Multiple-Breath Nitrogen Washout

Measurements of the uneven distribution of ventilation are also sensitive to mild airway obstruction. If the resident nitrogen in the lung is washed out by breathing 100% oxygen, the concentration of nitrogen decreases as cumulative expired volume decreases. If the distribution of ventilation is normal and uniform, the lung appears to behave as a single compartment that produces a relatively fast, single-exponential washout curve for nitrogen ( Fig. 26-16 ). In the abnormal patient with lung disease and nonuniform ventilation, the curve deviates from the single exponential and appears to contain more than one ventilatory compartment. Different lung units have their nitrogen diluted at different rates. The fast, well-ventilated alveoli cause a rapid decrease in expired nitrogen, whereas slow, poorly ventilated areas produce a prolonged tail on the washout curve. Unequal time constants throughout the lung may explain the phenomena. Although multiple-breath nitrogen washout appears to be a sensitive test, appropriate analysis of the curve is tedious and requires computer analysis to be practical as a screening test.