Biomedical Engineering Reference
In-Depth Information
The ratio of the inspiratory-to-expiratory period is often used to adjust the respiration
rate. This is represented by the I : E ratio and is usually expressed in terms normalized to
the inspiratory period. This makes the ratio 1:
R
, where
R
t
i
.
A number of control strategies are used to control the oxygen and air delivery valves.
One common type is the proportional plus integral (PI) controller, as discussed in Chapter
4, which can quickly adjust the amount of oxygen in the enriched breath gas, usually
within a single breath (Bronzino, 2006).
It is often desirable to maintain a slightly positive pressure in a patient's lungs rather
than allowing them to deflate completely during expiration. In this case, the controller
closes the expiration valve when the airway pressure reaches PEEP. In a microprocessor-
based system, the pressure is measured using a transducer and is monitored while the
expiration valve is open. The response time must be fast because no new air is being
provided, so any pressure overshoot will remain until the following inspiration phase.
=
t
e
/
9.8.6 Pressure-Controlled Mandatory Ventilation
In this mode, the respirator raises and maintains the airway pressure at the desired level
independent of patient compliance and resistance. The pressure level,
P
i
, shown in Fig-
ure 9-43, is set by the therapist. It should be noted that even though the pressure remains
constant the actual flow rate will be different for each patient.
This controller uses the following therapist-selected parameters to compute the desired
inspiratory pressure trajectory:
•
Inspiratory pressure
•
Respiration rate
•
I : E ratio
•
Delivered oxygen concentration
FIGURE 9-43
Inspiratory flow for
pressure-controlled
ventilation. [Adapted
from (Bronzino
2006).]
