Franks LJ, et al. - Researchers compared 6 positive expiratory pressure (PEP) devices (Flutter, Pari PEP S, Acapella Choice, Acapella DM, Acapella DH, and Aerobika) with respect to performance characteristics by varying resistance and flow. Across flows of 5, 10, 15, 20, 25 and 30 L/min and at low, medium, and high resistance, they obtained mean PEP, peak PEP, oscillation frequency, and amplitude PEP of devices, using an experimental apparatus custom-built. Mean PEP produced by the Acapella Choice, Acapella DH, Aerobika, and Pari PEP S increased with increasing flows at a fixed resistance. Mean PEP produced by the Flutter and Acapella DM showed minimal change due to increasing flow. Except the Acapella Choice, mean PEP produced by all devices was found to be increased due to increasing resistance at a fixed flow. The performance characteristics of these devices varied to a small extent across a range of flows and resistance settings but the variations were statistically significant. There seemed to be flow-dependent and non–flow-dependent devices. The production of mean, peak, and amplitude PEP and oscillation frequency was typically maintained or increased by varying flow or resistance.