Factors affecting carbon dioxide homeostasis during controlled ventilation with circle systems.

An experimental lung model was used, with controlled ventilation, to determine the effect of different circle arrangements and varying ventilatory frequencies on the efficiency of carbon dioxide removal from a circle system without carbon dioxide absorption. Greater efficiency was found when fresh gas entered the system between the unidirectional inspiratory valve and the subject that when the fresh gas inlet was on the ventilator side of this valve. At any fresh gas flow and minute volume, efficiency was greater at low respiratory frequencies. Good correlations existed between carbon dioxide concentration in the model lung, fresh gas flow and minute ventilation when respiratory frequency was constant. Paradoxical results were obtained when minute volume was varied by changes in frequency at a constant tidal volume. The major cause of the various differences in performance has been ascribed to variations in the degree of mixing of fresh and expired gas within the system.