A circle system without carbon dioxide absorption

An anaesthetic circle system without a carbon dioxide absorber is described. The efficiency of the circle, i.e., the fraction of alveolar gas in the outflow from the circle, was measured in 15 patients during halothane anaesthesia or neurolept analgesia. The fraction ranged from 0.88 to 0.95 (mean 0.91), while the ratio between the alveolar ventilation and the fresh gas inflow ranged from 0.97 to 1.71. The efficiency was not correlated to this ratio. There was no need for hyperventilation if the fresh gas inflow was 10% higher than the alveolar ventilation required to maintain normal PaCO2. The circle was used in 50 patients manually ventilated by nurse anaesthetists. Mean fresh gas inflow was 60 ml/kg. Mean PaCO2 was 5.47 kPa (41 mmHg). In a similar group of 50 other patients, in which the standard circle used in the department was employed, the mean PaCO2 was 4.80 kPa (36 mmHg). The frequency of hypercapnia was equal in the two groups, but hypocapnia was not seen when the circle without absorber was used.