Courtney Scales DipVN, NCert(Anaesth), RVN
Keith Simpson BVSc, MRCVS, MIET(Electronics) – Clinical Director
A circle breathing system, or rebreathing system, allows the patient’s expiratory breath to be recycled and rebreathed after the carbon dioxide (CO2) has been removed. It is removed via CO2 absorbing granules that are held within a canister, which the expiratory breath must pass through before being inhaled again. The unidirectional flow of gases is achieved with one-way inspiratory and expiratory valves.
This circle system does not require a high fresh gas flow (FGF) to remove the CO2-laden gases from it, such as in the non-rebreathing systems, because of the CO2 absorbing granules. Therefore, the maintenance FGF only needs to replace the oxygen that the patient uses.
The FGF that needs to be delivered only needs to replace the patient’s metabolic oxygen consumption rate (overestimated as 10ml/kg/min), with a minimum flow for vaporiser accuracy of 0.5L/minute. This lower FGF has many benefits for both the veterinary practice and the patient:
Whilst the benefits of a circle system outweigh any perceived disadvantages in the use of a circle system, the disadvantages can be easily managed. These include the slow change of volatile agent concentration within the system when using low FGFs, the fact that many parts need monitoring e.g. CO2 absorbing granules and one-way valves, and that it may cause overheating in large breed dogs with a thick coat. How to manage these are discussed throughout.
Patient Selection
Circle systems have historically been used in patients over 10kg for fear that if they were used in smaller patients, there would be an increase in the work of breathing or that their small tidal volumes wouldn’t generate enough of a pressure difference to move through the wide 22mm tubing and move the one-way valves.
These issues can be overcome by ensuring the reservoir bag is on the inspiratory side* of the system and holds gases that have already been through the CO2 absorbing granules (so that the patient doesn’t have to “pull” their fresh breath through the CO2 absorber) and by using smooth narrower bore 15mm tubing. This type of tubing can be used on patients up to 40kg and with cats from approximately 3kg.
The tubing most commonly comes in a parallel configuration, however a co-axial “universal F circuit” is available. This has the inner tube as the inspiratory limb and the outer tubing as the expiratory limb. This type of system theorises that the warm expiratory breath can wrap around the cooler-inspired gases. Care must be taken when using an F-circuit in patients that have a large tidal volume as there may be an increase in circuit resistance due to the smaller cross-sectional area of the inner tube when they take a breath, however, it is rarely of clinical significance.
*It can be difficult to see by simply looking at the circle system if the reservoir bag is on the inspiratory side or not of the canister, but if in doubt, contact the manufacturer for further details.
Circle Fresh Gas Flow Rates
As previously mentioned, a notable difference in how a circle system is used in comparison to its non-rebreathing counterparts is that it uses FGFs below the patient’s minute volume.
The FGF in a circle system is not as variable as in the non-rebreathing systems, however different FGFs are used at different points of the anaesthesia. Initially, high flows are required to purge the system of nitrogen-rich room air and fill it with a higher concentration of oxygen. This is called denitrogenation. It also fills the breathing system with the volatile agent. After the period of denitrogenation, the maintenance flow to replace the patient’s metabolic oxygen consumption can be provided:
It should be noted however that there is a minimum flow rate required for accurate delivery of the volatile agent from the vaporiser. If the FGF is too low, there isn’t enough turbulence from the airflow within the vaporiser to “pick up” and carry the anaesthetic vapour through it. Most modern plenum vaporizers (TEC 3+) can deliver accurate concentrations of volatile agents at flows >0.25L/min.
Additionally, if sidestream capnography is utilised, the sampling volume must be replaced. Sidestream sampling rates vary between 50-200ml/minute, so this should be added to the rate that is required by the patient, depending on what type of multiparameter you have.
By factoring in the metabolic oxygen consumption rate, sidestream capnography rates and the need for a minimum flow for most vaporisers – a maintenance FGF of 0.5L/minute – 1L/minute is adequate for patients in small animal anaesthesia.
Often low-flow anaesthesia is synonymous with circle breathing systems (maintenance FGF <1L/minute). Minimal flow anaesthesia is when the FGF is <0.5L/minute. (Nunn, 2008). When the exhaust valve is fully closed, there is no excess gas vented and only the patient’s oxygen consumption needs to be replaced. In the author’s opinion, this is rarely performed in veterinary anaesthesia and most practices will operate with the exhaust valve fully open or partially closed.
The Concentration of Anaesthetic Gases in a Circle System
The circle system holds a large volume of gas in the tubing, the CO2 absorbing canister and the reservoir bag – which together may total almost 5L in some cases.
As the patient expires, the expiratory breath will have less volatile agent in it than what was inhaled due to some absorption of the agent by the patient. Over time, the expiratory breath may dilute the volatile agent concentration throughout this large system volume.
Additionally, if only 0.5L/minute of fresh gas is entering a 5L system, it would take 10+ minutes for a full change of gases to occur – and this is without the patient continuously expiring into it and diluting it further.
This poses a problem where the patient’s plane of anaesthesia lightens over time, especially during long procedures. Unless an anaesthetic agent monitoring modality is being used on a multiparameter, you may find you need to increase the delivery of the vaporiser concentration delivery (%) over time to maintain an adequate depth of anaesthesia.
Changing the Concentration in the Circle System Quickly
If there is a need to change the concentration of the volatile agent in the circle system quickly, for example, if the patient is too deeply anaesthetised, in an emergency, or the patient needs to be recovered from the anaesthesia after the procedure, then the FGF should be increased after the vaporiser concentration has been changed. Additionally, “dumping” the reservoir bag by squeezing it so the gas can go out of the exhaust will increase the speed at which the concentration is changed.
Using The Circle System with Nitrous Oxide or Oxygen Concentrators
Due to the low FGFs in a circle system, some specific things must be considered if there is the potential for gases other than oxygen to be in the breathing system. The gases of concern are nitrous oxide (N2O) and argon.
When using N2O in a circle system, unless the concentration of oxygen being delivered can be monitored to ensure it is >30%, then a rate of 50:50 N2O: oxygen should be provided e.g. 0.5L/minute of both gases. This is because oxygen is continuously used by the patient, and the N2O is not, so over time it may deliver a hypoxic mixture of gases to the patient.
An oxygen concentrator has a zeolite sieve that absorbs nitrogen, but not the third most abundant gas in the atmosphere – argon. Even though there is only 1% of argon in the atmosphere, there is the potential for this to accumulate in the circle system when using an oxygen concentrator. Therefore, a minimum flow of 0.5L/minute is recommended.
Reservoir Bag Size
Veterinary anaesthesia is full of calculations, including how to determine what size of reservoir bag should be used. Some text will say that the calculation is the tidal volume x respiratory rate x 5. However, this would be excessive in a circle system as the CO2 canister and tubing hold a large volume of gas and act as a reservoir too.
A reservoir bag is exactly what it says it is – a reservoir for gases, either prior to being inspired or it holds expiratory gases. Its purpose is to accommodate volume changes in the breathing system as the patient breathes so there is always something there for inhalation (and protects the patient from excessive pressure being accidentally delivered to them if the gas flow suddenly increases).
Therefore, it only needs to accommodate around 2-5 tidal volumes. You may see that a reservoir bag should be 40ml/kg, which is effectively a safe overestimate of a tidal volume being estimated as 20ml/kg.
Let’s take the standard 2L bag:
If you have an open exhaust or adjustable pressure limiting (APL) valve, any excess gas will leave the system and be scavenged. If the reservoir bag is sucked flat by the patient between breaths, then the FGF or bag size may need to be increased. Additionally, check the power of your active scavenging if this is being used.
Carbon Dioxide Absorbing
The exhaled CO2 is removed from expired gases via a chemical reaction with an absorbent within a canister of the circle system.
The most common absorbent is calcium hydroxide (Ca(OH)2), with other constituents added to enhance the reactivity, most commonly sodium hydroxide (NaOH). This gives us the formula:
The chemical reaction requires water (H2O), which is present in the absorbent. This reaction will generate more moisture and also heat. The chemical reaction can be summarised as:
Changing the CO2-absorbing Granules
The absorbing granules have a pH indicator which changes colour as they absorb CO2.
As capnography is now a frequently used modality in practice, this can give us a clear indication of when the granules have lost the ability to absorb CO2, saving the unnecessary wastage in changing the granules based on a colour change or time used. Granules that have lost the ability to absorb CO2 will cause rebreathing to be seen on the capnograph (when the baseline does not return to zero). When persistent rebreathing is seen, the granules should be changed.
If capnography is not available and you must rely on the pH colour change, varying sources say the absorbent should be changed when 50% (Phillips, 2015) or two-thirds of the granules have expired (Geoffrey, 2019). Some practices may choose to change the absorber after a certain period, however, this could prove very wasteful as it doesn’t account for the varying canister sizes, the FGF used during use or how large the patient is that was expiring into it.
Rebreathing and the Circle System
Commonly we think of rebreathing seen on capnography as only a raised baseline of CO2, or an increase in the Fractional Inspired CO2 (FiCO2). This may however only be the case if there is a mixing of fresh gas with the expiratory breath and it is inhaled, which is the case when the one-way valves become stuck due to dust or moisture, or when the CO2 absorbing granules have expired.
If there is rebreathing of dead space gases, you are likely to see a prolonged phase III, or alveolar plateau on capnography and an increase in end-tidal CO2 (ETCO2), but not necessarily an increase in FiCO2.
Maintenance of Body Temperature
Anecdotally, it has been assumed a circle breathing system may improve the patient’s temperature by providing warmed inspiratory gases to the patient, however, there is a lack of evidence published to support this. There is promising evidence for using an actively heated breathing system to aid in the maintenance of a patient’s body temperature and airway humidity, but this is separate from a circle system.
Some supporting human and veterinary medicine studies found by the authors conclude the following:
It must be noted that with a circle system, if the FGF is excessive, then any moisture and humidity that is formed from the patient’s expiratory breath and reaction with the CO2 absorber is lost. Additionally, in human medicine where a circle system is used, sufficient moisture (>20mgH2O/l) was still not achieved in patients receiving minimal flow conditions (<0.6L/minute) for up to an hour of use (Kleemann, 1994).
For completeness, although not discussed in depth, if low-flow techniques are used with a circle breathing system, the additional use of a heat and moisture exchanger (HME) is probably not required (Wilkes, 2010) and may be more useful in non-rebreathing systems. An HME with a non-rebreathing system supports higher temperatures and absolute humidity of inhaled gases (Kanda et al., 2020) but not in the preservation of body temperature in dogs (Khenissi et al., 2017).
Usually, heat loss from the respiratory tract is less than 10% of total heat loss (Posner, 2007) and there are much higher heat losses via conduction (cold surgical tables) or radiation (a patient that is not covered with a blanket or drape, etc).
Pre-oxygenation
The correct breathing system must be used to perform pre-oxygenation. It can be provided with all types of non-rebreathing systems, however, not all circle systems will have oxygen flow down the inspiratory limb as it depends on where the fresh gas enters the system – either behind the inspiratory valve or on the patient side of the valve. To eliminate any confusion as to what type of circle system you have, the exhaust valve can be closed whilst delivering flow-by and reopened at the time of anaesthesia induction.
Conclusion
As circle systems are commonly found in veterinary practices, understanding their use and limitations is beneficial to ensure they are used correctly.
References
Bowling, P.A. et al. (2021) “Effects of a heated anaesthesia breathing circuit on body temperature in anesthetized rhesus macaques (macaca mulatta),” Journal of the American Association for Laboratory Animal Science, 60(6), pp. 675–680. Available at: https://doi.org/10.30802/aalas-jaalas-21-000058.
Geoffrey, T. (2019) How To Detect Soda Lime Exhaustion? [online], Dispomed, available: https://www.dispomed.com/detect-soda-lime-exhaustion
Jones, S. et al. (2023) “The effect of a heated humidified breathing circuit on body temperature in healthy, anesthetized dogs,” Veterinary Anaesthesia and Analgesia, 50(1). Available at: https://doi.org/10.1016/j.vaa.2022.09.012.
Kanda, T. et al. (2020) “Effect of a heat and moisture exchanger on temperature and humidity of inhaled gas in isoflurane-anesthetized dogs,” Veterinary Anaesthesia and Analgesia, 47(3), pp. 377–380. Available at: https://doi.org/10.1016/j.vaa.2020.02.003.
Kelly, C.K., Hodgson, D.S. and McMurphy, R.M. (2012) “Effect of anesthetic breathing circuit type on thermal loss in cats during inhalation anesthesia for ovariohysterectomy,” Journal of the American Veterinary Medical Association, 240(11), pp. 1296–1299. Available at: https://doi.org/10.2460/javma.240.11.1296.
Khenissi, L. et al. (2017) “Do heat and moisture exchangers in the anaesthesia breathing circuit preserve body temperature in dogs undergoing anaesthesia for magnetic resonance imaging?,” Veterinary Anaesthesia and Analgesia, 44(3), pp. 452–460. Available at: https://doi.org/10.1016/j.vaa.2016.05.016.
Kleemann, P.P. (1994) “Humidity of anaesthetic gases with respect to low flow anaesthesia,” Anaesthesia and Intensive Care, 22(4), pp. 396–408. Available at: https://doi.org/10.1177/0310057x9402200414.
Kwak, I.-S. et al. (2013) “The effect of heated breathing circuit on body temperature and humidity of anesthetic gas in major burns,” Korean Journal of Anesthesiology, 64(1), p. 6. Available at: https://doi.org/10.4097/kjae.2013.64.1.6.
Nunn, G. (2008) “Low-flow anaesthesia,” Continuing Education in Anaesthesia Critical Care and Pain, 8(1), pp. 1–4. Available at: https://doi.org/10.1093/bjaceaccp/mkm052.
Park, H.J. et al. (2017) “The effect of humidified heated breathing circuit on core body temperature in perioperative hypothermia during thyroid surgery,” International Journal of Medical Sciences, 14(8), pp. 791–797. Available at: https://doi.org/10.7150/ijms.19318.
Phillips, H. (2015) Pretty In Pink : Soda Lime. When Should It Be Changed? [online], Australian College of Veterinary Nursing, available: https://vetnurse.com.au/2015/09/14/changing-soda-lime
Posner, L. (2007) Perioperative hypothermia in veterinary patients, Clinician's Brief. Clinician's Brief. Available at: https://digital.cliniciansbrief.com/columns/40/perioperative-hypothermia-veterinary-patients (Accessed: April 11, 2023).
Wilkes, A.R. (2010) “Heat and moisture exchangers and breathing system filters: Their use in anaesthesia and intensive care. part 1 - history, principles and efficiency,” Anaesthesia, 66(1), pp. 31–39. Available at: https://doi.org/10.1111/j.1365-2044.2010.06563.x.