Closing Volume and Spirometry

Question:

Where would I locate the closing volume and capacity in a spirometry diagram?

Answer:

Typically, you cannot locate closing capacity (CC) or closing volume (CV) on a spirometry diagram. However, you can do specialized tests to determine your CC and CV.

First, let’s back up and establish our parameters for respiratory physiology.

Lung volumes and capacities

There are four standard lung volumes (which are not subdivided)

  • Residual volume (RV)
  • Expiratory reserve volume (ERV)
  • Tidal volume (TV)
  • Inspiratory reserve volume (IRV)

and four standard lung capacities, which consist of two or more standard lung volumes in combination

  • Functional residual capacity (FRC)
  • Vital capacity (VC)
  • Inspiratory capacity (IC)
  • Total lung capacity (TLC)

Normal respiration moves air in and out of the lungs using the tidal volume (TV). We cannot exhale below the residual volume (RV), which always becomes the smallest amount of gas we have in the lungs. Only by exerted effort are we able to exhale to this point (RV). We can measure all lung volumes except RV. Without measuring RV directly, we cannot measure FRC or TLC since these capacities include RV. To measure RV, we need to do specialized testing. There are three ways we can determine the RV

  • Nitrogen washout technique
  • Helium dilution technique
  • Body Plethysmography

Closing volume (CV) is the lung volume at which airway closure begins, and closing capacity (CC) is CV + RV. The most common method to determine CC/CV is a single-breath nitrogen washout called Fowler’s. With this technique, you can make a diagram showing when small airways begin to close.

Nitrogen washout technique

In the single-breath nitrogen washout technique, the person makes a complete exhalation (to RV). At this point, the air in the lungs is mainly in the upper part. The lungs' lower (dependent) part is closed and contains little nitrogen. The subject then inhales 100% oxygen to TLC. Now, the lungs are maximally filled with gas.

Nitrogen washout diagram

All expired gas is collected and analyzed during the following slow exhalation. The nitrogen concentration is plotted on a curve against the expired total volume of gas (fig 2). The exhalation is now separated into 4 phases: phases I – IV

  • Phase I: Gas from anatomic dead space, which will not contain any nitrogen
  • Phase II: Gas is a mixture of dead space and alveolar gas
  • Phase III: Gas is mixed alveolar gas from the upper and lower regions of the lungs, also called the alveolar plateau phase
  • Phase IV: This phase represents airway closures

Airway closures will begin in the dependent part of the lungs. In this part of the lungs, the alveoli have less elastic recoil and, therefore, will close first. Since this part of the lungs was mostly closed when the test person started the initial inhalation, there will be very little nitrogen (low concentration) in these dependent alveoli. Thus, as airway closure begins, the expired nitrogen concentration rises abruptly because more and more of the expired gas is coming from the alveoli in the upper parts of the lungs. These upper alveoli have the highest nitrogen concentration. Phase IV represents this airway closure. The curve (fig. 2) shows the sharp rise in nitrogen concentration when airway closures begin. The part following CV on the curve represents RV, which does not have a tracing since this air will always stay in the lungs.

Tracer gas analysis

CC/CV can also be determined using a tracer gas. The testing technique is similar to the single-breath nitrogen washout technique, except a tracer gas (often Xenon or helium) is added to the inhaled gas. The diagram may look slightly different because it tries to show the detected tracer gas and the lung capacities in the same picture (Fig. 3).

Tracer gas volumes

Technique

From end expiration (RV), the test person starts a complete inspiration. During the initial part of this inhalation from RV, the first gas to enter the alveolus is the dead space gas and the tracer bolus. The tracer gas will only enter alveoli that are already open (presumably the apices of the lungs [hatched lines fig. 3]) and does not enter alveoli that are already closed (presumably the bases of the lungs [no hatched lines fig. 4]). As the inhalation continues, apical alveoli complete filling, and basilar alveoli begin to open and fill, but with gas that does not contain any tracer gas. A differential tracer gas concentration is thus established; the gas in the apices has a higher tracer concentration (fig. 3 - hatched lines) than that in the bases (fig. 3 - no hatched lines). As the subject exhales and the diaphragm ascends, a point is reached at which the small airways just above the diaphragm start to close, limiting airflow from these areas. The airflow now comes more from the upper lung fields, where the alveolar gas has a much higher tracer concentration, which results in a sudden increase in the tracer gas concentration toward the end of exhalation (phase IV).

Analysis

As shown in Fig. 3, the bottom tracing resembles the single-breath nitrogen washout tracing. The added curve above shows lung capacities during breathing. Airway closures begin when the curve rises sharply (phase IV) on the lower graph. This can then be correlated to the upper diagram.

References

Michael Levitzky Pulmonary Physiology 9th edition p.59-62, 66, 85-87