The negative pressure relief valve: pressure-flow relationships.

The scavenging of gases from anaesthetic circuits may present hazards to the patient. The negative pressure relief valve prevents the generation of subatmospheric pressures in the circuit as a result of a discrepancy between the fresh gas flow and the gas evacuation rate. The ideal valve will open at a small negative pressure, and immediately permit a high gas inflow. Leakage with positive pressure in the circuit and admixture of atmospheric air during spontaneous respiration must not occur. Six different valves were studied. Two membrane valves came nearest to fulfilling the ideal requirements.