LOW FLOW ANAESTHESIA
Dr. Chandrashekharan Cham
Chief Consultant Anaesthesiologist & Interventional Pain Physician, Kingsway Hospital, Nagpur
Any technique that uses a Fresh gas flow that is less than the alveolar ventilation can be stated to as low-flow anaesthesia. (1) It is an inhalation anaesthetic technique in which the rebreathing fraction at least amounts to 50%, where at least 50% of the exhaled gas mixture is returned to the patient after CO2 removal in the next inspiration. The development of modern sophisticated anaesthesia machines, gas analyser monitors, precision vapourisers and introduction of more potent volatile agents with minimal uptake have encouraged low-flow anaesthesia enthusiasts. The staggering amount of environmental pollution due to anaesthetic gases should be impetus for every anaesthesiologist to take that extra bit of effort to implement low-flow anaesthesia. (2)
As early as 1850, John Snow had concluded that if the exhaled anaesthetic gases such as chloroform and ether can be re-inspired, their effect will be markedly prolonged. (3) Brian Sword, in 1930 first described the circle breathing system with soda lime absorber for closed circuit anaesthesia. (4) Highly explosive inhalational agents like cyclopropane compelled the users to minimise excessive flow of the agents. However, with the introduction of potent agents like halothane, having small therapeutic indices, use of high fresh gas flow with negligible rebreathing became the practice although circle systems were available. (1) In 1979, Aldrete et al took efforts to rekindle the use of low flow and closed system went futile and unacceptable to the anesthesia fraternity. (5) A decade later attention was drawn towards the environmental hazards of the anaesthetic agents by Logan M & Farmer JG.
In the beginning of inhalational anaesthetic administration, a Fresh gas mixture with a given composition of the anaesthetic gases is delivered. The uptake of agents into the body occurs as per the physical characteristics of each agent. In a closed circuit, the exhaled gas mixture, which eventually mixes with the fresh gas, will have a different composition due to the uptake of agents and addition of CO2. The rationale of LOW FLOW ANAESTHESIA is to replenish the consumed gases with as minimum fresh gas as possible; while making sure to remove CO2 before recirculating. This minimises the loss of anaesthetic agents into the environment. (2)
A gas flow in excess of the minute volume will provide a predictable inspired gas concentrations, which will be more or less the same for any patient, using any breathing system, at any stage of the anaesthetic and will be unaffected by agent uptake by the patient. However, as the fresh gas flow is reduced and more exhaled gas is retained within the breathing system (increased rebreathing fraction), gas uptake by the patient will increasingly affect the exhaled and hence the inspired gas mixture. Once the flow rate is reduced to near the patient’s requirements, the fresh gas mixture will closely reflect the uptake of each of its components by the patient. Hence, there is a difference between the inspired gas mixtures from that set at the rotameters. This implies that these techniques are critically dependent on gas monitoring and through frequent adjustments in the gas flow controls. This quantitative concept of gas delivery, by controlling the dynamic equilibrium of fresh gas composition and the consumption and production of gaseous components is the fundamental defining feature of low-flow techniques. (7)
- Flowmeters that are calibrated to as low as 50 ml/min.
- A leak proof breathing system and airway devices like cuffed endotracheal tube or well fitted supraglottic airway devices
- Gas monitoring system, especially inspired and end-tidal concentration of agents. The measurement of end-tidal concentration system should be as close to Y-piece as possible.
- Vapourisers which can deliver very high concentrations and calibrated accurately to deliver at low flows
- The circuit should have minimal internal volume to reduce to minimise reserve volume.
Who should avoid low flow anaesthesia?
- Anaesthesiologist not familiar with low flow anaesthesia
- Procedures with imperfectly gas tight airways e.g. during rigid bronchoscopy
- Unsatisfactory equipment with a high gas leakage
- Inadequate monitoring (i.e., malfunction of the gas analyser) or lack of machine/equipment suitable for leak-free closed breathing systems
- Low flow anaesthesia techniques should not be applied in situations when other clinical issues like haemodynamic instability require scrupulous attention of the anaesthesia provider
Advantages of low flow techniques: (2,8)
- Preserves heat and humidity of inspired gas, hence reduces water loss and maintains temperature.
- Improves mucocilliary clearance, reduces accumulation of dried airway secretions.
- Reduced anaesthetic agent consumption
- Significant savings in the order of 60-75% with regard to volatile anaesthetic agents
- Reduces operating room pollution
- Reduced emission of fluorocarbons and N2O into the atmosphere which damage the earth’s ozone layer
- Reduced green house effect due to fluorocarbons and N2O
Red flags of Low Flow Anaesthesia: (8)
- Dilution of anaesthetic agents: Low fresh gas flows are added to significantly large reserve volume (approximately 9–10 litres), consisting of breathing tubing, reservoir bag, anaesthetic ventilator, intergranular space, etc., in addition to functional residual capacity (FRC) of the patient.
- The rate of change of composition of gas in the reserve volume is exponential, which is related to the time constant.
O2 uptake is constant and given by Brody formula
VO2 = 10 x body weight (kg)3/4 or 3ml/kg/min and about 250 ml/min
Uptake of N2O is given by Severinghaus formula
VN2O = 1000 x t-1/2
Uptake of Inhalational anaesthetics may be calculated with Lowe’s formula
VAN = f x MAC x C X Q x t-1/2 (f x MAC is fraction of MAC, Q is cardiac output and t is time)
Thus, consumption of O2 is constant but the uptake of N2O and volatile anaesthetics follow a power function (exponential). Initially, it is high and declines sharply during the first 30 min, but it is comparatively low and decreases only slowly during the course of anaesthesia.
- It requires 3 time constants (Measure for the time it takes the alterations of the fresh gas composition to lead to corresponding change of the gas in the breathing circuit & is calculated by reserve volume divided by fresh gas flow) to effect 95% change in gas composition to occur. However, once steady state is achieved, low flow anaesthesia provides the most economic use of anaesthetic agents.
- Differential uptake of agents modifying the composition of gas mixture: This effect is particularly important while combining N2O as carrier gas along with oxygen. Uptake of N2O is high initially, followed by gross reduction in its uptake. This change in the trend in differential uptake may lead to hypoxic mixtures being delivered.
- Ensuring enough oxygen for metabolism: When wide range of variations in the gas composition is possible while reducing the fresh gas flow, scrupulous attention should be paid to provide enough oxygen to meet the metabolic demands. Pulse oximeter is a less sensitive surrogate monitor of tissue oxygenation, and an oxygen analyser is essential. Lower limit of FiO2 should be set as 0.30.
- There may be a danger of hypoxaemia if FiO2 is less than 33% when using N2 This is because with time N2O uptake decreases, while O2 continues resulting in relatively more N2O. Increasing O2 to 50% prevents this and hence O2 analysers monitor and display of FiO2 is must.
- Delay in recovery from anaesthesia: Long-time constant leads to slow reduction in concentration of volatile anaesthetic agents during the recovery phase. Change over to high fresh gas flows (to reduce time constant) and switching off vapourisers early can accelerate washout of anaesthetic agents.
How to conduct low flow anaesthesia? (2)
Premedication, preoxygenation and induction of anesthesia are performed according to the usual practice.
Initiation of low-flow anaesthesia
The objective is to achieve an alveolar concentration of the anaesthetic agent that is adequate for producing surgical anaesthesia. There are different methods of achieving this objective.
- Use of high flows during initial phase
This is done to bring the circuit concentration to the desired concentration rapidly. Often, a fresh gas flow of 10 L of the desired gas concentration and 2 MAC agent concentration is used. By the end of 3 min (i.e., 3 time constants), the circuit would be brought to the desired concentration. Some advocate high fresh gas flows of 4.5 – 5 L/min for 10 min. This ensures better denitrogenation and rapid achievement of desired concentration by counterbalancing the large uptake encountered at the start of the anaesthesia.
- Use of prefilled circuits
Here, we use a different circuit like Magill’s for preoxygenation. Simultaneously, the circle system is fitted with a test lung and the entire circuit is filled with the gas mixture of the desired concentration. After tracheal intubation, the patient is connected to the circle system and rapid achievement of the desired concentration in the circuit occurs.
- Injection of volatile agent into the breathing circuit
The usual requirement of anaesthetic agent is approximately 400–500 ml of vapour in the first 10 min (i.e., 40–50 ml/min). At 20°C, 1 ml liquid halothane yields 226 ml of vapour, 1 ml isoflurane yields 196 ml and 1 ml of sevoflurane yields 180 ml of vapour. About 2 ml of the liquid agent is injected in small increments into the expiratory limb of the circuit. The intermittent injections are often made in 0.2–0.5 ml aliquots manually. Alternatively, continuous infusion may be used with the added advantage of doing away with the peaks and troughs associated with intermittent injections. The accurate dose requirement is given by the formula:
Priming dose (ml vapour) = Desired concentration × ([FRC + circuit volume] + [cardiac output × blood gas coefficient])
Maintenance of low flow anesthesia:
A steady-state concentration of the anaesthetic agents needs to be maintained with a flow rate of 1 L/min. Although the oxygen uptake remains constant at 200–250 ml/min but to maintain a safe inspired concentration of about 30% O2 in Low flows, the fresh gas O2 concentration has to be increased to 50%. The amount of anaesthetic vapours delivered to the system is markedly reduced with low flows. So, this has to be compensated by a corresponding increase of the agent’s concentration in the fresh gas, e.g. 2 vol% of Isoflurane and 3 vol% for sevoflurane. The expired concentrations will thus be maintained in the desired range of 0.7 – 0.8 MAC.
Termination of low flow:
Because of long-time constants, recovery is delayed in low flow anaesthesia. However, switching over to high flows to accelerate the wash-out of anaesthetic agents or use of activated charcoal to remove the potent vapours by absorption can result in rapid recovery. N2O gets washed off while changing over to 100% O2.
Disadvantages of low flow techniques: (2,7)
- The need for capital investment for absorber breathing systems and dependence on gas monitoring may limit the use of low-flow methods in poorer countries. Similarly, the need for and increased consumption of absorbent at low flows may also be an issue.
- Higher consumption of CO2absorbents and risk of hypercarbia and CO2 rebreathing with frequent exhaustion of absorbers
- Limitations of currently available vapourisers
The need to increase the end-tidal volatile agent, in response, for example, to increased surgical stimulus, poses a problem because of lower flow rates and long ‘time constant’. This also leads to slower induction and emergence, too. Quick alteration of inspired concentrations, thus is not possible while on low flows.
– Accumulation of unwanted gases in the breathing system
Due to a complete leak proof system, there may be failure to flush gases out of the system and any gases introduced which are not taken up by the patient or absorbed chemically will tend to accumulate. Such gases may be exhaled by the patient, be a contaminant of the medical gases, or result from a reaction with the chemical agents used for carbon dioxide absorption.
– Substances exhaled by the patient
Substances exhaled by the patient include alcohol, acetone, carbon monoxide, and methane. Therefore, the use of low fresh gas flows is contraindicated in patients who are intoxicated, in uncompensated diabetic states, or who are suffering from carbon monoxide poisoning.
– Contaminants of medical gases
There is a sparing possibility of accumulation of lethal gases like Carbon monoxide and Nitric oxide. More benignly, nitrogen and argon may accumulate and cannot be detected by infra-red analysers.
– Products of reactions with absorbents
Sevoflurane reacts with sodalime to produce an olefin ‘Compound A’, which can cause renal toxicity. The issues of carbon monoxide and compound A production would have been a major concern, but with the use of a novel absorbent Amsorb, (Armstrong Medical) which contains no strong alkali and thus produces no carbon monoxide or compound A.
Low flow anaesthesia is the need of the day. Anesthesiologists should learn and preach low flow anaesthesia as a humane and professional obligation as an ecological contribution. Understanding the concept behind it would help safe and economical practise with this technique.
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