Distribution of Ventilation
Various pathologic processes alter the normal pattern of ventilation distribution (i.e., the uniformity with which an inhaled breath is distributed to all the alveoli). For this reason, tests that detect abnormal patterns of ventilation distribution are fairly nonspecific and rarely of diagnostic importance. Their major contribution is that such abnormal patterns almost always are associated with alterations in ventilation-perfusion relationships (see pages 66 and 55). Abnormal distribution of ventilation also contributes to the frequency dependence of compliance (see page 68).
There are several tests of ventilation distribution. Some are complex and require sophisticated equipment and complex analysis. This chapter discusses only the simplest procedure, the single-breath nitrogen (SBN2) test.
8A. Single-Breath Nitrogen Test
Procedure
The testing equipment and procedure are illustrated in Figure 8-1. The subject exhales to residual volume (RV) and then inhales a full breath of 100% oxygen from the bag on the left. A slow, complete exhalation is directed by the one-way valve through the orifice past the nitrogen meter into the spirometer. The orifice ensures that expiratory flow will be steady and slow (<0.5 L/s), and we recommend its use. The nitrogen meter continuously records the nitrogen concentration of the expired gas as it enters the spirometer. With simultaneous plotting of the expired nitrogen concentration against expired volume, the normal graph shown in Figures 8-1 and 8-2A is obtained.
Normal Results
The plot in Figure 8-2A is from a seated normal subject. There are four important portions of the normal graph: phases I through IV.
To understand this graph, we need to consider how the inhaled oxygen is normally distributed in the lungs of a seated subject. At RV, the alveolar nitrogen concentration can be considered uniform (roughly 80%) throughout the lung and alveolar gas is present in the trachea and upper airway (Fig. 8-3A). At RV, the alveoli (circles in Fig. 8-3A) in the more gravitationally dependent regions of the lung are at a smaller volume than those in the apical portions. Thus, the apical alveoli contain a larger volume of nitrogen at the same concentration. Therefore, as the subject inhales 100% oxygen, the apical alveoli receive proportionately less oxygen than the more dependent basal alveoli
and the alveolar nitrogen is less diluted than in the basal regions. Therefore, the nitrogen concentration is higher in the apical region. The result is a gradual decrease in nitrogen concentration farther down the lung, and the most diluted alveolar gas is at the base (Fig. 8-3B). At the end of inspiration, the trachea and proximal airways contain only oxygen.
and the alveolar nitrogen is less diluted than in the basal regions. Therefore, the nitrogen concentration is higher in the apical region. The result is a gradual decrease in nitrogen concentration farther down the lung, and the most diluted alveolar gas is at the base (Fig. 8-3B). At the end of inspiration, the trachea and proximal airways contain only oxygen.
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