8500 Ventilator
Training Manual
9
It will also be necessary to define an inspiratory time in which the pressure support is deliv-
ered. In this case the breathing rate and tidal volume are not relevant. The Inspiratory
flow rate will be a fixed value of 40 LPM in Adult and 30 LPM in Paediatric.
The detection of a “Patient Attempt” will be carried out by the control system to aid
the speed of support. A patient attempt will be indicated on screen by the negative
excursion of the pressure wave form being green rather than blue for the positive
phase.
SIMV
This mode is
S
ynchronised Intermittent
M
andatory
V
entilation. This is an extension of the
spontaneous mode in so far as the patient can take breaths on demand with pressure sup-
port but some mandatory breaths are included. To achieve this it is necessary to set a
tidal volume, a breathing rate and all of the features described above for pressure support.
To simplify things a little the inspiratory flow rate can be assumed from the TV / Inspiratory
Time set values.
The patient attempt or trigger signal will be used to synchronise the mandatory breaths to
the patients breathing pattern and initiate the pressure supported spontaneous breaths.
Thus allowing the patient to establish the breathing rate.
In this way a patient could be breathing at say 12 BPM with the settings such that 4
breaths of say 500 ml are mandatory and 8 breaths are taken spontaneously with pressure
support of up to 30 cmH
2
O above the PEEP level.
The ventilator will monitor all of the breaths, display the parameters TV / BPM / I:E and in-
dicate the patient attempt by showing negative excursions of the graph in RED. The set
breathing rate and measured rate will be displayed, the Set Inspiratory time and I:E ratio
will be displayed in the appropriate window.
A means of preventing hyperventilation will be implemented such that any change in
breathing rate is only allowed to take place slowly.
The method of inflating the patient's lungs and the operation of the valves and the gas
flows in the SIMV mode are the same as for the CMV mode.
The main difference between this mode and CMV is that a facility is provided for synchro-
nising the mechanical breaths given by the ventilator to the patient's own respiratory ef-
forts (spontaneous breaths).
If the patient fails to make any effort, then SIMV will default to ventilation functionally iden-
tical to CMV at the set SIMV rate.
The routine within the ventilator that detects the Patient Trigger is used to detect the in-
stant a patient starts to inhale and is thus ready for a synchronised breath to be given by
the ventilator. Whether a breath is spontaneous or mandatory depends on where it occurs
in the SIMV cycle.