IPPV - modes of ventilation

Controlled mechanical ventilation (CMV)

A preset number of breaths are delivered to supply all the patient’s ventilator requirements. Breaths may be at a preset VT (volume-controlled) or at a preset inspiratory pressure (pressure-controlled).

Assist-control mechanical ventilation (ACMV)

Patients can breathe, triggering the ventilator to determine the respiratory rate. However, a preset number of breaths are delivered if the spontaneous respiratory rate falls below the preset level.

Intermittent mandatory ventilation (IMV)

A number of breaths are preset, but patients are free to breathe spontaneously in between. Mandatory breaths may be synchronised with spontaneous efforts (synchronised IMV) to avoid mandatory breaths occurring on top of spontaneous breaths (‘stacking’). This may lead to excessive VT, high airway pressure, incomplete exhalation, and air trapping. Positive pressure support may be added to spontaneous breaths to reduce the work of breathing.

Pressure support ventilation (PSV)

A preset positive inspiratory pressure is added to the ventilator circuit during inspiration in spontaneously breathing patients. Preset pressures should be adjusted to an adequate (but not excessive) VT.

Volume support ventilation (VSV)

A target VT is set and pressure support is adjusted to achieve the target volume.

Choosing the appropriate mode

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

Apnoeic patient

Use of IMV or ACMV in totally apnoeic patients provides total minute volume requirement if the preset rate is high enough (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 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 pressure support level.