StandardsFor Adults and Children
Breathing systems have previously been divided into equipment designed for adults and that for children. Standard adult systems tend to be bulky, heavy and include valves which increase resistance to flow and make breathing more difficult. For younger patients under 25kg, the system needs to be lightweight and easy to breathe through.
This has been achieved by using smaller tubes and valveless systems such as the T-piece. Unfortunately the advantages of the T-piece are negated by three factors. Firstly the use of smaller tubes increases resistance, opposite to that required. Secondly very high fresh gas flows during spontaneous respiration increase costs. Finally pollution control is difficult to provide effectively since many scavenging devices have proved dangerous; the reservoir bag on the T-piece can easily be twisted and occluded with over-inflation and damage to lungs.
The unique features of the Humphrey ADE-circle system are that it has been designed to avoid these problems in all modes of use for adults and children. These include:
- the use of lightweight 15mm tubing but with a smooth internal bore. Resistance is much reduced such that it is suitable for both adults and children.
- a specially designed exhaust valve which is deliberately included even for children as it reduces the tendency to lung collapse under anaesthesia without increasing resistance. With this valve the system can be used with low fresh gas flows in all modes for all patients.
- connecting a standard scavenging system which is attached to the system at the anaesthetic machine with no danger of occluding flow.
Lightweight 15mm smooth bore tubing for all patients
The key to the solution to reducing resistance to flow within a tube lies in its size and internal construction. While the reduction in tube size from 22mm to 15mm tubing achieves a lightweight tube for children, it adversely affects resistance which is increased. The matter is made worse as corrugated tubing, commonly used because it resists kinking, causes considerable turbulence which increases resistance to flow within the tube. A lightweight low-resistance tube is required.
The solution to reducing resistance in a smaller tube is to use tubing that has a totally smooth bore throughout the tube rather than one which is corrugated. Flow generally becomes non-turbulent (laminar flow). The dramatic reduction in resistance is shown in the Fig 6 where resistance in 15mm ADE smooth bore tubing (red line) is reduced by fourfold when compared to the same size standard paediatric corrugated tube. This tube is therefore ideal for use in children, even under one year old. Furthermore the resistance of the ADE 15mm tube is lower than that in adult systems such as the Lack and Magill making it also very suitable for use in adults.
In conclusion the 15mm ADE tubing is ideal for general use throughout an operating list that includes both adults and children (including infants). ADE 15mm tubing is supplied in disposable or autoclavable forms. Two tubes may be joined together to make a total length of over 3 metres, this being especially useful in head and neck surgery and for MRI scanning.
New concepts in valve design for paediatric anaesthesia offering "PEEP", low resistance and pollution-free anaesthesia
In the sub-section "ADE as a Semi-closed system" , the use of the Humphrey ADE system for children is described. The deliberate use of an exhaust valve in the spontaneous breathing "A" mode is a now an accepted technique even though this contrasts with the alternative rationale of using the valveless T-piece. However, contrary to previous popular belief, a valve in a paediatric system can be beneficial provided it offers the physiological advantage of positive end-expiratory pressure ("PEEP"). The latter keeps that lungs more expanded at the end of expiration and so prevents alveolar collapse and the associated reduced gas exchange. As the lungs are held open, the generalized increase in airway size also reduces airway resistance and eases the work of breathing.
With these benefits in mind, the new valve on the Humphrey ADE system has been designed to open or close at a pressure of around 1cm of water; above this value it dumps excess flow. It thus offers the advantages of "PEEP" but without increasing resistance to gas flow through the valve itself (Fig 2).
Because of such advantages, the Humphrey ADE system can now be used for children, especially in its "A" mode for spontaneous respiration. Effectively this allows a reduction in the fresh gas flow required by up to 60% compared to the T-piece. While lower flows reduce theatre contamination, a pollution-free environment is further ensured by the connection of a standard scavenging device back at the anaesthetic machine away from the patient.
The Humphrey ADE system is therefore not only very cost-effective compared to the T-piece, but it is physiologically advantageous and environmentally friendly.