Predictable oxygen concentrations with the Bird respirator.

IN 1964, Fairley and Britt investigated the adequacy of the air-mix control of ventilators operated from an oxygen source. 1 The mean inspired oxygen percentages were measured at various cycling pressures, and with the Bird Mark 7 ventilator the oxygen levels varied from 51.5 to 96.8 per cent, being highest at low flows and at high cycling pressures. Consequently, these authors recommended that this type of equipment be driven by compressed air, the oxygen being added as required. Harrison 2 showed that as the pressure in the right-hand chamber of the ventilator increased, the flow through the venturi fell; therefore, as inflation proceeded, the amount of air entrained was reduced and the concentration of oxygen increased. The lower the flow setting, and hence the flow through the injector, the greater the percentage contribution of the nebulizer oxygen to the inspired mixture. in following up this problem, Sellery and Fairley published in 1967 a study of the inspired oxygen concentrations delivered by the Bird ventilator when operated from a compressed air source with oxygen added to the ambient or to the pressurized side of the ventilator. 3 They found that when oxygen was added to the ambient side of the ventilator at any given oxygen flow rate, the inspired oxygen concentrations decreased as the cycling pressure was increased (Fig. 1). When the oxygen was added to the pressmSzed side, high oxygen concentrations were delivered at low cycling pressures (Fig. 2), and this was thought to be due to the added oxygen accounting for a high proportion of the inspired volume when cycling occurred quickly. The inspired oxygen concentrations were seen to rise to a high level again at the high cycling pressures due to a decrease in the amount of air entraiamaent. In both instances little change was produced in the inspired oxygen concentration by changes in the flow rate within the clinically used range. In view of the above findings, it was thought advisable to investigate the adequacy of the Bird Parallel Inspiratory Flow Mixing Cartridge, as regards its ability to supply a constant predictable level of oxygen in the inspired gas mixture. This cartridge may be installed in the line of the primary pneumatic power source driving the Bird respirator, and is said by the manufacturer to furnish the following: