IPPV - adjusting the ventilator

Adjustments are usually made in response to blood gases, pulse oximetry or capnography, patient agitation or discomfort, or during weaning. ‘Migration’ of the endotracheal tube, either distally to the carina or beyond, or proximally such that the cuff is at vocal cord level, may result in agitation, excess coughing, and a deterioration in blood gases. Tube migration or obstruction should be considered and rectifi ed before changing ventilator settings or sedative dosing.

The choice of ventilator mode depends upon conscious level, the number of spontaneous breaths being taken, and blood gas values. Many spontaneously breathing patients can cope adequately with pressure support ventilation alone. However, a few intermittent mandatory breaths (SIMV) may be needed to assist gas exchange or slow an excessive spontaneous rate. The paralysed/heavily sedated patient will require either volume- or pressure-controlled mandatory breaths.

The order of change will be dictated by the severity of respiratory failure and individual operator preference. Earlier use of increased PEEP is advocated to recruit collapsed alveoli and thus improve oxygenation in severe respiratory failure.

Low PaO2 considerations

• Increase FIO2.

• Increase PEEP (may raise peak airway pressure or reduce cardiac output).

• Increase I:E ratio.

• Consider tolerating low PaO2 (‘permissive hypoxaemia’).

• Consider increasing pressure support/pressure control or VT.

• In CMV, consider increasing sedation 9 muscle relaxants.

• Prone ventilation, inhaled nitric oxide.

High PaO2 considerations

• Decrease FIO2.

• Decrease I:E ratio.

• Decrease PEEP.

• Decrease level of pressure control/pressure support if VT adequate.

High PaCO2 considerations

• Consider tolerating high level (‘permissive hypercapnia’).

• Increase VT (if low and peak airway pressure allows).

• Increase respiratory rate.

• Reduce rate if too high (to reduce intrinsic PEEP).

• Reduce dead space.

• In CMV, increase sedation 9 muscle relaxants.

Low PaCO2 considerations

• Decrease respiratory rate.

• Decrease VT.