Diagnosing Acidosis and Alkalosis

Examine the PCO2 and bicarbonate ion levels in arterial blood. A change in PCO2 indicates a respiratory problem, since carbon dioxide is regulated by pulmonary function. Conversely, a change in bicarbonate ion levels points to a metabolic cause, as bicarbonate is managed by renal function. This distinction guides diagnosis of the underlying disturbance.

Respiratory acidosis is identified when pH is low (acidosis) and PCO2 is elevated above 45 mmHg. This pattern shows the respiratory system is failing to eliminate carbon dioxide effectively, causing acid accumulation. The bicarbonate ion level typically remains normal in primary respiratory acidosis.

A low PCO2 below 35 mmHg during acidosis indicates respiratory compensation. The respiratory system is actively working to eliminate carbon dioxide and raise pH, suggesting the primary problem is metabolic acidosis. This compensatory response helps minimize the pH imbalance caused by a metabolic disturbance.

In metabolic alkalosis, the bicarbonate ion level is elevated while pH is high. In respiratory alkalosis, PCO2 is low while pH is high. Metabolic alkalosis reflects altered bicarbonate managed by renal function, whereas respiratory alkalosis reflects excessive carbon dioxide elimination by the lungs.

An elevated PCO2 during alkalosis indicates the respiratory system is compensating for a metabolic disturbance. Since PCO2 should normally be low in alkalosis, the elevated value shows the body is retaining carbon dioxide to lower pH back toward normal. This compensatory mechanism helps counteract the primary metabolic alkalosis.

First, assess pH to determine acidosis, alkalosis, or normal status. Second, examine PCO2 and bicarbonate values to identify which component is driving the pH change. Third, correlate the abnormal parameter with pH direction to identify the primary disturbance. Fourth, check whether the unaffected parameter has shifted compensatorily to minimize the imbalance.

Examine whether the unaffected parameter (bicarbonate in respiratory disturbances or PCO2 in metabolic disturbances) has shifted in a compensatory direction. If this value remains within normal limits, no compensation has occurred. If abnormal but its change corresponds to pH change direction, the body is attempting to compensate and partially correct the disturbance.