Biomedical Engineering Reference
In-Depth Information
The microprocessor then calculates the required inspiratory flow trajectory. In the
example shown in Figure 9-42, which uses a half-sinewave flow waveform, the therapist
has selected a tidal volume,
V
t
(L), and a respiration rate of
n
(breaths/min).
The total desired respiratory flow,
Q
d
(
t
)
, for a single breath can be determined from
the peak flow rate,
Q
i
(L/s).
Q
i
sin
π
t
t
i
,
≤
t
≤
t
i
0
Q
d
(
t
)
=
(9.13)
0
,
t
i
≤
t
≤
t
e
where
t
i
is the duration of inspiration, calculated from the tidal volume and the peak flow
rate.
t
i
Q
i
sin
π
t
t
i
V
t
=
dt
0
t
i
Q
i
t
i
π
cos
π
t
t
i
=−
0
2
Q
i
t
i
π
=
Therefore, the duration of inspiration is
π
V
t
2
Q
i
t
i
=
(9.14)
The duration of expiration in seconds is determined from the breathing rate and the inspi-
ration time
60
n
−
t
e
=
t
i
(9.15)
WORKED EXAMPLE
A ventilator has been programmed for the following:
•
Respiration rate: 12 breaths/min
•
Flow waveform: half sine
•
Tidal volume: 0.5 L
•
Peak flow: 0.4 L/s
The duration of inspiration is
π
V
t
2
Q
i
t
i
=
π
×
0
.
5
=
2
×
0
.
4
=
1
.
96 s
The duration of expiration is
60
n
−
t
e
=
t
i
60
12
−
=
1
.
96
=
3
.
03 s
