Frequently Asked Questions

As the ADE-circle system is safe and simple to use it is particularly useful in non-specialist environments, it is widely used in both human and veterinary practice. These questions are those most frequently asked – hopefully the reply will assist you in understanding the differences between the ADE-circle system and other commonly used equipment. Please feel free to add to this list of FAQs by sending in your observations to Dr Humphrey, by clicking here and selecting “Support” – they are here because you ask the important questions.

Questions on induction

Questions on maintenance of anaesthesia

Questions on the soda lime canister and its use

Questions on servicing and spare parts

Questions on induction:

Q. Induction seems to be slower with the ADE in the semi-closed mode than the T-piece and/or

Q. I have to use high flows to get the patient anaesthetized.

A. Yes, both observations are true. The ADE requires a different approach compared to the T-piece and Bain where the reservoir is on the expiratory . Firstly, with the reservoir bag on the inspiratory limb the ADE system must be primed with vapour at the normal % for induction BEFORE connecting to the patient. With the canister in the system the volume to be pre-filled is significantly greater at about 3 litres so priming needs to be longer using higher flows for 2-3 minutes to ensure that the air is removed. If the system is properly primed induction is much quicker, much as with the T-piece.

What happens if you don’t prime the system is that the patient will initially just re-breathe the air in the inspiratory limb. The reservoir bag will only fill up slowly with the low flows recommended and thus it will take some time for the inspired vapour tension to rise to that set. Thus, not surprising, induction is slow – and with propofol induction the animal may wake up before any significant amount of vapour is inhaled.

Thus it is important that the reservoir bag be pre-filled with the vapour at that normally used for induction – often 2-3% for halothane and 3-4% for isoflurane. Remember nothing goes to waste as the gas is stored on inspiratory side of the system. So at induction fill the bag fast by turning the oxygen flowmeter up – but don’t use the oxygen flush as this bypasses the vaporizer (breathing 0% vapour gets nowhere)! Once the bag is full, turn down the flows to those recommended for induction – and only reduce flows to maintenance flows once the animal is at or near the required degree of anaesthesia.

Another reason why induction may be slow is using too low vaporizer settings. If isoflurane is being used compared with halothane, remember that isoflurane is not as potent as halothane – 2% halothane is the equivalent of about 3% isoflurane. With larger animals who may take up significant amounts of anaesthetic into their tissues it may be necessary to induce with even higher dial settings for a short while. As a general rule, if induction is going too slowly, it is better to increase the vapour % rather than the flow which is just wasteful. As depth of anaesthesia can alter quickly, it is important to monitor the patient and adjust the vapour setting as required.

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Questions on maintenance of anaesthesia

Q. The patient is too light during a painful part of the procedure – how do I increase the inspired vapour % ?

A. The simple answer is to increase the vaporizer setting. However there are differences in response with the ADE system compared to the T-piece. The high flows of the T-piece immediately deliver the new concentration to the patient so that the change is rapid. The ADE response is much more gentle in that the new vapour mixes with the existing vapour in the reservoir bag before it gets to the patient. The mixture in the bag only rises to the new setting over a minute or two. This gradual rise is safer. However, if an immediate change is required, empty the bag by squeezing it and refill it with what is desired. Fill the bag quickly by increasing the flow and then turn it down again.

With the soda lime canister in place changes in vapour concentration are even slower as the new concentration mixes with about 3 litres of circulating volume. If only 300 mls/min are being added to a circle, it will take 10 mins just to re-fill 3 litre system. Again if an immediate change is required, empty the bag by squeezing it and refill it with what is desired. Fill the bag quickly by increasing the flow for about a minute (monitoring the patient’s response) and then turn it down again to that recommended.

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Q. When using the system the reservoir bag does not stay inflated as expected.

A. There are several answers.

The commonest is a leak in the system. This is either from the reservoir bag, tubing or soda lime canister. A leak test should always be performed prior to starting surgery each day. Perform the test first without the canister (this tests the bag + tubes) and then with the canister (this tests canister if the first leak test is OK). The reservoir bag often perishes around the neck where it is often not noticed – stretch it and look carefully! Plugging one end of a tube and filling with water usually identifies a leak when held up. Leaks after re-filling a canister are common and are usually around the lid – a retainer screw may be loose – or a granule of soda lime has got into the threaded socket thus preventing the screw from tightening on the lid. In this case the lid and soda lime will have to be removed and a pin used to extract the granule (best prevented by using a funnel during filling). Having completed the leak test the only place it can now be is beyond the end of the tubing.

The next place a leak can occur is around the endotracheal tube. If the cuff is under-inflated then gas can escape around the tube. Using an under-sized tube (especially in a large dog) can cause a significant leak especially in relation to the low flows used with the re-cycling system. So correct tube size and correctly inflating the cuff resolves this problem – but remember cuffs may leak and start fine but gradually leak during the operation (quite common). Using an un-cuffed endotracheal tube in a small animal is sensible – but if there is a leak this needs to be allowed for by increasing flows to keep the bag inflated.

A third reason why the reservoir bag may collapse is during light anaesthesia with rapid respiration rates and deep breaths. The bag may be emptied with the larger breaths while excess expiratory pressure generated in early expiration will lift the exhaust valve seat above the chimney such that excessive gas is lost from the system when in quiet respiration it is normally retained. Correcting and deepening anaesthesia is required, but until that is accomplished increase the fresh gas flow to fill the bag and temporarily screw down the valve to limit the loss of gas from the system. An analgesic drug often helps as this is synergistic with the vapour. Remember to open the exhaust valve up again and reduce the flows once the patient is settled.

Monitors such as a capnograph samples gases continuously. Some take out significant amounts such as 300-400 mls/min. As the flow is recommended for animals up to 3-4 kg on the ADE system and for 30kg dogs when re-cyling through the canister is itself 300 ml/min, the whole of the fresh gas flow supplied is being sucked out by the monitor. Thus this sampling flow must be added to the recommended fresh gas flow.

Surprizingly the last reason is more common than might be thought. Active scavenging systems assisted by fans may well be too strong causing a negative pressure at the exhaust valve. The ADE exhaust valve only needs minimal negative pressure on top of it to open it such that anaesthetic gas can be actively drawn out of the system causing the reservoir bag to collapse. The problem is easily resolved – simply put a T connector into the scavenge tube from the patient and attach a tube with a volume twice that of the patient’s tidal volume and leave it open to atmosphere. Gas from this side tube open to atmosphere will be scavenged all the time drawing with it anaesthetic gas from the system only when the valve opens.

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Q. The patient seems to recover more slowly at the end of anaesthesia.

A. Because the reservoir bag is on the inspiratory limb, anaesthetic gas stored there goes to the patient before being eliminated. So turning the vaporizer off does not reduce the concentration in the bag immediately – the fall in concentration in the reservoir bag will be therefore be a gradual process – and not surprizingly the wake-up time is will be slower if the patient continues to breathe this gas. Rather take advantage of this and turn the vaporizer off as skin closure commences or the last sutures are being put in – anaesthesia may last even 5 -10 mins but judge this time for yourself. When you have completed the operation, turn up the flows to at least double that recommended to ensure oxygenation and quick wash-out of any vapour remaining in the system and also that coming out of the patient’s tissues and lungs. Saving anaesthetic gas is not required at this point!

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Questions on the soda lime canister and its use:

Q. How do I know when to change the soda lime?

A. Soda lime exhaustion is indicated by a pH-sensitive dye colour change in the soda lime that responds to the change of pH in the granules as CO2 is absorbed. Some dyes change from white to purple/violet and others from pink to white. This can be confusing as white can indicate new or exhausted soda lime. So put a label on the canister with the colour changes expected and the date the soda lime is renewed.

The ADE canister holds about 500gms of soda lime and will absorb a finite amount of CO2. At the low fresh gas flows recommended a large amount will be absorbed especially from patients over 25kg. Consequently there is no time length the soda lime will last so observation of the colour change is required. The soda lime that first reacts with CO2 will exhaust first so colour changes will be seen early at the top of the expiratory side of the canister (below the blue one-way valve). This will gradually progress down this side and then begin to track up the other side especially around the middle division. Once this happens and half the soda lime has changed colour it is time to change the soda lime. At this point even if there is no colour change the soda lime is most used up as the colour change is a late change. Remember soda lime is cheap and the quicker it is exhausted the more oxygen and vapour has been saved!

It is also recommended that, even if the soda lime is not exhausted, it is changed at least once a month. There are some very poor soda limes on the market that hardly change colour and the exhausted state is not recognized. Exhausted soda lime gets wet and can completely clog the flow through it and cause obstruction – experience with Intersurgical UK “Spherosorb” and Drager soda lime is good.

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Questions on servicing and spare parts:

Q. How much servicing and general maintenance does the ADE system need?

A. All equipment needs some maintenance. With the ADE system you can go a long way to keeping the system fully functional by daily checks and careful handling.

The leak test prior to every anaesthetic will reveal perished reservoir bags and punctured tubes. It is a good idea to have 2 spare tubes – smooth bore tubing as recommended for the ADE as other corrugated tubing alters the efficiency and is more difficult to breathe through. This tubing is readily available from Anaequip or AES (addresses at the end). Also keep a range of reservoir bags as spares, the ½ litre and 1 litre being used the most.

The use of better grade of soda lime is recommended as it causes less dust. The dust from cheaper soda lime gets around the system and clogs up valves. Worse still it can be taken into the lungs by the patient (unless a filter is used which is uncommon in vet practice). So ensure you use the best dust free soda lime – it pays huge dividends in keeping the system functioning. Generally sphere or half-sphere shape soda lime granules are better than the irregular size sausage shaped granules (Spherosorb from Intersurgical UK and one from Drager (Germany) have been found to be good).

Sterilization: The ADE block (without the canister) is autoclavable, but generally it is easier to cold sterilize it periodically (monthly is a good interval). The canister should also be emptied and cleaned out and cold sterilized in the same way. Ensure to rinse the equipment well to be certain that no chemicals are left, and shake off all excess water. It does not have to be dried as gas from the patient is fully humidified anyway as can be seen by condensation in the expiratory limb. (See below for tube cleaning and sterilization)

Full servicing: This should not be overlooked and undertaken ANNUALLY. The two one-way valves on the canister and the exhaust valve and safety valve on the main block of the ADE should be undertaken once a year. This is generally done by Anaequip on an exchange basis – this involves inspection, cleaning and new parts where required such as silicone valve flaps. The canister itself does not need any attention unless it shows any obvious signs of damage or deterioration – the suggested time limit of 12 months on the canister base lid can be considerably extended in vet use as it is used so much less than in hospital theatres which function all day and all year. Any damaged canister or ADE system should be returned for repair or replacement. For Anaequip addresses see below.

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Q. How often should the breathing tubes be sterilized and how is it best done?

A. The ideal would be to have clean tubes for every patient – uncleaned the tubes breed bacteria and fungi such that they can go green and visibly discolour. Keeping the ADE tubes is particularly easy so that there is no reason why they should not be cleaned on a daily basis.

The tubing of the ADE system is different as it is not corrugated tubing. It is smooth inside with an outer ribbing for support and strength. Being smooth bore the chemicals do not accumulate in the corrugations so it is easy to put the whole tubing set in a standard sterilizing solution for the recommended time for that solution and then simply rinse them out. It is best to leave all the connections on the tubing during sterilization as constant removal stretches the tubes which then fall off easily. Hanging the tubes vertically on the “Y” connector will drain any excess water.

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For queries about the use of the Humphrey ADE-Circle system

Please contact Dr David Humphrey (South Africa) by clicking here, selecting “Support”. Please feel free to add to this list of FAQs by sending in your observations

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