A pulmonary shunt may be present.

The important thing to notice here is that the oxygen level in inspired air was dropped by more than half between the second specimen on day 1 and day 3, with essentially no change in the blood pO2. If the pO2 of 63 were due to a selective block in oxygen transfer (VQ mismatch or pulmonary fibrosis), decreasing the oxygen content of the inspired air should have dramatically affected pO2.

Remember that shunts normally do not affect pCO2 unless they are very large, because the lung has a lot of excess capacity to remove CO2. Thus in this case where CO2 retention is occurring, we would be postulating a very big shunt.

Hematocrit          52 %
Serum bicarbonate   24 meq/l

        Day 1, 63% O2 by hood      Day 3, 25% O2
------------------------------------------------
pO2         107      63                  57
pCO2         48      45                  36
pH         7.35    7.37                7.43