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Charles Gomersall
Modes
- called CMV if country setting is US English
- = volume preset
assist-control
mode
- the controls that need to be set are:
- the I:E ratio is determined by the frequency, which sets the cycle time
and the inspiratory time (Tinsp)
- Absolute inspiratory time = set Tinsp
- Cycle time = 60/f
- Absolute expiratory time = Cycle time - Tinsp
- Therefore I:E ratio = Tinsp: (Cycle time-Tinsp)
- Any change in frequency ⇒change
in cycle time ⇒change in I:E ratio
- the inspiratory pause time is determined by the inspiratory time, the
inspiratory flow and the tidal volume
- time required for inspiratory flow = inspiratory flow/tidal volume
- pause time = Tinsp - time required for inspiratory flow
- Therefore change in tidal volume ⇒change
in inspiratory pause time
- turning on Autoflow produces a mode which is similar to
Pressure Regulated Volume Control
- control breaths: ventilator adjusts the flow rate, dependent on the
patient's lung compliance and resistance, to deliver the tidal volume
with the lowest inspiratory pressure possible. The net result is a
constant pressure, decelerating flow pattern and can be thought of as
pressure control ventilation that is titrated on a breath by breath
basis to produce a set tidal volume
- assisted breaths:
- differ from assisted breaths in PRVC mode
- ventilator delivers an additional inspiratory flow but this is
flow is limited by the tidal volume alarm limit NOT by the set tidal
volume
- maximum inspiratory pressure that can be delivered in this mode is
determined by the upper pressure alarm limit. If this limit is reached
an alarm message is displayed
- = pressure support with PEEP
- the following controls need to be set
- ΔPASB = pressure support
above PEEP
- Ramp = rise time. Note that ramp is absolute
rise time in seconds and maximum is 2 secs. This is a long time in relation
to likely inspiratory time and may mean that the set pressure is not reached
until the end of inspiration, as in the example below:
- The following settings need to be set:
- I:E ratio and inspiratory pause time are determined by frequency, Tinsp,
flow and tidal volume in the same way as in IPPV mode
- the window for triggering a synchronized breath (rather than an
assist support breath) is 5 seconds long in adult
mode and 1.5 seconds in paediatric mode. The spontaneous breathing period
(when spontaneous effort triggers an assist support breath) is any remaining
time between the end of inspiration and the start of the trigger window
This long trigger window means that the patient may receive many more
than the set number of mandatory breaths. To prevent this happening the
ventilator increases the duration of the spontaneous breathing period during
subsequent breaths
BIPAP
To keep the driving pressure constant when changing PEEP select either
PEEP or Pinsp
Then select Link PEEP/Pinsp. Any change in PEEP will then be accompanied
by a corresponding change in Pinsp, thuus keeping the driving pressure
constant.
- the higher of the two levels of airway pressure is set by Pinsp and the
lower by PEEP
- time at high pressure is determined by Tinsp
- time at low pressure is Cycle time (60/f) - Tinsp
- the patient can take spontaneous breaths at both high and low pressures
but ASB is only provided a the low level of airway
pressure
- ramp determines the rise time for changes from low to high pressure and
for ASB breaths
BIPAP assist
- called PCV+ assist if country setting is US English
- this mode is similar to pressure preset assist-control except that the
patient can take (unassisted) spontaneous breaths at the high pressure.
Spontaneous breaths at the low pressure trigger a "pressure control" breath
APRV
- similar to BIPAP
- spontaneous breaths are possible at both high and low pressures but
these cannot be assisted
- controls are:
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