The most commonly used T-piece system is the Jackson-Rees' modification of the Ayre's T-piece (sometimes known as the Mapleson F system). This system connects a two-ended bag to the expiratory limb of the circuit; gas escapes via the `tail' of the bag.
Figure 1: Modifications to the Ayre's T-piece
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 (IPPV) 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.
Figure 2: Bag-tail valve
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.
Figure 3: The Kuhn bag
A number of different designs of T-piece are available, which function in essentially the same way.
Figure 4: T-piece designs
Modern T-pieces incorporate 15 mm fittings for the reservoir tube and endotracheal adapter.
The volume of the reservoir tubing should approximate the patient's tidal volume. If the volume is too large rebreathing may occur and if too small, ambient air may be entrained.
To prevent rebreathing, the system requires a minimal flow of 4 L/min, with a fresh gas flow of 2.5 to 3 times the patient's minute volume.
Advantages of T-piece systems
Minimal dead space
Minimal resistance to breathing
Economical for controlled ventilation
The bag may get twisted and impede breathing
High gas flow requirement
Children under 20 kg weight
Figure 5: Mapleson F system
[i] Anaesthesia in the newborn. Jackson-Rees G. BMJ 1950; 2: 1419.
[ii] Essentials of Anaesthetic Equipment, Churchill Livingstone.