Mechanical Ventilation- Modes of Ventilation II

Continuous Positive Airways Pressure (CPAP) & Positive End Expiratory Pressure (PEEP)

CPAP is the term commonly used when we are talking about non-invasive ventilation or the use of a mask rather than an endotracheal tube. PEEP is the term used when the patient is intubated, but the underlying principals are the same.

As the name implies this is a continuous pressure whether the patient is breathing in or out.

PEEP and CPAP can be set from 5-20 cm H2O

The key point behind these pressures is that when the patient breaths out there will still be a set pressure acting on the lungs over and above that maintained by the patient. This means that the alveoli have an increased pressure acting on them during expiration. 

This prevents them from collapsing down as much as they would if the pressure was not there. This is often referred to as the the alveoli being recruited. PEEP is almost always applied to the ventilated patient at a minimum of 5 cm H2O.

Pressure Support

Pressure support only occurs on inhalation. The patient initiates all the breaths where pressure support is involved (in pressure controlled ventilation the pressure is delivered even when the patient does not initiate the breath).

This is a good mode of ventilation to use when the patient is weaning from the ventilator.

You want the patient to start initiating the breaths, but you also still want to offer their weakened respiratory muscles some support.

As the patient becomes stronger you can reduce the pressure support you give them with each breath, which is again measured in cm H2O. So the patient has to work harder, exercising those muscles.