title: An experimental study of gaseous homeostasis and the Magill circuit using low fresh gas flows.
abstract: Gas concentrations and ventilation levels have been measured within a conventional Magill circuit when conscious volunteers breathed a non-narcotic gas mixture at varying fresh gas flows. When evidence of rebreathing of alveolar gas was detected, the fresh gas flow was kept constant until a steady state developed. All subjects showed evidence of rebreathing when the fresh gas flow approached the predicted alveolar ventilation levels. A variety of subject-circuit interactions was seen and shown to be precipitated by naturally occurring breath-to-breath variations in ventilation. A single large breath could perturb the system. This could have a temporary effect, when the fresh gas flow was sufficient to wash the increased aliquot of expired carbon dioxide from the circuit. At other times a progressive response occurred as ventilatory stimulation as a result of the increased inspired carbon dioxide concentrations caused alveolar gas to reach the reservoir bag and converted the system behaviour from that of a simple added deadspace to that of a total mixing device. Whilst marked changes occurred commonly in both ventilation and inspired gas concentrations, only slight changes in end-tidal gas concentrations occurred.
mesh_terms: D000769:Anesthesia, Inhalation / Q000295:instrumentation*; D002245:Carbon Dioxide; D006801:Humans; D009584:Nitrogen; D010100:Oxygen; D010992:Plethysmography, Impedance; D011650:Pulmonary Alveoli / Q000502:physiology; D012119:Respiration; D013990:Tidal Volume
chemical_list: D002245:Carbon Dioxide; D009584:Nitrogen; D010100:Oxygen