Constancy of air-oxygen mixtures in intensive care

The performance of ten high-flow Bird blenders (3M Company) was assessed to ascertain the stability of the oxygen delivery both over time and within a single respiratory cycle. Blended oxygen concentrations were assessed for both continuous low flow and for intermittent flow with variable tidal volumes as is seen with mechanical ventilation. Studies were repeated after the addition of a high flow bleed from the blender via a T-piece. We observed clinically significant variations in the oxygen concentrations delivered by several blenders when the relationship between air and oxygen supply pressures varied. This variability was greatest when the air and oxygen pressures were nearly equal. When the line pressures were stable, mixed oxygen concentrations were constant but variations in oxygen delivery were found within individual breath cycles. This could be explained by postulating that at the initiation of flow from the blender a small pocket of unblended gas (pure air or pure oxygen) was issued by the blender before the balancing mechanism stabilised to deliver the desired oxygen concentration. This variability of oxygen delivery may have considerable impact on the measurement of oxygen consumption using the open circuit technique. The addition of a high flow bleed completely ablated this blender-derived variation in oxygen delivery.