Diagnosing acid-base imbalances involves systematically analyzing arterial blood samples, focusing on three key measurements: pH, bicarbonate (HCO3−) concentration, and carbon dioxide partial pressure (PCO2). 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 PCO2 and HCO3− values are examined to identify which component is driving the change in pH. A deviation in PCO2 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:
The third step determines the primary disturbance by correlating the abnormal parameter (PCO2 or HCO3−) with the direction of the pH change. If the pH is acidic and PCO2 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 PCO2 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.
From Chapter 30:
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