Figure 1.
Two compartment lung model incorporating nitrogen excretion.
There are two alveolar compartments, A1 and A2, with volumes VA1 and VA2 respectively. The proportion of the ventilation and blood flow received by A2 are defined as X and Y respectively. A proportion of expiration representing dead space is re-circulated (DS). PA1, PA2 and PDS are the nitrogen partial pressures in the two alveolar and DS compartments respectively and FCorA1 and FCorA2 the rates of nitrogen excretion from the blood into the two alveolar compartments.
Table 1.
Effect of nitrogen excretion on LCI and FRC for different lung model scenarios.
Figure 2.
Effect of nitrogen (N2) excretion on expired N2 concentration from a two compartment lung model.
Expired N2 is shown with (solid line) and without (dotted line) N2 excretion for a model representing a healthy adult (A), and a CF adult (B). Washout ends at expired N2 = 1.975% (1/40th of the starting concentration). Increase in expired N2 at the true LCI point as a result of excreted nitrogen is indicated by the arrow.
Figure 3.
Impact of nitrogen excretion and increasing dead space fraction on functional residual capacity.
In the absence of nitrogen (N2) excretion, increasing dead space did not change the measured FRC (black diamonds). The effect of N2 excretion on FRC and the percentage error in measured FRC are shown by the grey diamonds and open circles respectively.
Figure 4.
Impact of nitrogen excretion and increasing dead space fraction on lung clearance index.
The impact of increasing deadspace on true LCI is shown by the lower curve (grey diamonds). The impact of nitrogen (N2) excretion on measured LCI is shown by the open circles, using true FRC. Measured LCI using measured FRC is also shown (black circles). The percent error in the measured LCI using measured FRC is shown by the black diamonds (right axis).
Figure 5.
Impact of increasing nitrogen excretion and ventilation heterogeneity on functional residual capacity.
In the absence of nitrogen (N2) excretion, increasing X did not change the measured FRC (black diamonds). The effect of N2 excretion on FRC and the percentage error in measured FRC are shown by the grey diamonds and open circles respectively.
Figure 6.
Impact of nitrogen excretion and increasing ventilation heterogeneity on lung clearance index.
The impact of increasing X on LCI is shown by the lower curve (grey diamonds). The impact of nitrogen (N2) excretion on measured LCI is shown by the open circles. The error is partially offset for by the additional error in FRC (black circles). The percentage error in LCI caused by nitrogen excretion is shown by the black diamonds (right axis).
Figure 7.
Contour map showing combined impact of nitrogen excretion, ventilation heterogeneity and dead space on nitrogen washout.
Impact of increasing ventilation heterogeneity (varying X between 0.5–0.9) and dead space (0.3–0.6) is shown on LCI (A) and the fractional error in LCI caused by nitrogen excretion (B). Numbers on contours represent the resulting LCI (top) or fractional error (bottom). Fractional error of 0.07 (pale grey) represents a 7% error in LCI due to nitrogen excretion.
Figure 8.
Contribution of excreted nitrogen to expired nitrogen concentration from a child lung model.
Expired nitrogen (N2) is shown with (solid line) and without (dotted line) N2 excretion for a two compartment lung model adjusted to represent a child with cystic fibrosis. N2 excretion has been reduced by a factor of 0.45 (A) to reflect the lower body mass and blood volume, or increased by a factor 1.6 (B) to reflect increased tissue perfusion and relative cardiac output. Washout ends at expired N2 = 1.975% (1/40th of the starting concentration). Increase in expired N2 at the true LCI point as a result of excreted nitrogen is indicated by the arrow.