BACKGROUND: High-flow nasal cannula (HFNC) is supposed to provide additional PEEP compared with conventional oxygen therapy. However, the exact determinants of this PEEP effect are unclear. We investigated the factors that might affect the PEEP and compared PEEP performance among 3 HFNC devices. METHODS: Three available HFNC devices were evaluated: the AIRVO 2 device and 2 mechanical ventilators (SV300 and Monnal T75). A device consisting of a test lung (5600i) and an airway model (AMT(IE)) was used to simulate spontaneous breathing. The flows ranged from 0 to their maximum flow with an interval of 10 L/min. The pressures were measured at 4 sites (nasopharynx, supraglottis, carina, and lung) under compliances of 50 and 100 mL/cm H2O and tidal volume of 300, 500, and 700 mL with the mouth closed or open. The influencing factors were determined by multiple linear regression. The sum of squares reduction test was used to compare working curves of PEEP effect among 3 devices. Pairwise comparisons were conducted by using Tukey's multiple comparisons test within an overlap of flow from 0 to 50 L/min. RESULTS: A quadratic curved relationship between PEEP and flow was observed (coefficients were 8.97 x 10(-3) for flow and 4.79 x 10(-4) for a quadratic element of flow, respectively) but evanished when the mouth was open. The PEEP increased along with lung compliance (coefficient was 2.58 x 10(-3)). Despite the difference in working curves, both the mechanical ventilators performed slightly better than the AIRVO 2 device at higher flows (40 and 50 L/min). CONCLUSIONS: The mouth status, flow, and compliance were the 3 major influencing factors of PEEP effect, whereas performance of the 2 mechanical ventilators was slightly superior to that of the AIRVO 2 device at higher flows.