Combined air and oxygen entrainment. Effect on the percentage output of fixed performance masks.
Secondary oxygen was added to air entrained by the primary jet of a fixed performance mask. Twenty such masks, in each of six groups rated at 24%, 28%, 31%, 35%, 40% and 50%, gave higher O2 percentages equivalent to those calculated from an equation that related the O2 concentration to the secondary flow. Actual values of both were closely correlated, with regression slopes and intercepts similar to those derived from this equation. This technique is simple and predictable in altering the composition of an air-oxygen mixture delivered to the patient.