The form in the flow administration depends on the ventilatory mode and of the selected flow
waveform.
In the VCV mode the flow waveforms, which can be selected, are: descending ramp (decelerating),
rectangular (continuous flow), sinusoidal, ascending ramp (accelerating). In the pressure modes
(PCV and PSV) the flow waveform is decelerating, except in PSV with volume assured where it
could be combined decelerating with continuous flow in the same inspiratory phase.
Layouts of pressure (up) and flow (below) curves. From left to right: Flow in
descending ramp, rectangular, sinusoidal, ascending ramp. Notice the
modifications of the curves of pressure according to the used flow.
End of inspiration
The suspension of the ventilator inspiratory flow depends on the time selected in the VCV
and PCV modes.
In the PSV mode depends on the fall of the inspired flow until a derivative percentage of the
initial flow of that same inspiration is reached. The regulation of this Expiratory Sensitivity can
be made from 5% up to 40% of the initial flow. The default percentage is 25%.
Flow curve during Pressure Support (PSV). In this case the inspiration finishes
when the flow has diminished to 25% of the initial flow (default value).
Expiratory phase
It begins when the expiratory valve opens up allowing escaping the flow exhaled by the
patient. This action is passive and it carries out by the elastic recoil of the lung and the
thoracic cage. Generally, the expiratory flow waveform shows an inverted peak which returns
with variable retard to the zero flow line. The delay in reaching the zero can be due to
expiratory retard of an obstructive lung disease or for breathing circuit defects.
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