Biomedical Engineering Reference
In-Depth Information
5. Instability Study
Our lung model was used to simulate spontaneously breathing patients with
obstructive lung disease ventilated with PSV using the following parame-
ters: R
i
= R
e
= 20 cmH
2
O/L/s, P
set
= 22 cmH
2
O, P
peep
= 5 cmH
2
O,
D = 0:75, C = 0:6; 0:08; 0:10; and 0.12 L/cmH
2
O, f = 14; 16; 18; 20; and 22
breaths/min, over a range of ow cut-o settings from 5% to 80% of peak
inspiratory ow (0:050:80), and pressure triggering levels of 1, 3,
5, 10 and 15 cmH
2
O. Instability is said to occur if the tidal volume (V
T
)
coecient of variation (CV) is greater than 10% where CV = 100
,
SD
Mean
or there is a positive integer i such that t
(i)
tot
6= t
(j)
tot
, if i 6= j which denes
\skipped breaths."
Figure 3 shows the tidal volume coecient of variation (CV) computed
from the lung volume, calculated from our mathematical model (4)-(5)
over 100 consecutive breaths. In Figure 3(a), CV is greater than 10% if
0:08 < < 0:09 and 0:31 < < 0:36. At these points, the tidal volume
V
T
is unstable. In Figure 3(b-d), skipped breaths are seen to occur for
0:08 < < 0:09 and 0:31 < < 0:36. For example, at = 0:36, there are
65 breaths with t
tot
n) =
120
and 35 breaths with t
(n)
tot
=
6
f
. At these points,
V
T
is unstable. Figure 3(e) shows the volume wave form for = 0:20, for
which CV < 10% and there is no skipped breath, that is, for all n, t
(n)
f
tot
=
12
f
,
so that V
T
is stable. Figure 3(f) shows the volume wave form for = 0:35,
for which CV > 10%, so that V
T
is unstable. There is an i = 14 such that
t
(14)
tot
= 6 6= 3 = t
(j)
tot
, j = 1; 2; :::; 13 .
We plotted the ow cut-o, , against lung compliance C in Figure 4 in
which P
sen
is 1:0 and 15:0 cmH
2
O. It is observed here that lower values of
P
sen
gives rise to more unstable regions at higher cut-o values. Moreover,
instability is greater with a higher compliance. Figures 5 and 6 show that
increasing respiratory frequency, resistance or compliance exacerbates the
instability of ventilatory support.
6. Comparison of Model Simulation with Experimental
Data
A comparison of model simulations and experimental data with C = 0:08
L/cmH
2
O, P
set
= 22 cmH
2
O, P
peep
= 5 cmH
2
O, t
tot
= 6 s, P
sen
= 15
cmH
2
O, f = 20 breaths/min, R
i
= R
e
= 15 cmH
2
O/L/s, is shown in Fig-
ure 7. In this gure, the tidal volume coecient of variation (CV) computed
from the lung volume calculated from our mathematical model (4)-(5) over
100 consecutive breaths. It is then plotted against the ow cut-o . The
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