The rebreathing characteristics of the Hudson oxygen mask

A lung model was used to study the effect of varying minute ventilation, oxygen flow and inspiratory : expiratory ratio on the rebreathing characteristics of the Hudson oxygen mask. The onset of rebreathing was detected using two different sets of criteria; those defined by Kain et al. and Humphrey. The onset of rebreathing was also quantified in terms of the inspired volume of carbon dioxide and increases in deadspace. The magnitude of rebreathing was increased by reducing oxygen inflow and increasing minute ventilation. Variation in I : E ratio between 1 : 1 and 1 : 4 had little effect on rebreathing. Using the clinical criteria of Kain et al, the onset of rebreathing was detected when oxygen inflow/minute ventilation ratio was reduced below 0.78 on average. With the other criterion, rebreathing occurred when the ratio was 0.46.