A ventilator works on the principle that a positive pressure on the airways fills up the lungs with air, and the elastic forces expel that air when the positive airway pressure goes away.
The breathing circuit
With the patients lungs at one end, the ventilator is connected by some tube to create a “circuit” with a passage for air to the lungs and another for air from the lungs. Various things are added to this circuit: filters, pressure sensors, gas sensors, or maybe a vaporiser.
Ventilator settings
Flow, pressure, and volume are the primary settings. Flow is just the volume over time.
Modes
To provide a gross oversimplification, the ventilator can be set to volume or pressure control. And it can also be set to mandatory or spontaneous, that is, the machine will deliver “breaths” or the patient can initiate breaths. The most common modes are
- Volume controlled (VCV)
- Pressure controlled (PCV)
- Synchronised intermittent mandatory ventilation (SIMV) The choice of mode is complex and depends greatly on the objective of ventilation. Generally volume control favours ventilation while pressure control favours oxygenation. Pressure control may be better for situations where the breathing circuit is not guaranteed to be sealed such as when using an LMA.
VCV
Volume controlled ventilation provides a constant (inspiratory) flow. It necessitates the selection of the variables required to calculate gas delivery.
- Tidal volume - generally around 6mL/kg (ideal body weight)
- Respiratory rate - about 12/min
- Inspiratory flow
- Fraction of inspired oxygen - depends on how good the lungs are
- Positive end expiratory pressure - probably about 5cm of water
They say a picture paints 1000 words… Here is a VAV vaveform.
Note the absence of PEEP in this image. Vale F - CC-BY-NC-SA 4.0 Link
PCV
Here the principle is providing a constant pressure leaving flow up to the circuit. The flow will taper off as the lungs fill to maintain pressure and if the set pressure reaches equilibrium with the alveolar pressure no gas will flow. Here we set the variable require for the pressure curve.
- Inspiratory pressure - usually between 10 and 20cm of water
- Inspiratory time (related to the respiratory rate and inspiration:expiration ratio) - often about one second
- Positive end expiratory pressure - about 5cm of water
- Fraction of inspired oxygen
Note the absence of PEEP in this image Vale F - CC-BY-NC-SA 4.0 Link
SIMV
Either pressure of volume controlled and similar to above. Now with the addition of patient initiated breaths.