For most of us, the terms CPAP (continuous positive airway pressure) and PEEP (positive end expiratory pressure) have existed for all of our careers. But this was not always the case. Although mechanical ventilation, including the positive pressure variant, was a child of the 1950s – PEEP was not described until the late 1960s and even then was seen as a therapy for postoperative atelectasis in cardiac surgery patients. PEEP subsequently became the mainstay of therapy for hypoxic respiratory failure, but was always used in associated with positive pressure breaths. CPAP was developed in the early 1980s as a therapy for sleep disordered breathing. Over two decades the non invasive CPAP therapy and the invasive ventilation (pressure targeted breaths with PEEP) coalesced such that CPAP became a therapy for hypoxic respiratory failure and congestive heart failure, and pressure support (BiPAP or NIV) became a therapy for sleep apnea.

Strictly speaking PEEP and CPAP are different. It is possible to apply PEEP at end expiration and then commence the next breath from atmospheric pressure (try slapping your hand over your mouth mid expiration – then remove it and take a breath) – spontaneous PEEP. However this is almost never used in clinical practice. In CPAP the patients sinusoidal respiratory pattern persists – but starts and ends at an elevated baseline pressure. In PEEP the positive pressure breath starts and ends at that pressure (i.e. pre inspiration and end expiration). So, these days, in most scenarios PEEP and CPAP are indistinguishable. How they are delivered is, of course, different. Nevertheless they serve the same functions 1. To overcome airway resistance that causes disrupted or obstructed gas flow in expiration; 2. To reduce the work of breathing by reducing the magnitude of negative pleural pressure required to generate a tidal volume; 3. Most importantly – to restore functional residual capacity (FRC); 3. To prevent derecruitment of vulnerable lung units in the posterior dorsal segments of the lungs.

PEEP does not easily re-expand collapsed lung tissue – this is usually achieved by applying a recruitment maneuver (30cmH₂O or more for 10 seconds during anesthesia, for 30 seconds in lung injury). The application of PEEP then prevents derecruitment. As such the majority of lung tissue may be re-expanded during anesthesia. This may not be the case in diseased lungs – the principle is to restore a functional residual capacity even if that effectively utilizes the inspiratory reserve volume.  

@ccmtutorials