Page 134 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF RESPIRATORY DEVICES 113

7

6

5 VC

Volume (L) 4 TLC TV

3
ERV
FRC
2

1 RV

0
FIGURE 4.2 Spirometer tracings showing the lung volumes. Note that since the zero point of
the spirometer may be set arbitrarily, only volumes not depending on zero (VC, TV, and ERV in
this figure) may be measured directly.

exhaled, starting from full inflation, in a single breath. Thus, TLC = RV + VC. Figure 4.2 shows a
diagram of the various lung volumes and capacities.
Unlike TV, which is determined in large part by effort and ventilatory drive, TLC, RV, and VC
are parameters that reflect certain properties of the pulmonary physiology. For example, a condition
tending to increase the propensity of the airways to collapse would tend to increase the RV, and
thereby decrease the VC, without changing the TLC. Asthmatics, in fact, may exhibit just this
propensity during an asthma attack. Likewise, a condition increasing the elasticity (or “stiffness”) of
the lung would tend to decrease the TLC, and perhaps the VC, as the muscles became unable to sup-
ply sufficient force for continued inspiration. One condition causing such a change is asbestosis.
Measurement of the various static lung volumes can be helpful both in classifying a lung disease and
in tracking its progress or responsiveness to treatment.
During the process of ventilation, gases must travel through the airways linking the alveoli with
the nose and mouth. These airways impose a pneumatic resistance to the flow of air, which, similar
to an electrical resistance limiting current, reduces the flow for a given pressure gradient. During
quiet breathing, most of the resistance to airflow occurs in the upper airway and not in the smaller
airways within the lung. The resistance of the airways themselves is affected by the volume of the
lung at the time of measurement, because as the lung approaches full inflation, the airways are
stretched radially, achieving their greatest diameter and hence their lowest resistance. Another lung
parameter affecting both static and dynamic changes is the lung compliance, a measure of tissue
elasticity. The compliance is defined as the change in volume divided by the change in pressure
across the wall of the lung, has units of volume over pressure, and is typically measured as the slope
of a plot of lung volume against transmural pressure. Even in a person having normal muscle
strength, it is possible to have marked reductions in the static lung volumes (such as TLC, VC, and
RV) because of increases in the stiffness of the lung tissue. Like the resistance, compliance depends
on the volume of gas contained within the lung. As the lung volume approaches TLC, it takes con-
siderably greater pressure to continue inflation when compared with similar inflations taken nearer
to FRC, representing a reduction in lung compliance at higher lung volumes.