The Bain Mapleson Circuit is a modification of the Mapleson D system. It is a coaxial system in which the fresh gas flows through a narrow inner tube within the outer corrugated tubing.
Essentially, the Bain circuit functions in the same way as the T-piece, except that the tube supplying fresh gas to the patient is located inside the reservoir tube.
Inspiration - The patient inspires fresh gas from the outer reservoir tube.
Expiration - The patient expires into the reservoir tube. Although fresh gas is still flowing into the system at this time, it is wasted as it is contaminated by expired gas.
Expiratory pause - Fresh gas from the inner tube washes the expired gas out of the reservoir tube, filling it with fresh gas for the next inspiration.
Alternatively, the circuit may be attached to a block assembly with a pop-off valve and mounted directly to the common gas outlet of the anaesthesia machine. This arrangement facilitates scavenging and intermittent positive pressure ventilation.
Advantages:
Low dead space.
Low resistance to breathing.
Facilitates scavenging of waste gases.
