Closing Volume and Spirometry
In a spirometry diagram where would I locate the closing volume and closing capacity?
Normally you will not be able to locate closing capacity (CC) or closing volume (CV) on a spirometry diagram. But, you can make specialized test to determine CC and CV.
First let’s back up a little and establish our parameters within 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 consists 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 is moving air in and out of the lungs using just the tidal volume (TV). We are not able to exhale below the residual volume (RV), which therefore become the smallest amount of gas we have in the lungs at all time. Only by exerted effort are we able to exhale to this point (RV).
We can measure all lung volumes with the exception of RV. Without being able to directly measure RV we cannot measure FRC or TLC either, 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 to occur and closing capacity (CC) is CV + RV.
Most common method to determine CC/CV is a single breath nitrogen washout, also called a Fowler’s method. With this technique you can make a diagram where it shows when small airways begin to close.
Nitrogen washout technique
In the single-breath nitrogen washout technique, the person makes a full exhalation (all the way to RV). The air present in the lungs at this point will be mostly in the upper part. The lower (dependent) part of the lungs will be closed and contain little nitrogen. The subject then inhales 100% oxygen to TLC. Now the lungs are maximally filled with gas.
Nitrogen washout diagram
During the following slow exhalation all the expired gas is collected and analyzed. The concentration of nitrogen is plotted on a curve against the expired total volume of gas (fig 2).
The exhalation is now separated into 4 phases: phase 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 were 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 stay in the lungs at all time.
Tracer gas analysis
CC/CV can also be determined using a tracer gas. The testing technique is similar to single-breath nitrogen washout technique, except there is added a tracer gas (often Xenon or helium) to the inhaled gas.
The diagram may look a little different because they are trying to show both the detected tracer gas and the lung capacities in the same picture (fig. 3).
Tracer gas volumes
From end expiration (RV) the test person starts a full 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).
As can be seen in fig. 3 the bottom tracing is similar to the single-breath nitrogen washout tracing. The added curve above shows lung capacities during the breathing.
When the curve rises sharply (phase IV) on the lower graph airway closures begin. This can then be correlated to the upper diagram.
Michael Levitzky Pulmonary Physiology 7th edition p.54-57, 59-61, 79-81