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simulation result yields consistently higher C than experimentally observed,
but predicts the rise in CV, becoming higher than 10%, in the proximity of
ow cut-o value of 0:2 which compares well with the laboratory data.
In [4], factors creating instability during pressure support ventilation
were studied by using a mechanical test lung (Hans-Rudolph) with ad-
justable resistance, compliance, and triggering frequency. Ventilatory pat-
terns were examined using PSV over a range of ow cut-o settings from 5%
to 45% of peak inspiratory ow. Instability was dened as a tidal volume
(V
T
) delivered with a coecient of variation greater than 10%. They found
that tidal volume delivery becomes unstable with clinically relevant combi-
nations of inputs. For each frequency, the region above the line is associated
with stable tidal volume delivery and region below the line is unstable (Fig-
ure 8). Increasing resistance shifted the area of instability giving rise to a
system more prone to variable delivery of support. Both Figures 5 and
8 indicate that the region of unstable ow cut-o values enlarges as the
breathing frequency increases.
7. Discussion and Conclusion
Although pressure support ventilation has been used for many years in dif-
ferent clinical settings, there have been relatively few investigations of the
dynamical behavior of this mode of ventilation
3
. A major limitation in the
employment of noninvasive ventilation, where PSV is frequently applied, is
lack of patient tolerance. It has been reported that up to 30% of patients
with chronic obstructive pulmonary disease (COPD) fail a trial of nonin-
vasive ventilation for reasons which are often unclear although the number
varies
3
. Since PSV should be capable of providing a level of inspiratory as-
sistance that is adequate for most relaxed COPD patients, it is likely that
intolerance of NIV may be related to the adverse patient-ventilator interac-
tions, namely, ventilator dyssynchrony
5
. Unstable inspiratory support may
require active patient eort to terminate inspiration, or inspiration may be
spontaneously terminated prematurely. Moreover, breath-to-breath varia-
tions in auto-PEEP will require dierent levels of patient eort to trigger
the ventilator. These phenomena may lead to failure of PSV.
Earlier studies
5;2;3
have revealed that application of pressure support
ventilation in the context of mask leaks, can lead to signicant breath
to breath variability in the duration of inspiratory support, to oscillations
in end expiratory lung volume, and to unstable tidal volumes. Although
an adequate seal can usually be obtained, leaks frequently develop between
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