Nitrogen washout lung volumes are still relatively new to my PFT Lab. The number of problems we’ve encountered has decreased substantially but we are still learning some of the idiosyncrasies of the system. Recently while trying to understand a test with odd results we were reminded by the manufacturer that during the washout period a patient’s inspiratory and expiratory flow rates should not exceed 1.5 liters/second. The reason this “speed limit” is necessary highlights some of the limitations of modern open-circuit lung volume measurements.
The basic concept behind nitrogen washout is relatively simple. The air we breathe contains 78% nitrogen which is a relatively inert, insoluble gas. If you have a patient breathe 100% oxygen and then collect their exhaled air you can calculate the volume of exhaled nitrogen by multiplying the concentration in the exhaled air by the total volume of air that was collected. Once you know the volume of nitrogen you can then calculate the lung volume.
Initially this was a laborious and cumbersome process. The patient’s exhaled breathing circuit and a Tissot Gasometer (a very large spirometer with a volume between 125 and 300 liters) are first flushed with oxygen several times to remove any nitrogen. Next, while breathing room air the patient exhales to RV and an end-expiratory gas sample is taken and used to estimate the patient’s alveolar nitrogen concentration. The patient is then switched to 100% oxygen and breathes for seven minutes. At the end of the washout period the nitrogen concentration of the exhaled gas in the gasometer is analyzed and the volume recorded.
