Toward the end of a nitrogen washout test for functional residual capacity, the expired concen-tration of N2 begins rising. What does this probably indicate?

a. The patient is trying too hard.
b. There is a leak in the system.
c. The test is nearing completion.
d. N2 is being trapped in the lungs.

ANS: B
The test must occur in a leak proof circuit because the presence of any air increases the measured nitrogen percentages and results in grossly elevated measurements of lung volume.