StandardsTHE ADE SYSTEM AS A CIRCLE SYSTEM
Re-cycling mode using the soda-lime canister
Simplicity: the key to the function of the Humphrey ADE - Circle System
Special design features of the ADE - Circle System
- Easily and quickly attached or removed; no change in basic instructions for use compared to the semi-closed mode.
- 500ms soda lime; lasts 8-12 hours depending on use.
- Small circulating volume; quick priming and rapid response to changes in gas composition compared to jumbo absorbers.
- Unique design with soda lime in "parallel" rather than in "series" - soda lime automatically bypassed at higher flows; induction fresh gas flows (4 l/min) lower than jumbo absorber (6-8 l/min).
- Low resistance - suitable for adults and children down to 15kg.
- MRI compatible.
- Re-fillable or disposable versions.
Attachment of the soda lime canister: the canister is easily attached to the main body of the ADE system in a matter of seconds by the use of two lock nuts on the inspiratory and expiratory limbs (see Fig 3 below).
The instructions for use of the ADE-circle system with the soda lime canister are the same as the ADE system without the canister.
The ADE-Circle for Adults
Spontaneous & Manual ventilation
The Humphrey ADE system has a single lever which is positioned UP for spontaneous respiration (Fig 3). For manual ventilation the lever position is the same as for spontaneous respiration. In both modes, waste gases are scavenged at the exhaust valve.
Controlled ventilation
The ADE lever is positioned DOWN to bring the ventilator into circuit. 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 and ventilator modes.
In either mode, the reservoir bag and ventilator are left permanently attached, the Humphrey ADE-circle system being ready for use in any mode at all times; this is of particular value in an emergency.
Fresh gas flow (FGF) in the Humphrey ADE-circle system
- Induction: Because at higher FGFs gases automatically pass over and not through the soda lime in the canister, the ADE behaves more like a semi-closed system. Consequently the FGF during the induction phase can be set at lower flows than with other circle systems. 4 l/min is now recommended for all patients for routine surgery.
- Maintenance: Induction flows can be reduced once an acceptable anaesthetic vapour tension has been achieved in the patient's tissues. This varies with the vapour used, but is generally after about 5 minutes with sevoflurane and desflurane, 10 minutes with isoflurane, 15 minutes with enflurane and 20 minutes with halothane. The FGF can be reduced to a flow with which the anaesthetist feels comfortable, but one litre/min achieves economy and is safe, provided the gas contains 50% oxygen.
- Monitoring of inspired vapour tension is recommended as re-cycled gases dilute the vapour delivered from the vapourizer. With sevoflurane and desflurane the inspired tension may only be 10% less than that set on the vapourizer, but with isoflurane the difference may be 2-3 times less, while with enflurane and halothane it is even greater and unpredictable.
Ventilator settings
As inspired gas contains no carbon dioxide, ventilation volumes must be carefully controlled to avoid over or under-ventilation. For normal arterial carbon dioxide tensions in adults, ventilation volumes should be set around 70 ml/kg/min (ref 9), increased by about 10-20% for younger male patients aged below 35years. For children capnograph monitors are recommended as ventilation volumes are difficult to predict.
The ADE-circle in paediatrics
The ADE system as a semi-closed system without an absorber has become an accepted low flow system. It uses as little as one third of the FGF compared to the T-piece and providing a means of scavenging (see menu at the top of this page in the section "The ADE system as a semi-closed system - paediatrics). Thus FGF can be reduced to 3 litre/min or less. However, for children weighing 15kg or more, greater savings can be achieved by re-cycling vapours and reducing FGFs to just one litre/min. Sevoflurane is particularly suited to this technique but adequate monitoring must be available. The canister offers low resistance, but for children below15kg the ADE without the canister is recommended.
Humphrey ADE system is set identically as for adults for both spontaneous and controlled ventilation (see Fig 3 above). 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 4 l/min during the induction phase for all patients and reduced for maintenance once an equilibrium is reached. In children this is achieved earlier than in adults. As for adults, the FGF can be reduced to any flow with which the anaesthetist feels comfortable, but one litre/min achieves economy and is safe, provided the gas contains 50% oxygen. Monitoring of inspired vapour tension and oxygen is even more important in children.
Comment on valve in paediatric use (Ref 7): The deliberate use of an exhaust valve in the spontaneous breathing 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-circle system has been designed to open or close at a "PEEP" pressure of around 1cm of water; above this value it dumps excess flow with minimal resistance. It thus offers the advantages of "PEEP" but without increasing resistance to gas flow through the valve itself (Fig 2).
Standard tubing for the ADE-circle system - lightweight 15mm smooth bore tubing for all patients (Ref 5)
The ADE-circle 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 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.
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 with the ADE-circle system: As the ADE-circle unit has no materials in it 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 outside the magnetic are and the ADE system left inside clamped to a post (see information on clamp kit below).
Clamp kit for ADE-circle system: The ADE-circle system normally attaches directly to a standard 22mm fresh gas outlet on the anaesthetic machine. When the soda lime canister is used, it may be less liable to accidental damage if it is securely fixed or placed in a less vulnerable position. 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. The fresh gas is supplied through a hose connected from the anaesthetic machine to the fresh gas inlet on the ADE-circle system; this 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.