In cardiothoracic critical care, the vast majority of patients require mechanical ventilation at some point in time. It is therefore important to have a good understanding of the underlying mechanical technology of ventilators and the physiological effects of mechanical ventilation.
Mechanics of ventilation
All ventilators require five key components:
• a power source (pneumatic, electrical or combined);
• a drive mechanism, which transforms the energy source into direct gas flow to the patient;
• the ability to generate a pressure gradient (positive or negative) for flow delivery;
• control mechanisms (mechanical, pneumatic, electrical – closed or open loop); and
• a pneumatic circuit, which directs gas flow within the ventilator (internal circuit) and from the ventilator to the patient (external circuit).
Basic components of breath delivery
All breaths delivered by the ventilator can be broken down into four parts:
the end of expiration and beginning of inspiration;
the delivery of inspiration;
the end of inspiration and beginning of expiration; and
the expiratory phase.
Transition between the phases is controlled by ventilator settings known as ‘phase variables,’ which are responsible for each part of the breath.
The triggering variable that begins the inspiratory gas flow can be time, pressure, flow or volume. The patient may control the beginning of inspiration (patient triggering). This usually requires the operator to set a ‘trigger sensitivity’ level. When time is the trigger variable, the breath is considered mandatory.