<40 mm Hg >24 mEq/L >40 mm Hg <24 mEq/L

Figure 30-10

Analysis of simple acid-base disorders. If the compensatory responses are markedly different from those shown at the bottom of the figure, one should suspect a mixed acid-base disorder.

The second step is to examine the plasma Pco2 and HCO3- concentration. The normal value for Pco2 is about 40 mm Hg, and for HCO3-, it is 24 mEq/L. If the disorder has been characterized as acidosis and the plasma Pco2 is increased, there must be a respiratory component to the acidosis. After renal compensation, the plasma HCO3- concentration in respiratory acidosis would tend to increase above normal. Therefore, the expected values for a simple respiratory acidosis would be reduced plasma pH, increased Pco2, and increased plasma HCOf concentration after partial renal compensation.

For metabolic acidosis, there would also be a decrease in plasma pH. However, with metabolic acidosis, the primary abnormality is a decrease in plasma HCO3-concentration. Therefore, if a low pH is associated with a low HCO3- concentration, there must be a metabolic component to the acidosis. In simple metabolic acidosis, the Pco2 is reduced because of partial respiratory compensation, in contrast to respiratory acidosis, in which Pco2 is increased. Therefore, in simple metabolic acidosis, one would expect to find a low pH, a low plasma HCOf concentration, and a reduction in Pco2 after partial respiratory compensation.

The procedures for categorizing the types of alkalo-sis involve the same basic steps. First, alkalosis implies that there is an increase in plasma pH. If the increase in pH is associated with decreased Pco2, there must be a respiratory component to the alkalosis. If the rise in pH is associated with increased HCO3-, there must be a metabolic component to the alkalosis. Therefore, in simple respiratory alkalosis, one would expect to find increased pH, decreased Pco2, and decreased HCOf concentration in the plasma. In simple metabolic alkalo-sis, one would expect to find increased pH, increased plasma HCOf, and increased Pco2.

Complex Acid-Base Disorders and Use of the Acid-Base Nomogram for Diagnosis

In some instances, acid-base disorders are not accompanied by appropriate compensatory responses. When this occurs, the abnormality is referred to as a mixed acid-base disorder. This means that there are two or more underlying causes for the acid-base disturbance. For example, a patient with low pH would be categorized as acidotic. If the disorder was metabolically mediated, this would also be accompanied by a low plasma HCO3- concentration and, after appropriate respiratory compensation, a low Pco2. However, if the low plasma pH and low HCO3- concentration are associated with elevated Pco2, one would suspect a respiratory component to the acidosis as well as a metabolic component. Therefore, this disorder would be categorized as a mixed acidosis. This could occur, for example, in a patient with acute HCO3- loss from the gastrointestinal tract because of diarrhea (metabolic acidosis) who also has emphysema (respiratory acidosis).

A convenient way to diagnose acid-base disorders is to use an acid-base nomogram, as shown in Figure 30-11. This diagram can be used to determine the type of acidosis or alkalosis, as well as its severity. In this acid-base diagram, pH, HCO3- concentration, and Pco2 values intersect according to the Henderson-Hassel-balch equation. The central open circle shows normal values and the deviations that can still be considered within the normal range. The shaded areas of the diagram show the 95 per cent confidence limits for the normal compensations to simple metabolic and respiratory disorders.

When using this diagram, one must assume that sufficient time has elapsed for a full compensatory response, which is 6 to 12 hours for the ventilatory compensations in primary metabolic disorders and 3 to 5 days for the metabolic compensations in primary respiratory disorders. If a value is within the shaded area, this suggests that there is a simple acid-base disturbance. Conversely, if the values for pH, bicarbonate, or Pco2 lie outside the shaded area, this suggests that there may be a mixed acid-base disorder.

It is important to recognize that an acid-base value within the shaded area does not always mean that there is a simple acid-base disorder. With this reservation in mind, the acid-base diagrams can be used as a quick means of determining the specific type and severity of an acid-base disorder.

For example, assume that the arterial plasma from a patient yields the following values: pH 7.30, plasma HCO3- concentration 12.0 mEq/L, and plasma Pco2 25 mm Hg. With these values, one can look at the diagram and find that this represents a simple metabolic acidosis, with appropriate respiratory compensation that reduces the Pco2 from its normal value of 40 mm Hg to 25 mm Hg.

A second example would be a patient with the following values: pH 7.15, plasma HCO3- concentration 7 mEq/L, and plasma Pco2 50 mm Hg. In this example, the patient is acidotic, and there appears to be a metabolic component because the plasma HCO3- concentration is lower than the normal value of 24 mEq/L. However, the respiratory compensation that would normally reduce Pco2 is absent, and Pco2 is slightly increased above the normal value of 40 mm Hg. This is

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