IPPV - description of ventilators

Classifi cation of mechanical ventilators

Modern ventilators deliver a gas flow with a cycling mechanism to cut flow during expiration. They may be classified by the method of cycling from inspiration to expiration, i.e. when a preset time has elapsed (time-cycled), a preset pressure is reached (pressure-cycled), or a preset volume delivered (volume-cycled). The ventilator breath may be volume-controlled (a predetermined tidal volume (VT) is delivered), pressure-controlled (gas flow is at a pre-determined pressure), or volume controlled with a limited pressure (the ventilator delivers a preset VT within a pressure limit unless the lungs are non-compliant or airway resistance is high). The latter is useful to avoid high airway pressures. Various ‘mixed’ modes are available. In volumecycled mode with a time limit, the inspiratory flow is reduced; the ventilator delivers the preset VT unless impossible at the set respiratory rate (limiting airway pressure). In time-cycled mode with pressure control, preset pressure is delivered throughout inspiration with cycling determined by time. VT is dependent on respiratory compliance and airway resistance.

Setting up the mechanical ventilator

Tidal volume

Values of 6–7mL/kg ideal body weight are related to better outcomes in severe acute respiratory failure, by reducing ventilator-associated trauma and distant inflammatory effects. 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. Higher fl ow rates are more comfortable for alert patients. This allows for longer expiration in patients with severe airflow limitation, but may result in higher peak airway pressures. The fl ow pattern may be adjusted on most ventilators. A square waveform is often used, but decelerating fl ow may reduce peak airway pressure.

I:E ratio

A function of respiratory rate, VT, inspiratory fl ow, and inspiratory time. Prolonged expiration is useful in severe airfl ow 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 FIO2 = 0.6–1, then adjust according to arterial blood gases and pulse oximetry.

Airway pressure

In pressure-controlled or -limited modes, a peak airway pressure (circuit rather than alveolar pressure) can be set (ideally ≤30cmH2O). PEEP is often increased to maintain FRC when compliance is low.

 Initial ventilator set-up

• Check for leaks.

• Check O2 is flowing.

• FIO2                               0.6–1.

• VT                                 5–10mL/kg.

• Rate                              10–15/min.

• I:E ratio                         1:2.

• Peak pressure                 <35cmH2O.

• PEEP                             3–5cmH2O.