Page 150 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF RESPIRATORY DEVICES 129

A B
FIGURE 4.15 Diagram illustrating the helium-dilution lung-volume test. Prior to the test (a), the spirometer system
is filled with a known concentration of helium. When the test is completed (b) after the helium has distributed and
reached equilibrium with the gas in the lungs, the ratio of final to initial concentration of helium is equal to the ratio of
spirometer volume to spirometer plus lung volume. In practice, the spirometer used generally has two ports, not only
one as pictured, and allows for better mixing of the gases.

In the helium-dilution method, a two-port spirometer is connected with tubes to a breathing valve
and a blower motor. The spirometer is filled with a known concentration of helium and the patient,
with no helium in the lungs, is attached at FRC by turning the breathing valve. At the beginning of
the test, the total amount of helium contained within the system is equal to the initial concentration
times the system volume. After 3 to 7 minutes of rebreathing, when the helium has been distributed
evenly between the patient’s lungs and the system, the final helium concentration is recorded
(Fig. 4.15). Then, owing to the fact that helium is inert and does not leave the lung via solution in
tissues or diffusion into the blood,
F IHE − F FHE
FRC = V S (4.11)
F FHE
where V = system volume
S
F = initial helium concentration
IHE
F = final helium concentration
FHE
This test requires that some of the gas be circulated through a CO -removal canister to prevent
2
exhaled CO from rising to uncomfortable or dangerous levels during the rebreathing period. Oxygen
2
is added periodically to maintain a constant total system volume at end expiration. Once FRC is
obtained, TLC and RV may be calculated, after the patient performs one or more VC maneuvers, as
described earlier. The helium-dilution system, since it is a closed-system test, is almost always per-
formed using a volume-displacement spirometer. It also requires the use of a helium analyzer.
The nitrogen-washout method attempts to “wash out” all of the nitrogen from the lung during a
period of 100 percent oxygen breathing (Fig. 4.16). All of the exhaled gas is analyzed for volume
and N concentration, which is integrated to give the total volume of nitrogen washed from the lung.
2
By assuming a known initial concentration of nitrogen within the lung, and having measured its vol-
ume, the total volume of gas contained within the lung may be calculated as follows:
FRC = V E N 2 (4.12)
F A
N 2
is the initial alveolar N concentration (typically
2 2
where V EN 2 is the total volume of exhaled N and F AN 2
taken to be ~0.79).