Mechanisms of expiratory valves resistance.

Expiratory valves are the major source of resistance in ventilator circuits. Some valves, particularly under certain conditions, cause high resistances. To establish the mechanisms of expiratory valve resistance (R), we measured R of 4 commercial valves under several experimental conditions. The pressure above the diaphragm of one of the valves was also measured. The results reveal that all valves produce significant resistance to air outflow. This resistance is the result of the physical and functional interplay of several valve components. (1) In most valves, the dimensions of the air pathway through the valve contribute minimally to resistance in spite of its tortuosity. (2) The diaphragm adds a major contribution to valve resistance particularly at low flows. The diaphragm effect has 2 components: its weight--a gravity-dependent, fixed effect most predominant at very low flows and evident in all the valves--and a spring-like effect--a component related to the resistance of the diaphragm to deformation and of variable magnitude in different valves. Because of diaphragm-related components at low flows, valves behave as variable threshold resistors. (3) There is limitation to air egress from above the diaphragm at higher exhaled flows, which further contributes to valve resistance. The first and last components cause the valves to function also as flow resistors. Constant flows effectively counteract the diaphragm spring-like effect. Measuring valve resistance during constant flow underestimates the resistance that occurs under dynamic flow conditions characteristic of clinical situations.