Fundamental Acidbase Disorders

Any condition that disturbs acid-base balance by increasing [H +]—whether through endogenous production, decreased buffering capacity, decreased excretion, or exogenous addition—is termed acidosis. Similarly, any condition that decreases [H+] is termed alkalosis. The terms acidemia and alkalemia refer to the net imbalance of [H+] in the blood. The difference between acidosis and acidemia is not merely semantic but of great clinical importance. For example, a patient with an acidosis and an alkalosis of equal magnitude will have a normal pH. A patient with these disturbances, then, has neither acidemia nor alkalemia but has both an acidosis and an alkalosis. It is therefore important to appreciate that while acidemia is diagnostic of acidosis and alkalemia of alkalosis, a normal or high pH does not exclude acidosis and a normal or low pH does not exclude the presence of an alkalosis.

Acid-base disturbances are further classified as either respiratory or metabolic. Respiratory acid-base disorders first affect P co2, while metabolic disorders first alter bicarbonate concentration. If there were no compensatory mechanism, the Kassirer-Bleich equation states that the magnitude of change in either P co2 or [HCO2-] would directly determine the magnitude of change in the pH. However, physiologic mechanisms tend to mitigate pH changes by effecting offsetting changes in bicarbonate and CO2 levels. For example, the Pco2 (and thus [H+]) is elevated in respiratory acidosis, but the corresponding fall in pH is attenuated over time by renal retention of bicarbonate.

Compensatory mechanisms are, by definition, not "disorders." Such mechanisms constitute normal physiologic responses to acid-base derangements, and such terms as a compensatory respiratory alkalosis are therefore not only confusing but also misleading. The clinician is concerned with the adequacy of compensation, however, because failure of adequate compensation implies the presence of another primary acid-base disturbance. For example, a patient with metabolic acidosis who does not exhibit adequate respiratory compensation has a superimposed respiratory acidosis. Normal compensatory responses to each primary acid-base disturbance have been established through careful study and are presented later in this chapter.

It is important to note that compensatory mechanisms return the pH toward but not to normal.^ The fact that compensatory mechanisms cannot become complete is evident when one considers that complete compensation would necessarily remove the (physiologic) stimulus driving the compensation. 1

The "normal" values of pH, Pco2 and [HCO3-] for a given laboratory are ranges, intended to include 95 percent of patients without an acid-base disorder. The normal pH range is 7.35 to 7.45, the normal Pco2 range is 35 to 45 mmHg, and the normal [HCO3-] is usually 21 to 28 meq/L. However, a patient may have no value outside the "normal range" and still have significant acid-base disturbances. As detailed further below, a patient with a wide anion gap acidosis and a concomitant metabolic alkalosis of near-equal magnitude will have a normal pH, Pco2 and [HCO3-]. On the other hand, abnormal-appearing values may, in fact, be appropriate for a given simple acid-base disturbance. For example, in the presence of a metabolic acidosis ([HCO 3-] = 15; pH = 7.3), an appropriate respiratory compensation should result in a Pco2 of about 30 mmHg. The Pco2 is below the "normal" range but at the expected level for the degree of (metabolic) acidosis. In this example, the finding of 40 mmHg for Pco2, a "normal"-appearing value, actually implies the presence of a respiratory acidosis, since the expected physiologic respiratory response is inadequate.

The Anion Gap

The principle of electroneutrality requires that plasma have no net charge. The charge of the predominant plasma cation, sodium, must therefore be "balanced" by the charge of plasma anions. Although bicarbonate and chloride constitute a significant fraction of plasma anions, the sum of their concentrations does not equal that of sodium. There must therefore be other anions present in the serum that preserve electroneutrality. These anions are mostly serum proteins, phosphate, sulfate, and organic anions such as lactate and the conjugate bases of ketoacids. Because these substances are not commonly measured, they are termed unmeasured anions (UA). Unmeasured cations (UC) also exist largely in the form of Ca2+ and Mg2+. Since all cations, both measured and unmeasured, must equal all anions (both measured and unmeasured),

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