A co-axial modification of the Mapleson A system, designed to facilitate scavenging of expired gas. (Dr A Lack, retired Salisbury anaesthetist).
Figure 1: The Lack circuit
A four-way block is attached to the fresh gas outlet (F). This block is connected to an outer reservoir tube (R) attached to the patient (P), an inner exhaust tube (E), a breathing bag (B) and a spring-loaded expiratory valve (V).
Figure 2: Function of the Lack circuit
The Lack circuit is essentially similar in function to the Magill, except that the expiratory valve is located at the machine-end of the circuit, being connected to the patient adapter by the inner coaxial tube.
Inspiration - The valve closes and the patient inspires fresh gas from the outer reservoir tube.
Expiration - The patient expires into the reservoir tube. Towards the end of expiration, the bag fills and positive pressure opens the valve, allowing expired gas to escape via the inner exhaust tube.
Expiratory pause - Fresh gas washes the expired gas out of the reservoir tube, filling it with fresh gas for the next inspiration.
As for the Magill circuit.
The location of the valve is more convenient, facilitating intermittent positive pressure ventilation and scavenging of expired gas.
In common with other co-axial systems, if the inner tube becomes disconnected or breaks, the entire reservoir tube becomes dead-space. This can be avoided by use of the 'parallel Lack' system, in which the inner and outer tubes are replaced by conventional breathing tubing and a Y-piece:
Figure 3: 'Parallel Lack' system