Nitrogen washout test

Nitrogen washout (or Fowler's method) is a test for measuring anatomic dead space in the lung during a respiratory cycle, as well as some parameters related to the closure of airways. Nitrogen washout (or Fowler's method) is a test for measuring anatomic dead space in the lung during a respiratory cycle, as well as some parameters related to the closure of airways. A nitrogen washout can be performed with a single nitrogen breath, or multiple ones. Both tests use similar tools, both can estimate functional residual capacity and the degree of nonuniformity of gas distribution in the lungs, but the multiple-breath test more accurately measuresabsolute lung volumes. The following describes a single-breath nitrogen test: A subject takes a breath of 100% oxygen and exhales through a one-way valve measuring nitrogen content and volume. A plot of the nitrogen concentration (as a % of total gas) vs. expired volume is obtained by increasing the nitrogen concentration from zero to the percentage of nitrogen in the alveoli. The nitrogen concentration is initially zero because the subject is exhaling the dead space oxygen they just breathed in (does not participate in alveolar exchange), and climbs as alveolar air mixes with the dead space air. The dead space can be determined from this curve by drawing a vertical line down the curve such that the areas below the curve (left of the line) and above the curve (right of the line) are equal.