The Controller and Acid-Base Physiology: An Introduction to a Complex Process

Analyzing Acid-Base Disorders: An Initial Approach

In the initial approach to simple acid-base disturbances, you must decide if the patient has one or more primary problems and whether the disorder (or disorders) are acute or chronic, that is, whether compensation has occurred (Table 7-3). The data upon which we rely to make these determinations are the ABG results, which tell us the pH and the P_aCO₂, and the serum electrolyte levels, which provide us with the serum bicarbonate and allow us to calculate the anion gap.

The first step is to examine the pH. If the pH is outside the normal range (7.36-7.44), an acidemia or alkalemia is present. Next you must determine what process, or -osis, is accounting for the abnormal pH. Look at the P_aCO₂—is it in the direction that you would expect if there were a primary respiratory process to account for the change in pH? For example, if the pH is 7.30, you know that the patient has an acidemia. When you look at the rest of the ABG results, you note that the P_aCO₂ is 53 mm Hg (significantly above a normal value of 40 mm Hg), indicative of respiratory acidosis. Thus, the change in the P_aCO₂ in this case is in the direction that indicates that the primary disturbance is consistent with a respiratory acidosis contributing to an acidemia. (Note: when there is a primary respiratory system cause of an acid-base abnormality, the P_aCO₂ moves in the opposite direction of the pH; in this example of a primary respiratory acidosis, the pH is down from normal, and the P_aCO₂ is elevated.)

Now you must make a calculation to determine if the respiratory acidosis is acute or chronic. The pH change associated with an acute respiratory acidosis is greater than for a chronic respiratory acidosis because the kidney has not yet had time to compensate for lowered pH by retaining more bicarbonate and eliminating protons (as ammonium NH₄⁺). With acute respiratory acidosis, the pH decreases approximately 0.08 units for every increase in P_aCO₂ of 10 mm Hg. In contrast, a chronic respiratory acidosis is associated with a decrease in the pH of approximately 0.03 units for every 10 mm Hg increase in the P_aCO₂. The serum bicarbonate also changes acutely as a reflection of initial buffering of the change in pH, and to a greater degree, chronically, as the kidneys compensate for the acute respiratory disorder (see Table 7-3).

If there is evidence of an abnormal pH but the P_aCO₂ has moved in a direction that is opposite of what you expect for a primary respiratory disorder, then you are dealing with a primary metabolic disturbance with respiratory compensation. For example, if the pH is 7.30, an acidemia is present. If the P_aCO₂ is 28 mm Hg, the acidemia cannot be attributable to a primary respiratory process; the patient is hyperventilating (which, with no other disturbance present, would tend to make the blood more alkalemic). This means that the patient has a primary metabolic acidosis with a respiratory compensation. You would then look at the patient’s serum electrolytes and calculate the anion gap to determine if the metabolic acidosis is in the anion gap or non-anion gap category. With primary metabolic disturbances, the respiratory system, if it is normal, can compensate almost instantaneously. Thus, having identified that there is a primary metabolic abnormality, the question you must address is whether there is an appropriate respiratory system response rather than whether the problem is acute or chronic (Table 7-4). For metabolic acidosis, the appropriate respiratory system response results in a decrease in P_aCO₂ of 1.2 mm Hg for every 1 meq/L decrease in the concentration of bicarbonate in the blood. In patients with metabolic alkalosis, the P_aCO₂ should increase 0.7 mm Hg for every 1 meq/L increase in bicarbonate concentration.

Remember that abnormalities in any of the components of the respiratory system—the controller, ventilatory pump, or gas exchanger—may prevent an appropriate response to a primary metabolic disturbance.

We have presented an approach that is useful in understanding simple acid-base disturbances. You must remember, however, that it is possible for two or three primary abnormalities to be present simultaneously. In these circumstances, your clue to a complex acid-base disturbance is that the simple rules we have described do not seem to explain the data before you. Your understanding of the physiology of the respiratory, renal, and GI systems will then guide you to making correct diagnoses of your patient's problem.