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Evita 4 continuously balances the difference between
spontaneous breathing and the set minimum ventilation.
As soon as the balance becomes negative, because
spontaneous breathing is no longer sufficient, Evita 4
applies a mandatory ventilation stroke at the set tidal
volume V
, so that the balance is again positive.
T
Experience shows, patients breathe very irregularly.
Phases of weak breathing alternate with phases of heavy
breathing. In order to allow for these individual fluctua-
tions, the balancing process also takes account of the
extent by which the set minimum ventilation has been
exceeded.
This positive allowance is progressively reduced to zero
by Evita 4 within a maximum of 7.5 seconds after apnoea.
In other words, the response time of Evita 4 before
activating mandatory ventilation is automatically adapted
to the preceding cycle of spontaneous breathing:
If this spontaneous breathing was close to the minimum
ventilation, the machine responds rapidly within the IMV
time. By contrast, if the patient's spontaneous breathing
was much higher than the set minimum ventilation,
Evita 4 tolerates a longer breathing pause. In extreme
cases of sudden apnoea after a phase of heavy
breathing, the response time will be 7.5 seconds plus
the trigger time, with a minimum of 1 IMV cycle time.
Response times longer than 15 seconds may only occur
if the minimum ventilation with a low IMV frequency f is
set to correspondingly low values.
In this case, Evita 4 triggers an apnoea alarm that is
cancelled again as soon as the mandatory ventilation
strokes have been applied. If the IMV is set to a longer
period than the T
> alarm limit, and if there is no
Apnoea
spontaneous breathing between the mandatory ventilation
strokes, the apnoea alarm will be regularly triggered.
Example: f = 3/min = IMV time = 20 seconds
T
> = 15 seconds
Apnoea
This system is designed to prevent mandatory ventilation
being prematurely triggered in the event of irregular
spontaneous breathing, whilst at the same time giving
an alarm for any long period of low ventilation.
Paw
Pmax
PEEP
T
insp
1
f
Flow
V
T
Insp. Flow
MV
set
MV
mandatory MV
no spontaneous
beginning spontaneous
breathing
breathing
Description
Ventilation Modes
ASB
pressure support
short
long
rise time
rise time
t
trigger phase
t
spontaneously
breathed MV
t
sufficient spontaneous
breathing
169
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