Choosing the appropriate mode

Pressure controlled ventilation avoids the dangers associated with high peak airway pressures, although it may result in marked changes in VT if compliance alters. Allowing the patient to make some spontaneous respiratory effort may reduce sedation requirements, retrain respiratory muscles and reduce mean airway pressures.

Apnoeic patient

Use of IMV or ACMV in patients who are totally apnoeic provides the total minute volume requirement if the preset rate is high enough (this is effectively CMV) but allows spontaneous respiratory effort on recovery.

Patient taking limited spontaneous breaths

A guaranteed minimum minute volume is assured with both ACMV and IMV depending on the preset rate. The work of spontaneous breathing is reduced by supplying the preset VT for spontaneously triggered breaths with ACMV, or by adding pressure support to spontaneous breaths with IMV. With ACMV the spontaneous tidal volume is guaranteed whereas with IMV and pressure support spontaneous tidal volume depends on lung compliance and may be less than the preset tidal volume. The advantage of IMV and pressure support is that gradual reduction of preset rate, as spontaneous effort increases, allows a smooth transition to pressure support ventilation. Subsequent weaning is by reduction of the pressure support level.

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