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.

Enclosed Afferent Reservoir Breathing Systems

“…These may be described as the "Enclosed Mapleson A" (EMA) and the "Enclosed Preferential Flow" (EPF) systems. The conventional Mapleson A system and its modifications have been shown to require a minimum fresh gas flow almost equal to the alveolar minute volume of ventilation during spontaneous ventilation [9][10][11][12][13]. During controlled ventilation the flow dynamics change completely as gas is eliminated from the system during the inspiratory phase instead of during the expiratory phase as in spontaneous ventilation.…”

Comparison of the Magill and Lack Anaesthetic Breathing Systems in Anaesthetized Patients

“…The fresh gas flow rate required to prevent rebreathing in the Mapleson A (Magill) anaesthetic breathing system has been studied extensively in anaesthetized patients [1] and conscious volunteers [2,3]. These authors agreed with the conclusions of the theoretical analysis performed by Mapleson [4] and found that rebreathing did not occur until fresh gas flow was reduced to approximately alveolar minute volume.…”

Comparison of the Fresh Gas Flow Requirements and Resistance of the Preferential Flow System With Those of the Magill System

“…The virtue of the Magill (Mapleson A) anaesthetic system with its low resistance to gas flow and its economy of fresh gases in spontaneously breathing patients is well established (Mapleson, 1954;Kain and Nunn, 1968;Norman, Adams and Sykes, 1968;Sykes, 1968;Conway et al, 1976). The Mapleson D (Bain Modification) system with the expiratory control valve situated remote from the patient and its economy of fresh gas flow with controlled ventilation is useful in many situations (Bain and Spoerel, 1972;Conway, Seeley and Barnes, 1977).…”