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14.2 Alarms
14.2.1 High and Low Alarm Operation
The high and low alarm thresholds warn the user to a positive or negative
increase in the pressure waveform.
The high and low alarms are limited so that the user cannot set them so far above
or below the pressure waveform that in practice they are turned Off. These alarm
thresholds are limited to 15mbar above and below the pressure waveform for
conventional ventilation and 20mbar for HFO or HFO+CMV.
The high and low alarm thresholds autotrack the pressure waveform. If the user
has set a high/low threshold and then changes the PIP or Delta P (dependant on
mode of ventilation) the threshold will move to 5mbar of the pressure waveform
(conventional) and 10mbar (HFO). This will happen when any adjustment is
made to the waveform.
Note: The user must be made aware that the Low Alarm threshold will only
autotrack down to 1mbar in conventional ventilation. If the user wishes to
set the alarm lower than 1mbar then this has to be done manually. If the
alarm threshold is manually set lower than 1mbar and a pressure related
parameter is adjusted the low alarm threshold will return to 1mbar. The user
will have to manually re-adjust the alarm threshold to the required level.
Warning: If the user sets the low alarm threshold below 1mbar the ventilator
will not be able to detect a patient circuit disconnection of the following
type: Inspiratory limb disconnection from the ET manifold complete with
coloured restrictor. (In this case the low alarm will not be triggered as it is
set to 0mBar or less, also the leak alarm will not activate as the restrictor is
still on the inspiratory limb.
The ventilator will also not instantly detect an ET tube disconnection from
the flow sensor, if the low alarm is set to 0 mbar or less. The ventilator will
alarm with a “Breath Not Detected” alarm after 10 seconds.
If a user set/autotracked high or low alarm threshold is crossed, a high/low alarm
is triggered but ventilation is continued.
The high and low alarms also have two invisible ventilator set thresholds at 5mbar
and 20mbar above the high and below the low user set thresholds.
Crossing the 5mbar ventilator set threshold
If the high or low alarm threshold is exceeded by more than 5mbar the ventilator
drops the fresh gas supply for 3 seconds. It maintains the mean pressure and
stops ventilating and this is true for all modes of ventilation. The ventilator will re-
instate the fresh gas supply after 3 seconds and then restart ventilation a further 5
seconds after re-instatement of the fresh gas. The high or low pressure alarm will
sound until the condition is cleared. If the ventilator encounters the same
conditions after restarting ventilation the fresh gas pressure relief cycle is
repeated.
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