Ayre's T-Piece

The Mapleson E is a modification of Ayre's T-piece which was developed in 1937 by Phillip Ayre (a Newcastle anaesthetist) for use in paediatric patients undergoing cleft palate repair or intracranial surgery.

Construction



A three-way T-tube whose limbs are connected to (F) the fresh gas supply from the anaesthesia machine, (R) a length of corrugated reservoir tube and (P) the patient connector. It has minimal dead space, no valves and minimal resistance.



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. An expiratory limb volume greater than the patient's tidal volume prevents entrainment of room air (which would dilute anaesthetic gases and oxygen).

Expiratory pause: Fresh gas washes the expired gas out of the reservoir tube, filling it with fresh gas for the next inspiration.

A fresh gas flow greater than three times the minute ventilation prevents rebreathing.


Spontaneous ventilation

The fresh gas and exhaled gas flow down the expiratory limb. Peak expiratory flow occurs early in exhalation. Thus, the proportion of fresh gas added to the exhaled gases increases. During the next breath, fresh gas is drawn from the fresh gas inlet and the expiratory limb.

The original analysis of the Mapleson E system suggested that a gas flow rate of 2.5 to 3 times the minute volume was required to prevent rebreathing of expired gas. However, this assumed a square-wave respiratory pattern, and investigations using a more realistic breathing pattern have suggested that 1.5 - 2 times the minute volume is acceptable in spontaneously breathing patients:



Fresh gas flow (L/ Body weight (kg)[quote]
5 1.4 - 1.8
10 2.4 - 3.2
20 4.1 - 5.4
40 7.2 - 9.6

Again, these values are guidelines only - if there is evidence of rebreathing, the flow rate should be increased.

Controlled ventilation

In contrast with Mapleson A systems, Mapleson D and E circuits are more efficient during controlled than spontaneous ventilation. This is because the tidal volume must be supplied during the expiratory pause. With the almost sinusoidal respiratory pattern of spontaneous respiration, there is relatively little time for this volume to be supplied, so the fresh gas flow rate must be high. The pattern of controlled ventilation, however, is usually one of a rapid inspiration, expiration and a relatively prolonged expiratory pause. This long expiratory pause gives ample time for the tidal volume requirement to be supplied, even with a fairly low fresh gas flow rate. Consequently, during controlled ventilation, the recommended fresh gas flow rate is similar to that of the Mapleson A systems during spontaneous ventilation (see above).

Intermittent positive pressure ventilation may be performed by intermittently occluding the end of the reservoir tube.