Which of the following are true statements regarding the volume of tissue nitrogen excreted dur-ing the N2 washout test for measuring functional residual capacity (FRC)?

1. It is not a factor.
2. It varies with the length of the test.
3. It varies with the weight of the patient.
4. It cannot be correct.
a. 1 only
b. 2 and 3 only
c. 2, 3, and 4 only
d. 4 only


ANS: B
The volume of tissue nitrogen excreted (Vtis in milliliters) is directly related to the duration (t in minutes) of the test and weight (W in kilograms) of the patient. A correction for this extra nitro-gen should be made according to the following formula:
Vtis (ml) = (0.1209 – 0.0665) ⋅ (W/70). Vtis (ml) is subtracted from the BTPS-corrected lung volume. The residual volume is the difference between the expiratory reserve volume and the FRC.

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