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Charles Gomersall

Modes

IPPV

  • 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

CPAP/ASB

  • = 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:

SIMV

  • 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:


©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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