Respiratory alkalosis is converted to mild metabolic acidosis.

On initial presentation (room air) the patient is alkalotic (the pH is above 7.4). The pCO2 is less than 40, indicating that the lungs have removed acid from the body. Thus the alkalosis is respiratory alkalosis. We don't have the bicarbonate value here, but since this lung problem is chronic, the kidney has probably lowered the bicarbonate (base) concentration in blood in an attempt to compensate for the elevated pH. This pattern is common in patients with COPD, who must breath hard to maintain oxygenation. The "excess" breathing removes extra CO2 (which is equivalent to acid in the body), producing alkalosis.

When he is placed on oxygen, the oxygen-induced respiratory drive decreases and the pCO2 rises some. However, the declining pH actually passes 7.4 before the pCO2 reaches the normal level of 40 mm Hg. This is because the renal compensation is "uncovered" when the pCO2 rises and it can't respond fast enough to avoid an overshoot in pH. Now the tables are turned and the kidney is causing a mild metabolic acidosis (decreased pH because the bicarbonate level is too low). This will resolve over a few hours as the kidneys respond.

In patients with severe COPD and severe respiratory alkalosis, a sudden treatment with pure oxygen resulting in a sudden drop in breathing rate has been known to produce dangerous levels of acidosis by this mechanism.

Blood gases: 

            Room Air          100% oxygen
pO2            43 mm Hg           452 mm Hg
pCO2           22                  34
pH           7.51                7.38