disorder is accurate in defining the disturbance. This methodology does not, however, reveal the mechanisms that have led to a particular [H+] in blood. The components of blood that contribute to acid-base balance are

1. Water

2. Strong cations (Na+, Mg2 +, Ca2 +, K+) and strong anions (Cl−, lactate−)

3. Bicarbonate ion (HCO3−)

4. Weak acids and their conjugate bases (HA + A− = Atot) (Atot is the total independent variable, and HA + A− are dependent variables.)

5. Partial pressure of carbon dioxide (PCO2)

6. Carbonate ion (CO32−)

7. Hydroxyl ion (OH−)

8. Hydrogen ion (H+)

The difference between the strong cations and strong anions (the strong ion difference [SID]), PCO2, and the total amount of weak acids and their conjugate bases (Atot) are the only independent variables.1 All the other components are, by definition, dependent, including HCO3−, HA, A−, CO32−, OH−, and H+. Because the concentrations of each of these six variables are dependent on one or more of the independent variables, we must solve separate equilibrium equations for each. Water itself is minimally dissociated despite the importance of [H+] and can be considered a constant. The six equations are as follows:

Water dissociation:

Weak acid dissociation:

Weak acid conservation:

HCO3− formation:

CO32− formation:

Electrical neutrality:

K1 through K4 represent constants for the individual reactions. Now that we have six equations and six unknowns, we can arrange any unknown as a fourth-order polynomial and solve ...