Page 155 - Biomedical Engineering and Design Handbook Volume 2, Applications

134 MEDICAL DEVICE DESIGN

180
160
10 –4 140

× 120
Resistance (N . s/m 5 ) 80
100

60

40
20
0
0 20 40 60 80 100
Age
FIGURE 4.20 Respiratory resistance with age as measured by the APD.

4.5.6 Oxygen Consumption and Carbon Dioxide Production

The simplest method for measuring carbon dioxide production (V CO 2 ) is to collect a timed sample of
expired gas in a balloon or spirometer, measuring the total volume of gas collected and its CO con-
2
centration. If the inspired gas is assumed to contain no CO (a reasonable assumption for most mea-
2
2
surement purposes), then all of the CO contained within the expired gas came from the V CO 2 which
may be calculated as
VF ECO 2
V CO 2 = (4.21)
Time

Unfortunately, a similar equation does not hold for measurements of V O 2 . The reason for this is
that the inspired gas does contain oxygen, and thus there must be included a term to account for the
fact that the oxygen consumed is the relatively small difference between the large amounts of total

oxygen inspired and expired. Thus, an equation for V O 2 is

e
iIO 2
V O 2 = V F − V F EO 2 (4.22)

where V e is average expiratory ventilation in liters per minute and V i is average inspiratory venti-
are the inspired and expired oxygen concentrations,
lation in liters per minute and F IO 2 and F EO 2
respectively.

Note that, as described earlier, the V O 2 does not ordinarily equal the V CO 2 , which means as a con-
sequence that V e does not equal V i . Some devices do measure separately the V e and the V i , either
with two flow sensors or a single flow sensor measuring in both directions (inspiration and expiration).

However, it is possible to obtain a reasonably accurate calculation of V O 2 by noting that, in the steady
state, there is no net consumption nor production of N . Thus, the following equation for N con-
2 2
sumption may be set to zero:

e
V N 2 = V F − V F EN 2 = 0 (4.23)
iIN 2