Commonly adopted ventilation modes include pressure limited and time cycled (pressure control or bilevel ventilation) or volume limited and time cycled modes (synchronised intermittent mandatory ventilation, SIMV).28 In pressure limited modes, maximum airway pressure is set and the tidal volume delivered depends on respiratory system compliance. Volume cycled modes, however, deliver a set tidal volume and can be successfully and safely used provided an airway pressure limit is set appropriately. As the patient improves and begins to breathe, spontaneously triggered modes of ventilation such as pressure support ventilation (PSV) can be introduced.
The major variables that need to be set when placing a patient on mechanical ventilation include the oxygen concentration of inspired gas (Fio2), tidal volume (Vt) or inspiratory pressure, ventilator rate, inspiratory to expiratory time ratio (I:E ratio), and PEEP. The aims are to minimise airway pressure, allowing sufficient time for completion of expiration while achieving adequate alveolar ventilation. Suggested initial ventilator settings are shown in box 13.3.
Outcome is improved in mechanically ventilated asthmatics by limiting airway pressure using a low respiratory rate and tidal volume while permitting a moderate degree of hypercar-bia and respiratory acidosis.67 Hypercarbia has not been found to be detrimental except in patients with raised intracranial pressure or severe myocardial depression. Moderate degrees of hypercarbia with an associated acidosis (pH 7.2-7.15) are generally well tolerated. Reducing the respiratory rate to 8 or 10 breaths/min prolongs expiratory time so that I:E ratios of greater than 1:2 can be achieved. An attempt to increase minute ventilation (to reduce Paco2) by increasing the ventilator respiratory rate invariably reduces the expiratory time and I:E ratio, increases air trapping, and may paradoxically cause an increased PaCO2. This has resulted in perceived failure of mechanical ventilation.68
Humidification of inspired gas is particularly important in asthmatic patients to prevent thickening of secretions and drying of airway mucosa, a stimulus for bronchospasm in itself.69
These include peak pressure and low tidal volume/low minute ventilation alarms. If exceptionally high airway pressures occur or there is a sudden fall in Vt, blockage of the endotra-cheal tube, pneumothorax, or lobar collapse should be excluded. Plateau rather than peak airway pressure may provide the best measure of alveolar pressure and provide the best predictor of barotrauma, together with measures of hyperinflation such as PEEPi.70
Low level CPAP may be beneficial in spontaneously breathing, mechanically ventilated patients, especially if expiratory muscle activity is contributing to dynamic airways collapse. However, in mechanically ventilated paralysed patients extrinsic PEEP was of no benefit at low levels and was detrimental at high levels because the fall in gas trapping was outweighed by the rise in functional residual capacity (FRC).22 However, in this study large Vt were used (up to 18 ml/kg); furthermore PEEPi and arterial blood gases were not measured. Changes in FRC and gas trapping may guide the level of PEEP. Applied extrinsic PEEP should not exceed PEEPi.
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