Optimum design parameters for a therapist-constructed positive-expiratory-pressure therapy bottle device

Positive-expiratory-pressure (PEP) therapy uses positive airway pressure generated by a either a fixed-orifice resistor or a threshold resistor. We hypothesized that tubing diameter and length, and the diameter of the PEP bottle's air-escape orifice would impact the PEP pressure delivered to the airway and determine whether the PEP bottle acts as a threshold resistor or a fixed-orifice resistor.

We designed a model composed of a bottle partially filled with water, a compressed air source, a pneumotachometer, and a manometer, to evaluate the effects of various tubing diameters (range 2-25 mm inner diameter) and lengths (range 20-80 cm long). In the first set of experiments, the PEP bottle had an open top, so there was no pressure other than the atmospheric pressure against the air escaping from the immersed tubing. The distal tip of the tube was 10 cm below the surface of the water (ie, a water-column pressure of 10 cm H(2)O), and we tested flows of 1, 5, 10, 15, 20, and 25 L/min. In the second set of experiments we tested a PEP bottle, the top of which was closed except for an air-escape orifice (4, 6, 8, 9, or 10 mm).

With tubing of 2-6 mm inner diameter, the length of the tubing and the flow significantly affected the PEP pressure (ie, the system was not a threshold resistor). With tubing > or = 8 mm inner diameter there were no significant PEP-pressure differences with any of the tubing lengths or flows tested, which indicates a threshold-resistor system. The 4-mm and 6-mm air-escape orifices significantly increased the PEP pressure, whereas the 8 mm air-escape orifice did not increase the PEP pressure.

To obtain a threshold-resistor PEP bottle system (ie, the PEP pressure is generated only by the water-column pressure), the tubing must be > or = 8 mm inner diameter, and the air-escape orifice must be > or = 8 mm.