ABG interpretation: pH 7.50; pCO2 60; HCO3- 40. Which pattern best fits this result?

ABG interpretation: pH 7.50; pCO2 60; HCO3- 40. Which pattern best fits this result?

• A. Mixed metabolic alkalosis and respiratory acidosis
• B. Compensated respiratory alkalosis
• C. Uncompensated metabolic acidosis
• D. Compensated nonrespiratory alkalosis

Answer: (A)

When interpreting ABG, start by pH to see the direction of deviation, then identify the primary disturbances with pCO2 (respiratory) and HCO3- (metabolic), and finally assess whether there is compensation or a mixed disorder. Here the pH is 7.50, showing alkalemia. The pCO2 is elevated at 60 mmHg, which points to a respiratory disturbance causing acidemia if acting alone. The HCO3- is also elevated at 40 mEq/L, indicating a metabolic alkalosis. Having both an elevated pCO2 and an elevated HCO3- in the setting of alkalemia means two disturbances are present at the same time: a metabolic alkalosis and a respiratory acidosis. The respiratory system is not simply compensating for the metabolic alkalosis; instead, an additional respiratory problem is present, producing a mixed pattern. This isn’t a compensated respiratory alkalosis, because that would require a low pCO2 as the primary issue (with metabolic compensation lowering bicarbonate). It isn’t an uncompensated metabolic acidosis, since the pH is alkaline, not acidic. It isn’t best described as compensated nonrespiratory alkalosis because, although metabolic alkalosis is present, the level of the respiratory "compensation" (the rise in pCO2) is larger than what would be expected from compensation alone, indicating a concurrent respiratory acidosis as well.

When interpreting ABG, start by pH to see the direction of deviation, then identify the primary disturbances with pCO2 (respiratory) and HCO3- (metabolic), and finally assess whether there is compensation or a mixed disorder. Here the pH is 7.50, showing alkalemia. The pCO2 is elevated at 60 mmHg, which points to a respiratory disturbance causing acidemia if acting alone. The HCO3- is also elevated at 40 mEq/L, indicating a metabolic alkalosis.

Having both an elevated pCO2 and an elevated HCO3- in the setting of alkalemia means two disturbances are present at the same time: a metabolic alkalosis and a respiratory acidosis. The respiratory system is not simply compensating for the metabolic alkalosis; instead, an additional respiratory problem is present, producing a mixed pattern.

This isn’t a compensated respiratory alkalosis, because that would require a low pCO2 as the primary issue (with metabolic compensation lowering bicarbonate). It isn’t an uncompensated metabolic acidosis, since the pH is alkaline, not acidic. It isn’t best described as compensated nonrespiratory alkalosis because, although metabolic alkalosis is present, the level of the respiratory "compensation" (the rise in pCO2) is larger than what would be expected from compensation alone, indicating a concurrent respiratory acidosis as well.