Biomedical Engineering Reference
In-Depth Information
Asynchrony is a term that denotes conict between the patient and the
ventilator 14;10;23 . In many cases, failure to synchronize is due to inadequate
ow delivery from the ventilator. A number of technological solutions have
been proposed to solve the problem 1;22;15;11;29;21 . One of the more com-
monly used is pressure augmentation 19;1;22;6;26;15;11;21 .
Patients at risk of elevated auto-PEEP, which occurs when there is
insucient time for exhalation and the next ventilator breath stacks on
the previous breath, typically are characterized by high minute ventila-
tion, high respiratory rate, short expiratory times, and airway obstruc-
tion. Auto-PEEP can cause severe hyperination, discomfort and ventilator
asynchrony. Patients with high levels of auto-PEEP may fail to trigger the
ventilator. This is because auto-PEEP represents an inspiratory threshold
load that the patient must rst overcome before a ventilator breath can
be triggered. Accordingly, the ventilator fails to sense the patient's eort.
Such unintended instability in the level of ventilatory support can lead to
dyspnea and/or complicate weaning. Moreover, the stability of tidal volume
(V T ) may be related to the coordination of patient's eort with the pres-
sure trigger level (P sen ). The eect of trigger levels on the stability of tidal
volume has not be studied. To explore the potential for such variability
in NIPSV, we investigate the roles of compliance (C), resistance (R), fre-
quency (f), the inspiratory ow cut-o (), and pressure trigger level
(P sen ) on the stability of ventilation support.
2. Mathematical Model
We rst consider a mathematical model for the respiratory system con-
sisting of two ordinary dierential equations, each describing a dierent
phase of a breathing cycle. To obtain dierential equations for inspiration
and expiration volumes, we employ a \conservation of pressure" that states
that the applied pressure (P vent ) is the sum of resistive (P resistive ), elastic
(P elastic ), and residual pressures (P residual ), assuming that inertial losses
are negligible. That is,
P vent = P resistive + P elastic + P residual :
In their study, Crooke 3 proposed linear and nonlinear mathematical models
for noninvasive ventilation, addressing pressure support ventilation (PSV)
applied to a one-compartment lung (Figure 1) with a constant compliance
C. One can then write the following set of differential equations for
the nth breath:
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