In this video, from our Pulmonary Function Testing Essentials course, you'll learn all about the nitrogen washout test and the concept of closing volume when assessing small airway function in your patients.
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[00:00:00] Exhaled gas concentrations can also provide information on the uniformity of inspired gas distribution throughout the lungs. When we exhale, the concentration of gases in the expirate changes from beginning to end of expiration. If a gas is distributed non-uniformly throughout the lungs, the changes and concentration of the gas, throughout expiration, will be greater than normal.
[00:00:30] The single-breath nitrogen washout test is based on this hypothesis. The seated patient takes two breaths of air and exhales to residual volume. Then, the valve on the mouthpiece, is turned to administer 100% oxygen. The patient takes a deep breath of the oxygen to total lung capacity and then exhales slowly to residual volume. The concentration of nitrogen in the expired
[00:01:00] gas is measured continuously using a nitrogen meter. Additionally, exhaled volume is measured using a pneumotachograph. A plot of exhaled nitrogen concentration versus exhaled volume is generated. A typical plot appears as follows: four relatively discrete phases of the curve, I, II, III, and IV are depicted. Phase
[00:01:30] one represents the initial portion of the expirate, which is devoid of nitrogen, since pure oxygen was inhaled during the previous breath. This portion represents gas, residing in the proximal airways, from the end of the prior inspiration. Phase two constitutes a mixture of gas, from the central airways and the alveoli. Phase three is defined by a mixture of gases from the lung apices, mid lung zones,
[00:02:00] and lung bases. And phase four constitutes gas arising predominantly from the apices, as the airways at the bases close, as lower lung volumes are approached. With inspiration, most inspired gas goes to the bases where there are more alveoli and where the alveoli are less distended than those in the apices, and hence, can expand more. As a result,
[00:02:30] the inhaled pure oxygen is primarily distributed to the bases, diluting the nitrogen in that region and accounting for a concentration difference in nitrogen between the apices and the bases. During exhalation, the decreased elastic recoil of the lung and narrowing of small airways cause closure of small airways at the bases of the lungs. As this happens, the contribution of gas from
[00:03:00] basilar alveoli to the expired gas concentration decreases. And as exhalation progresses, more exhaled gas comes from the apices, which represent the less diluted alveolar air, which is higher in nitrogen concentration. The exhaled volume from the onset of phase four, to the end of the expiratory maneuver, is known as closing volume. Remember, this is the
[00:03:30] portion of the exhaled air after the closure of the small airways so a change in this value reflects changes in the small airways. Normal closing volume is about 10% of vital capacity. With small airways disease, aging, premature closure of airways, or cigarette smoking, closing volume is increased. In patients greater than 50 years of age, closing volume may be
[00:04:00] 25% of vital capacity.