8500 Ventilator
Training Manual
10
This can be explained as follows :-
1. The frequency control sets up an internal clock tick in the ventilator. The time be-
tween clock ticks is the Set breathing period and is equal to a time in seconds of
60/frequency in BPM. ie. 4 BPM = 1 tick every 15 seconds
With no patient triggers detected, the ventilator will deliver a mechanical breath at
the start of each respiratory period on the clock tick.
2. Preceding each clock tick is a time window in which the ventilator will be looking
for a patient trigger. If a trigger occurs within the window the patient will be given a
synchronised mechanical breath. If it occurs outside the window it will be a sponta-
neous or Pressure Supported spontaneous breath depending on the ventilator set-
ting.
In practical use of the SIMV mode, the ventilator should first be set at an adequate
RATE, TIDAL VOLUME, I:E RATIO, PEEP and PRESSURE LIMIT for controlled
ventilation, say 600 ml, 12 BPM and 1.5 secs for an adult patient. When patient
trigger signals are being displayed regularly the mandatory rate can be reduced
and the total breathing rate will be controlled by the patient. If this rate is inade-
quate the the low MV or Rate alarm will be activated.