Setting up the mechanical ventilator Tidal volume

Conventionally set at 7-10ml/kg, though recent data suggest lower values (6-7ml/kg) may be better in severe acute respiratory failure, reducing barotrauma and improving outcome. In severe airflow limitation (e.g. asthma, acute bronchitis) smaller VT and minute volume may be needed to allow prolonged expiration.

Respiratory rate

Usually set in accordance with VT to provide minute ventilation of 85-100ml/kg/min. In time-cycled or time-limited modes the set respiratory rate determines the timing of the ventilator cycles.

Inspiratory flow

Usually set between 40-80l/min. A higher flow rate is more comfortable for alert patients. This allows for longer expiration in patients with severe airflow limitation but may be associated with higher peak airway pressures. The flow pattern may be adjusted on most ventilators. A square waveform is often used but decelerating flow may reduce peak airway pressure.

I:E ratio

A function of respiratory rate, VT, inspiratory flow and inspiratory time. Prolonged expiration is useful in severe airflow limitation while a prolonged inspiratory time is used in ARDS to allow slow reacting alveoli time to fill. Alert patients are more comfortable with shorter inspiratory times and high inspiratory flow rates.

FIO2

Set according to arterial blood gases. Usual to start at FI02 = 0.6-1 then adjust according to arterial blood gases.

Airway pressure

In pressure-controlled or pressure-limited modes the peak airway pressure (circuit rather than alveolar pressure) can be set (usually <35-40cmH20). PEEP is usually increased to maintain FRC when respiratory compliance is low.

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