San Sebastian 2004 Session 1-3
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Anaesthetic methods with reduced fresh gas flow in veterinary practice / Técnicas anestésicas con flujos reducidos de gas fresco en la práctica veterinaria.

Dr. Rafael Cediel, Universidad Complutense, Madrid, España.

Low flow Anaesthesia in veterinary clinical practice / Anestesia con flujos bajos en clínica veterinaria.

Prof. Pablo Otero, Facultad de Ciencias Veterinarias, Universidad de Buenos Aires, Argentina)

 

Summary

"Low-flow anaesthesia" is a term applied to techniques in which fresh gas flows are less than the alveolar ventilation used. Depending on the specific technique, fresh gas flows may vary between 30 mL/min and 200 mL/min for an adult. Methods of anaesthesia with  reduced fresh gas flow, including the technique of “quantitative anaesthesia” with a completely closed rebreathing system have gained more and more interest in recent years. The potential for continuous and comprehensive analysis of anaesthetic gas composition, mandatory safety pharmacokinetics and pharmacodynamics of the inhalation anaesthetics justify the revival of these methods in veterinary anaesthesia.

Introduction

In the last years, there have been significant advances in veterinary anesthesiology. However, due to economic reasons the incorporation of new drugs has been deleted. Although this reality affects most countries around the world, in Latin America, the situation becomes more critical with time and thus becoming more necessary to resort to new techniques leading to greater efficiency. In the context of inhaled anaesthesia, the use of circular circuits appears to be a practical and safe alternative. The possibility of making the patient rebreathe part of or, even better, of all the totality of the exhaled gases decreases the anaesthetic vapour intake and therefore reduce the consumption of the selected agent. When working with low flow, even without completely closing the circuit, heat (1) and humidity (2) are conserved within the circuit, improving the clinical condition of the anaesthetized patient. A reduction in the anaesthetic gas flow would also leads to a lower level of contamination within the operating room and onto the environment. Although the amount of anaesthetic gases delivered to the atmosphere as a result of veterinary anaesthesia is insignificant compared to the amount release from human operating rooms, it is crucial to take into consideration the growing pollution problem and thus consider the improvement of environmental conditions. The use of anaesthetic circuits that allow the rebreathing and the reduction of fresh gas flows are as old as inhaled anaesthesia itself (3), but its incorporation to the practice is nowadays a reality in anaesthesiology. It allows the incorporation of new generation agents to routine procedures without significantly modifying the cost of the anaesthesia, it increases the advantages of hemodynamic stability, it demonstrates a marked analgesic effect and reduces the induction and recovery times. Although it is difficult to incorporate low flow techniques into the routine- veterinary practice due to the lack of infrastructure, we will try to define the basis for a safe and predictable procedure.

Oxygen and low flow

The total oxygen intake as well as the  diluted ratio of the gas mixture inhaled by the patient have to be taken into account when designing low flow anaesthesia. The circle rebreathing circuits can operate half-opened, half-closed or completely closed, varying in the level of exhaled gases that are re-inhaled and the amount of fresh gas administered. It is generally accepted that in low flow anaesthesia the level of rebreathed gases are above 50% (4). This varies with the size of the animal and the circuit volume. The primary obstacle in the use of low flow techniques is that the traditional vaporizer is flow dependent. Thus, working with small animals, the flow needed to maintain an appropriate level of re-inhalation is out of the denoted range.  On the other hand while working with large animals, it is not possible to reach an adequate amount of anesthetic vapor when using a fresh gas flow (FGF) similar to the animals metabolic O₂ consumption due to a preset  maximum vaporizing level of 5 %.  This observation occurs during the first 30-40 minutes of anesthesia which is where the greatest uptake of anesthesia occurs.

It is extremely important to ensure an oxygen supply that fulfils metabolic needs in order to reach an adequate inhaled rate (F_iO₂) and thus avoiding hypoxemia. The metabolic oxygen consumption is highly influenced by the patient’s weight, its body size, its body temperature, the degree of the CNS depression and the anesthetic agent. A quick and simple method to calculate the oxygen consumption is the formula suggested by Brody (5) for all homoiotherms:

VO₂ = 10,15 x PC(kg)^0,75 (mL/min)                     (Equation 1)

Where, VO₂ is the oxygen uptake (mL/min) and BW is the body weight (kg).

According to the studies performed by Lowe (6), in patients depress by anaesthetics, the oxygen consumption is about 30 % lower than that calculated by Brody’s formula. Studies performed in anaesthetized animals suggest that an oxygen flow of 3 to 4 mL/kg/min is enough to reach the metabolic oxygen requirements during the anaesthesia in small (7) as well as in large animals (8).

When the reservoir bag is inside the circuit, an easy and simple way of adjusting the FGF is by providing a fresh gas volume to maintain the bag’s volume unchanged. An increase in the volume of the bag would involve a gas supply greater than what the animal intakes, yet a decrease in the volume is associated to an insufficient fresh gas supply. It is assumed that the volume of gases added to the circuit equals the volume of gases incorporated by the patient plus the concomitant leaks.

Depending on the equipment, the evaluation could be complicated by the use of artificial ventilators. When working with a descending (during expiration) bellow system where the accumulation of excess gas can not occur, the airway pressure must be under control at all time. Sub zero pressures at the moment of inhalation is a result of a contraction of the system due to insufficient FGF, while positive pressures denotes an excess in FGF.  The latter, only happening when the ventilator pop off valve is closed. When working with an ascending (during expiration) bellow system, the bellow is maintained between 100 and 200 mL above the base. Therefore, changes in the height of the bellows will show a deficit or an excess of fresh gas, which can be easily detected and corrected by adjusting the FGF. To guarantee enough oxygen supply and a correct oxygenation of arterial blood, a F_iO₂ of at least 30% must be maintained during the totality of the procedure. When oxygen is the carrier gas, it is unlikely that a hypoxic mixture is generated, even if extremely low flows are used and more so if the circuit has a reservoir bag. In large animals, the recumbency is frequently associated to severe ventilation disorders. In these patients, hypoxemia is a frequent complication that demands ventilation control and a F_iO₂ greater than the percentage previously suggested. The use of oxygen as a unique carrier gas is considered more and more as a better option (9). The F_iO₂ in circuits that work with low flows begin with O₂ levels close to 100% but after approximately 5 to 10 minutes this percentage decreases to about 60% due to the elimination of N₂ accumulated in the body.

Numerous studies coincide in concluding that a hyperoxemic mixture yields a high security margin. The incidence of atelectasia observed in patients exposed to high and low oxygen concentrations is not statistically significant (10), while the activity of the immune system is improved (11), as shown by a lower incidence of post-surgery infections in patients who received O₂ as a unique carrier gas. Respiratory complications are also less frequent in patients inhaling high concentrations of O₂ during the anaesthetic procedure (12). In patients with no lung damage, the potential adverse effects that may result from a high exposure of O₂ would only be present after many hours, but this is extremely uncommon in everyday practice (13).

The use of nitrous oxide is not commonly utilized in veterinary medicine. This gas has a low potency in most animal species (MAC above 200%). In addition, the use of low solubility anaesthetics such as isoflurane, sevoflurane or desflurane, avoids the collateral effect of nitrous oxide as a “second gas”. The presence of nitrous oxide in the anaesthetic mixture of a circuit working with low flows demands rigorous monitoring due to variations in its uptake along the anaesthetic procedure.

When planning low flow anaesthesia, it is important to know the pharmacokinetics of anaesthetic gases and the rate between their pressures in different sections of the system. The objective of a general anaesthetic is basically to reach an adequate concentration in the CNS in order to perform different surgical maneuvers without producing pain or movements. The volume of the anaesthetic vapour to achieve this purpose is introduced into the body through the respiratory system. In the lungs, the blood collects the anaesthetic and carries it to different tissues, including the CNS. The magnitude and speed of this process depend on some factors. The most important parameter that conditions the kinetics of anaesthetic gases is the volume of alveolar ventilation and the alveolar concentration of the anaesthetic. The concentration gradient through the alveolar-capillary membrane and the solubility of the agent will determine the partial pressure of the anaesthetic in arterial blood. Therefore, it is necessary to take into account, during the totality of the procedure, the relationship between the variables that determine the concentration of the agent in the body.

Lowe, based on Züntz and Severinghaus studies, developed a mathematics equation to calculate the anaesthetic uptake in each anaesthetic phase (14).

V_AN = Ca x Q x t –½                                 (Equation 2)

Where V_AN is the total anaesthetic uptake, Ca is its arterial concentration, Q is the cardiac output and t is the time.

The arterial concentration can be calculated multiplying the alveolar concentration (C_A) by the blood/gas solubility coefficient of the agent (l_B/G)

Ca = C_A x l_B/G                                          (Equation 3)

Finally, the alveolar concentration can be calculated via the following equation,

C_A = f x MAC                                (Equation 4)

Where MAC is the Minimum Alveolar Concentration of the selected anaesthetic in that species and f is the fraction of the MAC (Table 1).

 

DRUG	Canine	Feline	Equine	Bovine	Ovine	Swine
Halothane	0.87	1.19	0.88	0.76	0.97	0.91
Isoflurane	1.28	1.61	1.31	____	1.58	1.45
Enflurane	2.06-2.2	2.4	2.12	____	2.0	____
Sevoflurane	2.34	2.58	2.34	____	3.3	1.97
Desflurane	7.20	9.80	7.23	____	9.5	10.0
Nitrous oxide	188-200	150	190	223	____	195-277
Metoxiflurane	0.29	0.23	0.22	0.26	0.26	____

Table 1: MAC of volatile anaesthetics (%vol).

* One time the MAC produces a mild anaesthesia, 1.5 times the MAC produces a moderate surgical anaesthesia, two times the MAC, produces a deep anaesthesia.

 

As can be inferred from the equation, when there is a constant anaesthetic supply, the uptake vs. time curve describes a decreasing exponential curve (Figure 1)

 

 

 

 

 

 

 

Figure 1: Figure A; isoflurane uptake (F_A = 1.3 %vol) in a 510 kg B.W. horse, calculated by means of Lowe’s formula. Figure B; cumulative isoflurane doses in the same patient.

 

As can be observed in figure 1, The uptake of anaesthetic during the first minute of anaesthesia is equal to the uptake in different periods (minutes 1 to 4, minutes 4 to 9 and minutes 9 to 16) and it continues indefinitely until it reaches a theoretical balance, which is difficult to reach in a typical anaesthesia.

The dose to be administered after the first minute was named by Lowe as “unit dose” (UD), which is expressed in mL of anaesthetic vapour and can be calculated by means of the following equation,

UD = 2 x Ca x Q                         (Equation 5)

The unit dose represents double the volume of vapour circulating in the system and must be administered at 1, 4, 9, 16, 25, 36 etc. minutes, to maintain an anaesthetic supply that keeps the alveolar concentration at a stable level.

The amount of anaesthetic vapour uptaken by the tissues can be calculated by multiplying the UD by the square root of time. This is known as cumulative dose (CD).

CD = DU x Öt + c                            (Equation 6)

where c represents the arterial concentration after the first dose.

At the beginning of the anaesthesia, the dose should be calculated considering the volume in which the anaesthetic will have to be diluted in order to reach equilibrium quicker. This dose is called “initial” or “priming dose” (PD) and is calculated by the following equation,

PD = C_A x (V_S + V_L) + Ca x Q                   (Equation 7)

Where V_S represents the volume of the anaesthetic circuit and V_L is the volume of lungs and airways. In both cases, these volumes are expressed in deciliters (dl).

It is essential to use these equations along with the volumetric addition of the anaesthetics in closed circuits. It is important to emphasize that there are some objections to Lowe’s theory. Lin suggests to keep closed circuit anaesthesia by means of the volumetric addition of some anaesthetics. Lung uptake of the anaesthetic agent is constant, at least during the first 2 hours of administration. Therefore, in a 70 kilogram individual, after an initial period of high uptake related to the central compartment saturation, the halothane uptake is maintained between 15 and 20 ml/min of vapour for each 1% of anaesthetic in the F_A. For isoflurane and enflurane, this rate is reached with 10-15 ml/min and 30 ml/min, respectively. Eger also suggests that the desflurane uptake is constant during anaesthesia (34). Studies performed in animals by Prof. Moens et al. (35), demonstrated that alveolar sevoflurane concentration shows an evident decreasing tendency when Lowe’s administration diagram is used, suggesting that uptake does not decrease as a function of the square root of time, as occurs with other anaesthetics like halothane e isoflurane (8).

The low flow technique always obliges one to constantly have in mind the uptake diagram. One should try to reach equilibrium between the volume of the anaesthetic vapour supplied and the uptake of this volume, creating a procedure extremely efficient. Therefore, if the O₂ supply is adjusted constantly during the anaesthesia procedure, a quantitative anaesthesia can be obtained.

Fractions

It is extremely important to know the ratio between the selected agent and the anaesthetic mixture in each part of the system. The fractions to take into account during anaesthesia are the vaporized fraction (F_V), the inhaled fraction (F_i) and the alveolar fraction (F_A).

The F_V represents the volume of vapour released by the anaesthetic machine and it is the result of the dilution between the column of gas that goes through the vaporizer without contacting the anaesthetic (diluent flow) and the column that goes into the vaporizing chamber to collect the anaesthetic (vaporizer flow). Most of the vaporizers have a maximum vaporizing limit that, for security reasons, is about 4-5 times the MAC of the anaesthetic agent. Thus when working with an agent specific vaporizer, the volume of vapour added to the system is the result of multiplying the FGF, expressed in deciliters (dl), by the percentage delivered by the vaporizer. Thus, if we work with an FGF of 0.5 liters/min with the dial at 5%, we will be releasing 25 mL of anaesthetic vapour per minute. This is valid for any anaesthetic agent.

When working with VOC, the volume of anaesthetic vapour to be released is a function of FGF and it is limited by the vaporizing maximum of the equipment. It is quite difficult to reduce the FGF during long uptake periods or in large animals.

The F_i represents the anaesthetic concentration inhaled by the patient, expressed in volume percentage (%vol). When working with non-rebreathing circuits, the F_i is equal to the F_V. However, when working with circuits that allow rebreathing, the F_i is the result of the mixture between the rebreathing gas column and the fresh gas column. Therefore, the final composition of the mixture will depend on how much anaesthetic vapour is delivered into the system and how much remains within the system after each uptake period. It is important to keep in mind that, as the FGF is reduced, the number of the vaporizer dial must be increased to maintain a constant anaesthetic supply.

The F_A or anaesthetic alveolar concentration represents the percentage of vapour that returns to the lungs with venous blood and it is closely related to the partial pressure of the anaesthetic in tissues, including the CNS. The F_A is always smaller than the F_i due to the tissue uptake process. The ratio (F_A/F_i) basically depends on the blood/gas solubility coefficient of the agent and it is smaller when the solubility is smaller. The magnitude of alveolar ventilation and the cardiac output also has influence on this gradient (15).

The ratio F_A/F_i is independent on the anaesthetic system because once the drug enters the body, its kinetics is only influenced by factors that does not depend on the anaesthetic technique (14).

Semi closed Circuits

The nomenclature clearly defines the different anaesthetic systems based on total FGF in human medicine (Table 2).

Table 2: suggested nomenclature to classify systems with more than 50% of rebreathing.

On the other hand, to classify the system in veterinary medicine, the FGF must be related to the patient’s weight (table 3).

Table 3: flows suggested in veterinary medicine for different anaesthetic systems.

 

In summary, the percentage of re-inhaled and how much anaesthetic vapour is being added to the circuit every minute must always be kept in mind.

In patients who are well ventilated and hemodynamically compensated, the desired alveolar concentration will depend on the F_i, and the latter will depend on the mixture between the fresh gas column and the rebreathing gas column.

The decrease in the halogenated agent concentration in the circuit has two origins. One being the tissue uptake and the other being the lost anesthetic gases, which are expelled outside the system through the pop off or relief valve and leaks of the circuit. No matter what the cause of the vapor loss, the same amount must be replenished every minute to keep the F_i constant. If we multiply the patient’s respiratory volume by the percentage of anaesthetic in the alveolar and the inspired fractions, we will obtain the volume of vapour to be replenished every minute. Therefore, if the patient’s respiratory volume is 2 liters/min and has a F_A of 1.5% and a F_i of 2%, the inspiration of 40 mL of anaesthetic vapour per minute and an expiration of 30 mL occurs (23). Therefore, 10 mL of vapor should be added to keep the concentration constant. In the same patient, if we were working with a vaporizer outside the circuit, a FGF of 0.5 L/min. with the vaporizer dial at 2% would be enough to guarantee the stability of a system with rebreathing of 100%. The same would happen with a FGF of 0.25 L/min and the dial at 4% or a FGF of 0.1 L/min and the dial of a hypothetical vaporizer at 10%.

Circuits that can work with low flows, although not completely closed, create an easier manipulation, primarily in patients that do not exceed 40-50 kg of body weight. The possibility of using low solubility compounds like sevoflurane or desflurane, whose vaporizers allow releasing a high percentage of anaesthetic vapour, simplifies our job.

The use of an almost closed circuit system is the most appropriate for a typical anaesthetic procedure because it can be used without integrating new equipment. In order to avoid the excess gas to increase the pressure of the circuit, one should leave the expiratory valve open slightly. In these cases, the anaesthesia could be planned as follows:

In order to calculate the volume of vapour necessary to saturate the system at the desired concentration, the first thing to know is the total volume of the system.

V_T = V_S + FRC + V_CC + V_CB                  (Equation 8)

Where:

V_T: is the total volume of the system.

V_S: is the volume of the anaesthetic circuit.

FRC: is the functional residual capacity (see table 4).

V_CC: is the volume of the central compartment (calculated as 10% of the patient’s body weight).

V_CB: is the volume of circulating blood (calculated as 60% of total blood volume of that species). Aldrich and Haskins (24) estimated this volume at 9-10% of body weight in dogs and 5% in cats.

Once the total volume of the system and the desired anaesthetic concentration is known, the volume of vapour to reach the desired concentration can be calculated. For instance, if the patient weighs 40 kg and the circuit volume is 5 liters, the total volume will be 13.4 liters. Therefore, if one desires 1.5% of anaesthetic vapour diluted in 13.4 liters, we must add 201 mL of anaesthetic vapour. If the vaporizer releases a maximum of 25 mL of vapour with a FGF of 5 mL/min, the system will be loaded with the desired concentration in 8 minutes. Once the system is saturated and the desired anaesthetic depth is achieved, the induction or impregnation phase is completed and the maintenance phase begins.

The vaporization level has to be simultaneously adjusted by regulating the FGF and the vaporizer dial, taking into account the tissue uptake and the anesthetic leaks. To estimate this, it is necessary to calculate the vapor deficit every minute. Although these calculations could only be made by means of halogenated gases analyzers, the relationship between the anesthetic level and the parameters used when monitoring the patient allows the utilization of this diagram with confidence.

Regardless of the system used, keeping the patient in a light anesthetic plane by performing a balanced anesthesia, decreases the amount of vapor to be added and thus increases the safety of the procedure.

The minimum flow level of the vaporizer has to be taken into account to avoid mistakes. In large animals, when working with vaporizers outside the circuit with a low vaporization limit (less than 20%), the reduction in FGF to metabolic levels will lead to drug deficit. This will prevent us from keeping the patient in an adequate anesthetic plane. The situation becomes more critical when the anesthetic agent has a high solubility and/or the equipment has a low vaporization limit. In these cases it is advisable to use methods that supplies the anesthetic vapor independently from the FGF (25). The volumetric addition has proved to be an effective and safe method to deal with this difficulty.

 

TABLE 4: Respiratory parameters in different species (approximate values).RF: Breathing frequency TV: Tidal Volume; VMR: Volume Minute Respiratory ; CFR: Functional Residual Capacity; RV: Residual Volume.

Closed Circuits

According to Lowe, there are two principles related to closed circuits:

1. The volume of the circuit must be kept constant

2. The oxygen fraction expired (F_EO₂) must remain constant, at a value previously established.

A circuit is considered “closed” when the volume of oxygen and anaesthetic gas supplied is similar to the volume uptake by the patient. Therefore, the oxygen supply must equal the metabolic consumption of the anaesthetized animal and the anaesthetic gases supply (N₂O and halogenated) must equal the uptake in each period of the procedure.

To corroborate the impermeability of the circuit, after filling the system with oxygen and closing both the relief valve and the gas exit (at the “Y” piece), the pressure in the vacuomanometer must remain at 30 cm of H₂O. The loss must remain under 100 mL/min to be acceptable. However, in small patients or for low volume circuits, this leak may complicate the use of this technique. Again, these circuits need methods to supply the anaesthetic vapour independently from the FGF.

Volumetric addition

The volumetric addition is an option that demands great attention of the anaesthesiologist, especially during the first half-hour of the procedure. The injection of the anaesthetic liquid at previously established times into the inspiratory or expiratory side of the breathing system will allow maintaining the desired anaesthetic level. The expiratory side presents some advantages at higher temperatures and a better dilution of the anaesthetic liquid. Injection times can be easily calculated as the square of the number of administration. Thus, the Unit Dose (UD in equation 5) should be injected at 0, 1, 4, 9, 16, 25 minutes and so on, which yields 0², 1², 2², 3², 4², 5², etc. Previous to the injection of the first UD, an initial dose (PD, equation 7) should be added. Although this method allows keeping an adequate average of the anaesthetic concentration, there are fluctuations between peaks and valleys that can alter the anaesthetic depth.

To avoid fluctuations, it may be more practical to constantly inject the liquid through an infusion pump. In these cases, the total volume to be administered in the procedure must be proportionally divided and the pump must be set according to anaesthetic expired levels (25).

The anaesthetic liquid must be compatible with the plastic materials of the system. Halogenated compounds rapidly disintegrate polycarbonate, which is known to be resistant. Other materials, as polyurethane and nylon are resistant and can be used. With these techniques, the drug is only used to saturate the peripheral compartments and keep the FA in the desired level. A thus we observe a significant reduction in the anaesthetic consumption.

In a horse weighing 550-600 kg, the PD would be about 5 ml and the UD to keep an isoflurane FA similar to the MAC (1.3 %vol) is about 4 mL (17). In a procedure of 100 minutes, 10 UD (40 mL) would be consumed. Therefore, the total consumption would be about 45 mL of isoflurane (PD plus 10 UD) in 100 minutes. This volume is much less than the volume consumed in a system where the vaporizer is outside the circuit that works with a FGF of 8-10 L/min during the induction phase (about 15-20 min) and 4-5 L/min the rest of the procedure.

In-the-circle vaporizer (VIC)

The use of closed circuits with in-the-circle vaporizer is an excellent option that is utilized in veterinary medicine (26, 27). Stephen’s, Komesaroff’s and Ohio #8, are some of the vaporizers used with these circuits. These vaporizers are generally located in the inspiratory side of the circuit. These vaporizers are not precise, have low resistance, are not temperature compensated and have a glass vaporizer chamber which allows the alternative use of diverse volatile agents. Although the anaesthetic concentration in the inspired fraction will depend on many factors, like room temperature, the ventilation pattern (spontaneous vs. controlled) and the FGF, the vaporization rate is constant and predictable (28). The advantages are a decrease in heat and humidity loss, less pollution and a decrease in anaesthetic consumption. With the exception of desflurane, which can reach high concentrations at the beginning of the procedure with a flow of 5-10 mL/kg/min.

Stephen’s vaporizer has been designed for veterinary use and has a dial graduated in eight eighths, which represent the percentage of the gas column that will pass through the glass vaporizer chamber to collect the anaesthetic vapour. Usually, the flow draw over vaporizer chamber is regulated in ¾ ± ¼ during the induction phase and in 3/8 ± 1/8 during the maintenance phase. Fresh gas supply is about 5-10 mL/kg/min during the first half-hour and between 2.5 and 5 mL/kg/min for the rest of the procedure. When an in-the-circle vaporizer is used, the inspired concentration increases with increased temperature and ventilation  (spontaneous or mechanical), while the F_i decreases if the fresh gas flow increases. Mechanical ventilation can be utilized with an in-the-circle vaporizer but it is necessary to strictly control all vital signs.

In small animals (up to 35 kg), the cumulative amount of oxygen and anesthetic used once equilibrium is obtained is sufficient to last about 20–30 minutes without additional supplementation. Thus, with a reservoir bag of 3-4 L, the oxygen and anaesthetic supply can be suspended after 40-60 minutes for an interval of ±20 minutes with only small changes in the anaesthetic depth. The F_iO₂ is maintained over 45%, which guarantees a correct haemoglobin saturation. Once the vaporizer is closed, the anaesthetic F_i decreases very slowly, producing very slow changes in the anaesthetic level. This process becomes more efficient when the anaesthetic agent has higher blood solubility and the circuit has greater volume (26).

Accumulation of gases and degradation compounds in closed circuits

The gases that accumulate in closed circuits can be divided in 4 groups:

1. Compounds formed in the body: hydrogen, acetone, carbon monoxide (CO) and methane.

2. Compounds absorbed by the body: ethanol (uncommon in animals), CO and nitrogen.

3. Compounds produced in the circuit: CO and Component A from halogenated agents.

4. Compounds that enter the system due to permeability: nitrogen.

From the above mentioned compounds, the most important ones are those that can alter the patient’s health. The accumulation is not clinically outstanding in routine practice because the average duration of the procedure is relatively short. Nevertheless, purging the system can reduce the concentration of any of the inert gases.

Sevoflurane may react with CO₂ absorbents forming the Compound A, which is potentially nephrotoxic and neurotoxic (31). This compound can accumulate in closed circuits. Therefore, the use of this agent in procedures involving closed circuits is not recommended. Although concentrations of the Compound A are generally below the suggested limits (32), the accumulation of the Compound A can be avoided by using absorbents like calcium hydroxide, which is free of sodium hydroxide and potassium hydroxide (33).

Conclusion

The reduction in FGF is related to a decrease in halogenated agent consumption. Because of this phenomenon, some compounds which have previously been avoided due to a lack of economic means can now be integrated into the routine practice. Numerous studies demonstrate the feasibility of this technique and the few side effects associated with it.

We consider the low flow technique to be a valuable therapeutic resource, bringing closer the gap between modern standards and the daily veterinary practice.

References

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