Biomedical Engineering Reference
In-Depth Information
FIGURE 9-18
Volume fractions
measured in
spirometry [Adapted
from (Bronzino
2006).]
9.5.1.1 Volume Fraction Definition
There are nine different volume fractions as characterized in spirometry; they are repro-
duced in Figure 9-18. The residual volume (RV) represents the volume of air left in the
lungs after a maximal expiration. The vital capacity (VC) is the maximum volume of
air that can be exhaled after a maximal inspiration. VC has three components. The first
is the inspiratory reserve volume (IRV), which is the quantity of air that can be inhaled
from a normal end inspiratory position. The second is the tidal volume (TV), which is the
volume of air inspired and expired with each breath (about 0.5 L during normal breathing).
The third is the expiratory reserve volume (ERV), which is the amount of air that can be
exhaled from the lungs from a normal end-tidal expiratory position characterized by a
relaxed expiratory pause. This is the easiest position to reproduce, and the lung volume
in this position is called functional residual capacity (FRC
RV). The total lung
capacity (TLC) is the total volume of air in the lungs when they are maximally inflated
(RV
=
ERV
+
+
VC) and is approximately 6 L of air.
9.5.1.2 Volume Tests
Several timed respiratory volume tests are used to determine the ability of the respiratory
system to move air. These include the forced vital capacity (FVC), forced expiratory
volume in
t
sec (FEV
t
)
, the maximal voluntary ventilation (MVV), and the peak flow (PF).
These measurements are obtained using a spirometer without valves or CO
2
absorber or
a pneumotachograph and an integrator.
The FVC test is performed by taking the maximum inspiration and forcing all of the
inspired air out as rapidly as possible. It uses all the expiratory and accessory muscles.
When the strong expiratory accessory muscles are contracted, high airflows at lung vol-
umes near total lung capacity are generated. However, just following peak-expiratory flow
(PEF) the airflow velocity decreases linearly with volume no matter how hard the subject
tries. This is the effort-independent airflow and is caused by dynamic airway compression.
