IPPV - weaning techniques

Patients may require all or part of their respiratory support to be provided by a mechanical ventilator. Weaning from mechanical ventilation may follow several patterns. In patients ventilated for short periods (no more than a few days), it is common to allow 20–30min breathing on a ‘T’ piece before removing the endotracheal tube. For patients who have received longer term ventilation, it is unlikely that mechanical support can be

withdrawn suddenly; several methods are commonly used to wean these patients from mechanical ventilation. There is no strong evidence that any technique is superior in terms of weaning success.

Intermittent ‘T’ piece or continuous positive airway pressure (CPAP)

Spontaneous breathing is allowed for increasingly prolonged periods with a rest on mechanical ventilation in between. The use of a ‘T’ piece for longer than 30min may lead to basal atelectasis since the endotracheal tube bypasses the physiological PEEP effect of the larynx. Therefore, it is common to use 5cmH2O CPAP as spontaneous breathing periods

get longer. In the early stages of weaning, mechanical ventilation is often continued at night to encourage sleep, avoid fatigue, and rest respiratory muscles.

Intermittent mandatory ventilation (IMV)

The set mandatory rate is gradually reduced as the spontaneous rate increases. Spontaneous breaths are usually pressure-supported to overcome circuit and ventilator valve resistance. With this technique, it is important that the patient’s required minute ventilation is provided by the combination of mandatory breaths and spontaneous breaths without an excessive spontaneous rate. The reduction in mandatory rate should be

slow enough to maintain adequate minute ventilation. It is also important that the patient can synchronise his own respiratory efforts with mandatory ventilator breaths; many cannot, particularly where there are frequent spontaneous breaths, some of which may ‘stack’ with mandatory breaths causing hyperinflation.

Pressure support ventilation

All respiratory efforts are spontaneous but positive pressure is added to each breath, the level being chosen to maintain an appropriate tidal volume. Weaning is performed by a gradual reduction of the pressure support level while the respiratory rate is <30/min. The patient is extubated or allowed to breathe with 5cmH2O CPAP when pressure support

is minimal (<10–15cmH2O with modern ventilators).

Choice of ventilator

Modern ventilators have enhancements to aid weaning; however, weaning most patients from ventilation is possible with a basic ventilator and the intermittent ‘T’ piece technique, provided an adequate fresh gas flow is provided. If IMV and/or pressure support are used, the ventilator should provide the features listed in the table opposite.

Key features in the choice of ventilator

• Ventilator must allow patient triggering (i.e. not a minute volume divider).

• Fresh gas flow must be greater than spontaneous peak inspiratory flow.

• Minimum circuit resistance (short, wide bore, and smooth internal lumen).

• Low resistance ventilator valves.

• Sensitive pressure or flow trigger (ideally monitored close to the ET tube).

• Synchronised IMV (avoids ‘stacking’ mandatory on spontaneous breaths).