30.20: Diagnosing Acidosis and Alkalosis

Diagnosing acid-base imbalances involves systematically analyzing arterial blood samples, focusing on three key measurements: pH, bicarbonate (HCO₃⁻) concentration, and carbon dioxide partial pressure (P_CO2). This analysis follows a four-step process that helps identify the imbalance's underlying cause and nature.

First, the pH level is assessed to determine whether the blood pH is normal (7.35–7.45), low (acidosis), or high (alkalosis).

Next, the P_CO2 and HCO₃⁻ values are examined to identify which component is driving the change in pH. A deviation in P_CO2 reflects a respiratory contribution, as carbon dioxide is regulated by pulmonary function. In contrast, an alteration in HCO₃− points to a metabolic cause, as bicarbonate is a key buffer managed by renal function. . For instance:

1. Acidosis with an elevated P_CO2 suggests respiratory acidosis, while reduced HCO₃− indicates metabolic acidosis.
2. Alkalosis with a low P_CO2 suggests respiratory alkalosis, while elevated HCO₃− indicates metabolic alkalosis.

The third step determines the primary disturbance by correlating the abnormal parameter (P_CO2 or HCO₃−) with the direction of the pH change. If the pH is acidic and P_CO2 is high, the disturbance is respiratory acidosis. Conversely, if the pH is acidic and HCO₃− is low, the disturbance is metabolic acidosis.

The final step involves examining whether the unaffected parameter (HCO₃− in respiratory disturbances or P_CO2 in metabolic disturbances) has shifted in a compensatory direction to minimize the pH imbalance. The body has not compensated for the imbalance if this value remains within normal limits. However, if this value is also abnormal but its change does not correspond to the change in the pH, it indicates that the body is attempting to compensate and partially correct the pH disturbance.