What PCO2 value would be seen in maximally compensated metabolic acidosis?

In maximally compensated metabolic acidosis, the body attempts to offset the elevated acidity (low pH) due to a decrease in bicarbonate (HCO3-) concentration by increasing respiratory rate to blow off carbon dioxide (CO2). This compensation mechanism leads to a reduction in arterial PCO2 levels.

The value of 15 mm Hg indicates a significant decrease in PCO2, which reflects hyperventilation and is characteristic of respiratory compensation in response to metabolic acidosis. Under these circumstances, the body's effort to lower CO2 levels serves to mitigate the effects of acidosis, promoting a shift back toward a more normal pH.

Values such as 30 mm Hg may suggest some compensation is occurring but are not indicative of maximal compensation. A PCO2 of 40 mm Hg is considered normal, suggesting no compensation from respiratory efforts has taken place. A PCO2 of 60 mm Hg would indicate hypoventilation, which is counterproductive in the scenario of metabolic acidosis. Thus, the significant reduction in PCO2 to 15 mm Hg precisely reflects the body’s maximal compensatory response to metabolic acidosis.