Mapleson's classification of an anaesthetic breathing system

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. Function: 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. Spontaneous ventilation Normocarbia requires a fresh gas flow of 200-300 ml/kg. Controlled ventilation 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