Ayre's T-Piece

The original Mapleson E system.

Construction

A three-way T-tube whose limbs are connected to (F) the fresh gas supply from the anesthesia machine, (R) a length of corrugated reservoir tube and (P) the patient connector.

Function

Inspiration -The patient inspires fresh gas from the 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 washes the expired gas out of the reservoir tube, filling it with fresh gas for the next inspiration.

Operational requirements

• The volume of the reservoir tube must be greater than the patient's tidal volume, otherwise the inspired gas will be contaminated by the surrounding air.
• Fresh gas flow requirements--see separate article.
  
• Intermittent positive pressure ventilation may be performed by intermittently occluding the end of the reservoir tube.

Modifications
Jackson-Rees' modification of the Ayre's T-piece (somethimes known as the Mapleson F system) connects a two-ended bag to the expiratory limb of the circuit, gas escaping via the `tail' of the bag

This allows respiratory movements to be more easily seen and permits intermittent positive ventilation if necessary. The bag is, however, not essential to the functioning of the circuit.
     Intermittent positive pressure ventilation may be performed by occluding the tail of the bag between a finger and thumb and squeezing the bag. Alternatively, a `bag-tail valve', which employs an adjustable resistance to gas flow, may be attached to the bag tail. This causes the bag to remain partially inflated and so facilitates one-handed performance of IPPV.

Another aid to IPPV is the Kuhn bag, which has the gas outlet on the side of the bag, rather than the tail. This allows the outlet to be occluded with the thumb during IPPV, but leads to difficulties in scavenging the waste gases.

A number of different designs of T-piece are available, which function in essentially the same way.

Classic T-piece	Modified T-piece	Washington T-tube	Norman elbow

Modern T-pieces incorporate 15 mm fittings for the reservoir tube and endotracheal adapter. The Normal elbow incorporates a right-angle bend which is convenient for use in human patients but has little advantage in veterinary anesthesia.

Advantages of T-piece systems

• Compact
• Inexpensive
• No valves
• Low dead-space
• Low resistance to breathing
• Economical for controlled ventilation

Disadvantages

• Some T-pieces are rather heavy and difficult to keep connected to the endo-tracheal tube in small animals.
• The bag may get twisted and impede breathing.
• High gas flow requirement in larger animals.

Uses

• Small animals under 7 kg body weight.

Bain