The Mapleson D may be described as a co-axial modification of the basic T-piece system, developed to facilitate scavenging of waste anaesthetic gases.
Figure 1: Mapleson D system
A tube carrying fresh gas (F) travels inside an outer reservoir tube (R) to the endotracheal tube connector (P).
Figure 2: Function of the Mapleson D system
The Bain circuit is a modification of the Mapleson D system. It is a co-axial 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.
Normocarbia requires a fresh gas flow of 200-300 ml/kg.
A fresh gas flow of only 70 ml/kg is required to produce normocarbia.
Bain and Spoerel have recommended the following:
2 L/min fresh gas flow in patients <10 kg
3.5 L/min fresh gas flow in patients 10-50 kg
70 ml/kg fresh gas flow in patients >60 kg
The recommended tidal volume is 10 ml/kg and respiratory rate is 12-16 breaths/minute.
A bag may be added to the tail of the reservoir tube, as in the T-piece.
Figure 3: Modified Mapleson D system: Addition of bag to the tail of the reservoir tube
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.
Figure 4: Modified Mapleson D system: Attachment of circuit to a block assembly with a pop-off valve and mounted directly to the common gas outlet of the anaesthesia machine
Low dead space.
Low resistance to breathing.
Facilitates scavenging of waste gases.
High fresh gas flow requirement.
High gas flow rates - for example, if the oxygen flush valve is used, it may cause lung barotrauma.
As with other co-axial systems, if the inner tube becomes disconnected or breaks, the entire breathing tube becomes dead space, leading to severe alveolar hypoventilation. This may be detected in systems fitted with a bag by closing the valve and activating the oxygen quick-flush. If the inner tube is intact, the Venturi effect of the rapidly moving stream of gas leaving the inner tube will suck gas out of the bag and the bag will empty. If the inner tube is damaged, the stream of gas will be directed into the bag and it will fill.
Figure 5: Scenario of damaged inner tube in modified Mapleson D system with attachment of circuit to a block assembly (see text)
Alternatively, the so-called 'parallel Bain' system may be used. In this system, the inner and outer tubes are replaced by conventional circle-absorber-type tubing and Y-piece. This arrangement is also used in the Humphrey ADE system.
Figure 6: Parallel Bain system
[i] Flow requirements for a modified Mapleson D system during controlled ventilation. Bain JA, Spoerel WE.
Can Anaesth Soc J 1973; 20(5): 629-36.