Humphrey ADE Circuit

It is not a difficult matter to construct the three most useful Mapleson non-rebreathing circuits (A, D and E) from a couple of lengths of breathing tube, a Y-piece, a breathing bag, an expiratory valve and a few connectors, as follows:

Mapleson A:

Mapleson D:

Mapleson E:

Although these look rather different from their more usual incarnations (for example, in most Mapleson E systems, such as the Ayre's T-piece, the fresh gas supply tube is small-bore tubing), they will function in exactly the same way.

The Humphrey ADE system makes use of this to provide a simple way of switching between these configurations whilst using the same apparatus.

Construction

The circuit consists of a block with a fresh gas inlet (FG), two breathing bag connectors (B1 and B2), an adjustable expiratory valve (V), connectors for the patient (P) tubing (which may be parallel, as shown above, or co-axial), and a rotary selector valve, shown here schematically in red. Smooth-bore tubing is used in order to reduce resistance compared with the normal corrugated tubing.

The circuit to be used (i.e. Mapleson A, D or E) is determined by the position of the selector valve and which bag connector is used.

Mapleson A

Here, the selector valve is set to connect the bag connector B1 and disconnect B2. It will be seen that the circuit is now identical to the Magill or Lack, except that the expiratory valve is connected to the patient via the expiratory limb of the circuit.

Mapleson D

The selector valve is set to connect the bag connector B2 and disconnect B1. The inspiratory limb of the patient circuit acts as the fresh gas supply tube, and the circuit is now functionally a Mapleson D.

Mapleson E

The selector valve is set as above, but the bag is removed and the expiratory valve is closed. The circuit is now a Mapleson E. A ventilator may be attached to the bag port in order to perform intermittent positive pressure ventilation.

Function

In each of its configurations, the circuit functions in the same way as the conventional circuits of each type.

Uses

This circuit provides a convenient method of switching between the Mapleson A and D/E arrangements. This makes it particularly easy to perform IPPV, making use of the economy of fresh gas flow provided by the Mapleson A in spontaneous breathing and the Mapleson E during IPPV.

Rebreathing circuits