AN EXPERIMENTAL STUDY OF GASEOUS HOMEOSTASIS AND THE MAGILL CIRCUIT USING LOW FRESH GAS FLOWS

• Published in: British journal of anaesthesia : BJA Vol. 48; no. 5; pp. 447 - 455
• Main Authors: CONWAY, C.M., DAVIS, F.M., LEIGH, J.M., TENNANT, R., KNIGHT, H.J., PRESTON, T.D.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.05.1976; Oxford University Press
• Subjects: Anesthesia, Inhalation - instrumentation; Carbon Dioxide; Humans; Nitrogen; Oxygen; Plethysmography, Impedance; Pulmonary Alveoli - physiology; Respiration; Tidal Volume
• ISSN: 0007-0912; 1471-6771
• DOI: 10.1093/bja/48.5.447

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.