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Canadian Journal of Anesthesia, Vol 28, 321-328, Copyright © 1981 by Canadian Anesthesiologists' Society

Rebreathing and Coaxial Circuits: A Comparison of the Bain and Mera F

R. J. BYRICK 1, E. JANSSEN 2, and MASAO YAMASHITA 3

1 Dept. of Anaesthesia, St. Michals's Toronto, Ontario
2 Dept. of Anaesthesia, St. Michals's Hospital, Toronto, Ontario
3 Dept. of Anesthesiology, Hirosaki University, Hirosaki, Aomori-Ken Japan

Dr. R.J. Byrick, Dept. of Anaesthesia, St. Michael's Hospital, 30 Bond Street, Toron-to Ontario, M5B 1W8.

This study compares the rebreathing characteristics of the Bain modification of the Mapleson ‘D’ type of T-piece circuit with those of the MERA F system which is used with the standard "circle" anaesthetic machine. Six healthy adults anaesthetized with halothane were studied breathing spontaneously. The volume of inspired carbon dioxide was measured on each breath as a measure of rebreathing. The tidal volume (Vt) frequency of respiration (f) and blood Pco2 were also noted. These measurements were made initially with either the BAIN or the MERA F system and then changed to the alternate circuit for further studies. All measurements were made with a fresh gas flow rate (FGF) of 100 ml·kg-1·min-1 which is recommended with the Bain system.

The inspired volume of carbon dioxide (rebreathing) with the Bain system was significantly greater than when the MERA F was used. Although the mean blood Pco2 was not significantly lower when the MERA F was used, some patients who cannot adequately compensate for this inspired carbon dioxide volume did become hypercapneic (maximum 8kPa [60torr]). This hypercapnia could be reduced by using a MERA F system.

The MERA F is a co-axial system that combines the convenience of the tube-in-a-tube structure with the beneficial effects of controlled rebreathing during controlled ventilation. In these advantages it is no different from the Bain system. The MERA F however, has the advantage of being adaptable to the commonly used "circle" anaesthetic machines for spontaneous respiration in adults. This eliminates the rebreathing of carbon dioxide at a fresh gas flow of 100 ml·kg-1·min-1, which occurs in adults during spontaneous respiration. The only disadvantage of the MERA F system that we used in adults was its length (90cm). However, from a functional viewpoint, it can be lengthened without altering the rebreathing characteristics of the system.

Key Words: ANAESTHETIC EQUIPMENT, MERA F, Bain






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Copyright © 1981 by the Canadian Anesthesiologists' Society.