Continuous positive airway pressure (CPAP) is the application in the airways of continuous positive pressure, close to the positive end expiratory pressure. The two common available systems are by a continuous (high/low) flow system and by a mechanical ventilator. Aim of this study was to compare the mechanical performance of the CPAP systems in intubated and not intubated patients. Medical literature databases (MEDLINE and EMBASE) were searched for articles on "clinical trials" and "randomized controlled trials". The key words "continuous positive airway pressure" and "CPAP", were combined with any of these key words: adult, work of breathing, continuous flow, mechanical valve, water valve, balloon reservoir, mechanical ventilator, pressure triggering, flow triggering, lung model, demand valve, equipment. Thirty-two articles (18 human and 14 bench studies) met the inclusion criteria. The continuous flow systems are able to maintain acceptable airway pressure variations during normal breathing. The most recent mechanical ventilators equipped with flow by systems compared to the first one, presented a similar or better work of breathing compared to the continuous flow systems due to the application of a little amount of pressure support. Although the use of a continuous flow is cheaper compared to mechanical ventilators, it does not allow a continuous respiratory monitoring.