StandardsTHE ADE AS A SEMI-CLOSED SYSTEM
Use in adults and children without re-cycling
Simplicity: the key to the function of the Humphrey ADE - System
Advantages and disadvantages of semi-closed systems
What is required for anaesthesia is enough; any more than that is wasted and expensive. Re-cycling reduces costs but the technique requires greater experience and monitoring. Semi-closed systems that simply rely on adequate fresh gas are much easier to use and worldwide are used extensively. Without re-cycling, the efficiency of anaesthetic systems varies from about 75% to as little as 25%; in other words some run on three times less fresh gas than others and yet achieve the same effect.
A system may be efficient for spontaneously breathing patients and very in-efficient when used with a ventilator or vice-versa. Clearly the use of the most advantageous system is required to reduce costs. Table 1 below lists such advantages and disadvantages of various well known systems.
Comparisons of Semi-closed breathing systems
Since 1978, the design of a multipurpose system has been directed at including and improving on all the advantages of the alternative systems listed above, while avoiding all their disadvantages. After 20 years of research and development, the Humphrey ADE system has been independently confirmed to have achieved all these goals (see section "references/publications & design history" at top of page). It is efficient in all modes of use; low FGFs in both spontaneous & ventilated modes results in the greatest economy at all times with significant reductions in running costs especially in gas and vapour usage. The ADE system is equally suitable for all sizes of patient, from adult to child in whom it has particular advantages (see below). The system offers the same benefits in veterinary practice. Because of the high cost of newer anaesthetic agents, the recycling mode has been recently introduced (see "ADE as circle system" at top of this page). To date the ADE system is used in about 250 British hospitals and worldwide in an estimated 11 million anaesthetics.
The remainder of this section describes the use of the ADE system (as a semi-closed system) in clinical practice, the principle being the same in veterinary practice.
Use of the Humphrey ADE system in Adults
Spontaneous & Manual ventilation
The Humphrey ADE system has a single lever which is positioned UP (in the Mapleson "A" mode) for spontaneous respiration (Fig 1a). As the Humphrey ADE system has been confirmed to be more efficient than the Magill, the fresh gas flow (FGF) is set at an average of only 50ml/kg/min i.e. only 3-4 l/min (Refs 2, 8,10,11,12). Waste gases are scavenged at the exhaust valve. For manual ventilation the lever position and FGF are the same as for spontaneous respiration. In this mode the ADE functions efficiently, again improving on the Magill (Ref 4,15).
Controlled ventilation
The ADE lever is positioned DOWN (Fig1b) to bring the ventilator into circuit (in the Mapleson "E" mode), the FGF being set at 70ml/kg/min for normocarbia (Ref 2,11). Nothing else needs to be altered, as the reservoir bag, exhaust valve and ventilator are automatically included or excluded from the system when switched between spontaneous ("A") and ventilator ("E") modes.
Ready for use in all modes
As the reservoir bag and ventilator are left permanently attached, the Humphrey ADE system is ready for use in any mode at all times; this is of particular value in an emergency.
Paediatrics
Humphrey ADE system can be used in place of the T-piece for all children (including the new born) as it has special features which offer benefit (see below).
It is set identically as for adults for both spontaneous and controlled ventilation. A smaller reservoir bag (1litre or 500 ml) should be used, while an appropriate paediatric ventilator should be used. The FGF should be set at 3 l/min for all patients and adjusted (usually down) if capnography is available. The main advantages over the T-piece are that in the mode for spontaneous respiration the FGFs are one third of that required with the T-piece while gases are scavenged at the exhaust valve back at the anaesthetic machine (Ref 7,14).
Comment on the use of a valve in paediatrics: (Ref 7). The deliberate use of an exhaust valve for spontaneous breathing (Mapleson "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 "PEEP" pressure of around 1cm of water; above this value it dumps excess gas with minimal resistance. 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 is particularly 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.
Standard tubing for the ADE system - lightweight 15mm smooth bore tubing for all patients (Ref 5)
The ADE system is supplied with lightweight smoothbore tubing for adults and children. Compared with corrugated tubes of the same internal diameter, the use of smooth-bore tubing results in a four-fold reduction in resistance to flow. For adults, such characteristics allow the use of smaller 15mm tubes rather than 22mm tubes. These smaller 15mm tubes even offer a lower resistance than well-known adult systems such as the Lack and the Bain. For children the lower resistance also reduces the work of breathing.
In conclusion the 15mm ADE smooth-bore tubing is ideal for general use with or without the soda lime canister throughout an operating list that includes both adults and children (including infants).
ADE 15mm tubing is supplied in lengths of 1.5metres in both 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.
MRI use in the semi-closed mode: As the ADE-circle system is not made with any materials that are affected by magnets, it is MRI compatible with or without the soda-lime canister. It can be used in the scanning room right next to the patient. If no MRI compatible anaesthetic machine is available, a standard machine can be placed in the protected area away from the scanner while the ADE system is left inside clamped to a post (see information on clamp kit below).
Clamp kit
The ADE system normally attaches directly to a standard 22mm fresh gas outlet on the anaesthetic machine. It is less liable to accidental damage if it is securely fixed. An lSO lock nut is available to secure it to an anaesthetic machine but, as many machines do not have the complimentary locking thread on the fresh gas outlet, a clamp allows the system to be attached to a convenient round pole, rail or square post. Fresh gas is supplied to the ADE system through a hose connected from the anaesthetic machine; the hose can be as long as required, even up to 5-6 metres (as may be needed in MRI or X-ray environments). The clamp kit includes all components required, and like the canister, it can be easily attached or removed.