Nitrogen washout
= Nitrogen washout
\b[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.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Measurement of FRC using MBW, Introduction and theory; Figure 5]{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,
$$
Vf_0=Vf_T+W,\qquad V=\frac{W}{f_0-f_T}.
$$
For stipulated $W=2.10$ L, $f_0=0.78$ and $f_T=0.01$, $V\simeq2.73$ 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,
$$
N(t)=a e^{-t/\tau_1}+b e^{-t/\tau_2},\qquad \tau_2>\tau_1,
$$
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.