Lack Circuit
A co-axial modification
of the Mapleson A system, designed to facilitate scavenging of expired
gas.
Construction
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).
Very similar in appearance to the modified
Bain, except that the inner exhaust tube has a greater diameter
than the fresh gas supply tube in the modified Bain.
Function
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.
Toward 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.
Controlled
ventilation
As for the Magill
circuit.
Operational
requirements
Advantages
- As for the
Magill.
- The location
of the valve is more convenient, facilitating intermittent positive
presure ventilation and scavenging of expired gas.
Disadvantages
- As for
the Magill, except the position of the
valve is more convenient.
-
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:
Uses
Ayre's
T-piece
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