Philips Respironics V60, User Manual

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4-4
Principles of operation
Shape signal method of cycling and triggering. The shape signal or “shadow
trigger” method uses a mathematical model derived from the flow signal. A
new flow signal (shape signal) is generated by offsetting the signal from the
actual flow and delaying it (Figure 4-2). This intentional delay causes the flow
shape signal to be slightly behind the patient’s flow signal. If there is a sudden
change in patient flow, the patient’s flow signal crosses the shape signal; this
results in a trigger or a cycle. As a result, a sudden decrease in expiratory flow
from an inspiratory effort will cross the shape signal and create a signal for
ventilator triggering.
Figure 4-2: Shape signal
SET method of cycling. Patient flow reaches the spontaneous exhalation
threshold (SET); see Figure 4-3. The SET represents the intersection of the
flow waveform and a line of a given slope. SET is updated each breath.
Figure 4-3: Spontaneous exhalation threshold (SET)
Estimated
patient flow
Shape
signal
Cycle to exhalation
crossover point
Trigger to
inspiration
crossover point
Spontaneous
exhalation threshold