The two terms PEEP and CPAP are used interchangeably, and lead to inappropriate confusion: they are the same thing, although CPAP is a more technically correct term. The concept of PEEP is that a pressure is applied at the end of expiration to maintainalveolar recruitment. Airway pressure is kept positive, and is never allowed to return to atmospheric. When inspiration commences, it is on top of this the pressure that remains in the airway at the end of expiration-this is the baseline airway pressure (below which it cannot go) and lasts for the duration of the respiratory cycle. Therefore, all patients who have a resistance applied to expiration are on CPAP. Nevertheless, it is conventional to describe this mode as PEEP, and this is what you will read in books and journals. The current thinking on CPAP is that the level set should be above the lower inflection point of the pressure volume curve , although this is controversial . An easier strategy is to base PEEP on Oxygen requirements .

 

When CPAP is used as a mode, it usually described a mode of ventilation without additional inspiratory support. So if you put somebody on CPAP of 5cmH2O on a mechanical ventilator, this is 5cm of positive pressure applied to the airway in inspiration and expiration. Any pressure support level dialed up on a respiratory ventilator is above CPAP: 5cmH20 of PEEP (CPAP) and 5cmH20 pressure support leads to a peak airway pressure in inspiration of 10cmH2O: pressure support is always described as a pressure above PEEP/CPAP.