: Concentrations of oxygen and nitrous oxide delivered at low flow rates to a circle system and concentrations expired by the patient were studied over a range of clinical conditions. No useful relation was found between the delivered and mixed expired oxygen concentrations. Expired concentrations were also compared with those predicted by two different equations. No reliable agreement between observed and predicted concentrations was found below total fresh gas inflows of 1.2 liters per minute. A major factor in the lack of agreement was a variation in gas exchange. Oxygen uptake ranged from 125 to 230 ml./minute/sq. meter. Nitrous oxide exchange after one hour of inhalation varied more than 200 ml./minute uptake to 100 ml./minute excreted. One, therefore, cannot rely on the oxygen concentrations delivered to the circuit or on predictions based on average uptake values. The convenience of low inflow nitrous oxide anesthesia can be achieved only when oxygen concentration within the breathing circuit is monitored.
(C) 1966 American Society of Anesthesiologists, Inc.