Is It Helpful to Diagnose a Triple Acid-Base Disturbance? (Or Is It Just a Mental Exercise?)
If nephrologists love ratios, they get absolutely ecstatic over complex acid-base disturbances. Back in the days, the mettle of a nephrology attending was indicated by how well he (or she) could solve a complex electrolyte or acid-base disorder “on the fly.” Dr. Donald Seldin from Southwestern in Dallas was famous for his ability to teach “metabolic alkalosis” using “Socratic methods” to keep his audience focused!
The basic steps in acid-base analysis are straightforward, providing an orderly, logical plan is followed. An arterial blood gas (ABG) is necessary for definitive acid-base analysis.
These are the steps to follow:
1. Look at pH to determine if there is acidemia (pH <7.38) or alkalemia (pH >7.46)
If pH is low, there must be a primary acidosis.
If pH is high, there must be a primary alkalosis.
2. Look at HCO3– and Pco2 to determine the type of acidosis or alkalosis present
[HCO3–] is low in metabolic acidosis and high in metabolic alkalosis.
PCO2 is low in respiratory alkalosis and high in respiratory acidosis.
At this point, the primary acid-base process should be evident, that is, metabolic or respiratory acidosis or alkalosis.
3. Determine if compensation (adaptation) is appropriate
Henderson’s equation shows the relationship between pH, PCO2, and HCO3–:
[H+] = (24 × PCO2)/[HCO3–]
From this formula, it is evident that in order to maintain [H+] near-normal, a primary change in either [HCO3–] or PCO2 should result in a counterbalancing change of the other in the same direction, that is,
Metabolic acidosis is compensated by respiratory alkalosis (hyperventilation).
Metabolic alkalosis is compensated by respiratory acidosis (hypoventilation).
Chronic respiratory acidosis is compensated by metabolic alkalosis.
Chronic respiratory alkalosis is compensated by metabolic acidosis.
There is little adaptation to acute respiratory acidosis or alkalosis because it requires 24 to 48 hours for the kidneys to adapt to a primary respiratory disorder.
The expected compensations (adaptations) to primary acid-base disturbances have been determined empirically and are given in Table 6.1 (Bushinsky et al., 1982; Emmett, 2006).
When compensation is less or greater than expected, this suggests the presence of another primary disorder. For instance, in metabolic acidosis, if Pco2 is higher than expected from Table 6.1, there is evidence for a coexisting primary respiratory acidosis (even though the Pco2 may still be below the normal range). Another helpful thing to keep in mind: with a simple acid-base disturbance, the pH will rarely be in the normal range, as compensation for a primary disturbance is not “complete,” that is, normal pH is not completely restored. Either a normal pH or a very abnormal pH is frequently seen in double acid-base disorders. A normal pH can occur if the extent of acidosis is balanced by a similar extent of alkalosis. However, if both types of
acidosis or both types of alkalosis coexist, the pH may be very low or very high, respectively.
acidosis or both types of alkalosis coexist, the pH may be very low or very high, respectively.
