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Mechanical ventilation

Initial settings

FiO2

PEEP

Spontaneous or controlled mode

Pressure or volume controlled

Peak and plateau pressure

Tidal volume

I:E ratio

FiO2

Start at 1.0. It should be reduced quickly using SpO2 as a guide. If the patient is already established on lower levels of FiO2 (e.g. patient from theatre) and oxygenation is adequate, continue at that level.

High FiO2, or at least high tissue PO2, may contribute to VILI. FiO2 should normally be chosen to achieve a normal SpO2. If FiO2 is > 0.5, the aim should be SpO2 of 92%, once other ventilator settings have been optimised. This is termed permissive hypoxia. Relative hypoxaemia is also tolerated in some patients who are chronically hypoxic.

 

PEEP

Starting levels on arrival in ICU of 5 – 10 cmH2O.

In many patients, PEEP improves oxygenation by:

  • increasing functional residual capacity (FRC)
  • recruiting collapsed alveoli
  • reducing shunt

In addition it will reduce the opening and closing of alveoli that occurs with each breath (‘atelectrauma’), and therefore reduce VILI.

The precise level of PEEP is controversial and will depend on the underlying condition. It will be set between 5 – 15cm H2O in the majority of patients.

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