Nitrogen washout

Nitrogen washout is a lung-gas measurement method that follows removal of resident nitrogen while the inspired gas contains little or no nitrogen. Gas analysis and expired volume are combined over multiple breaths to estimate the communicating gas volume.^[ref] It differs from adding helium and waiting for equilibration in helium dilution measurement of lung volume.

A conservation argument supplies the basic calculation. In an ideal compartment with unchanged gas volume $V$, initial and final nitrogen fractions $f_0$ and $f_T$, and net nitrogen volume $W$ removed,For stipulated $W=2.10$ L, $f_0=0.78$ and $f_T=0.01$, $V\simeq 2.73$ L. This is a simplified bookkeeping model with gas volumes compared at consistent conditions. Real equipment space and physiological gas exchange need their defined treatment.

The removed tracer is a volume-weighted quantity. For a nitrogen fraction $f(t)$ and positive expiratory volumetric flow rate $Q(t)$, an interval contributes $\int f(t)Q(t)dt$, with any inspired tracer subtracted for net removal. Two constructed expired parcels of 1 L at fraction 0.4 and 3 L at fraction 0.1 contain 0.7 L nitrogen in total. Multiplying their unweighted mean fraction 0.25 by total volume 4 L would instead give 1.0 L. Correct synchronization of gas and flow signals is therefore fundamental.

A long tail need not imply one uniformly slow compartment. In an original two-compartment model,the late signal is increasingly dominated by the slow component even if its initial amount $b$ was small. A finite stopping interval can consequently miss slowly communicating material. The washout curve, its integral and the functional residual capacity estimate describe different aspects of the same transport measurement.