EVALUATION OF ACID-BASE STATUS

Clinical diagnosis of acid-base imbalance needs evalua­tion of three parameters on arterial blood gas (ABG) report-pH, HCO₃ and pCO2. Normal range of these parameters are given in Table 7.12.

Interpretation of an ABG report involves answers to following questions:

a. Whether blood pH normal or abnormal?

*Also denoted as torr units: 1 torr = 1 mm Hg.

Normal blood pH varies from 7.35 to 7.45. Lower pH lt;7.35 indicates acidosis, while higher pH gt;7.45 indicates alkalosis. However, normal pH does not exclude underlying acid-base imbalance, as it could return to normal after some time due to compensatory mechanisms despite persistence of physiological abnormality.

b. Whether pH abnormality is metabolic or respiratory in origin?

Answer to this question requires study of two major pH determinants—HCO₃ or pCO₂, to identify primary altered parameter and direction of change. Predominant changes in HCO₃ values indicate either metabolic acidosis (#936; HCO₃ ) or metabolic alkalosis (#8593; HCO₃). On the other hand, predominant changes in pCO₂ values indicate respiratory acidosis (#8593; pCO₂) or respiratory alkalosis (#936; pCO₂). In later stages however, it may be difficult to identify primarily altered parameter due to compensatory change in other parameter. In such situations, clinical clues help to identify origin of pH abnormality.

c. Whether any compensatory change has occurred?

As the pH is a primarily a ratio between HCO₃ and pCO₂ (excluding effects of other buffers), compensatory mechanisms attempt to change the secondary parameter in the same direction as the primary abnormality, to restore normalcy. For example, #936; HCO₃ in metabolic acidosis is compensated

by increased respiratory efforts to 4 pCO ₂; or #8593; pCO₂ in respiratory acidosis is compensated by excess renal bicarbonate reabsorption to #8593; HCO₃. Normal values of secondary parameter indicate absence of compensation, while altered values indicate partial or adequate compensation, discussed in next step.

d. Whether compensation is adequate or inadequate?

e. Efficacy of compensatory mechanisms depends on the time available for compensation and adequate pulmonary or renal functions. For example, metabolic acidosis may not be adequately compensated in a poorly ventilated child, who is unable to eliminate extra CO₂ for compensation. Compensation is considered as adequate if the secondary parameter has changed in following proportions to the change in primary parameter:

- In metabolic acidosis, 1 mEq/L drop in HCO₃^- is compensated by 1.2 mm Hg drop in pCO₂.

- In metabolic alkalosis, 1 mEq/L rise in HCO₃^- is compensated by 0.7 mm Hg rise in pCO₂.

- In respiratory acidosis, 10 mm Hg rise in pCO₂ is compensated by 3.5 mEq/L rise in HCO₃^-.

- In respiratory alkalosis, 10 mm Hg drop in pCO₂ is compensated by 5 mEq/L drop in HCO₃^-.

Proportionately lesser change in second parameter indicates inadequate compensation.

Following examples illustrate the interpretation of an ABG report:

• pH 7.25, pCO₂ 58, HCO₃^- 25: Low pH indicates acidosis, high pCO₂ indicates respiratory acidosis and normal HCO₃ indicates uncompensated respiratory acidosis.

• pH 7.48, pCO₂ 20, HCO₃^- 17: High pH indicates alkalosis, low pCO₂ indicates respiratory alkalosis and simultaneous reduction in HCO₃ indicates compensated respiratory alkalosis.

• pH 7.2, pCO2 40, HCO3^-18: Low pH indicates acidosis, low HCO₃^- indicates metabolic acidosis and normal pCO₂ indicates uncompensated metabolic acidosis.

• pH 7.5, pCO₂ 39, HCO₃^- 30: High pH indicates alkalosis, high HCO₃^- indicates metabolic alkalosis and normal pCO₂ indicates uncompensated metabolic alkalosis.

7.6.3