Low flow anesthesia

1. Safe and cost effective anesthetic gas
management with
low flow anesthesia
Helga Komen, MD
Ryan Guffey, MD
Ivan Kangrga
Department of Anesthesiology
Washington University in St Louis

2. BJH Anesthetic Gas Costs
In 2016 Barnes-Jewish Hospital spent nearly
1 million dollars on anesthetic gases
2/3 of total cost is related to desflurane
Wasted gas is literally vented out of the hospital chimney
Medication 11/2015 – 10/2016
Desflurane $686,621
Sevoflurane $307,036
Isoflurane $4,613
Total $998,271

3. Relative Cost
• Cost is directly proportional to fresh gas flow (FGF) rate
• Recommend using lowest safe flow to maximize efficiency
• Recommended maintenance flow rates are bolded.
Agent Fresh Gas Flow rate (L/min) Cost per MAC hour (340B - GPO)
Desflurane 2 $8 – 20.06
Desflurane 1 $4 – 10.03
Desflurane 0.7 $2.8 – 7.02
Desflurane 0.5 $2.0 – 5.02
Sevoflurane 2 $3 – 4.07
Sevoflurane 1 $1.5 – 2.03
Isoflurane 1 $0.21 – 0.39

4. Sevoflurane
• Minimal airway irritation
• Lower cost than Desflurane
• Fast wake up time
• Neither Compound A or carbon monoxide is
produced when absorbent Amsorb® is used (all
BJH machines)
• Recommended maintenance flow (FDA)
– 2 L/min for cases scheduled > 2 hours
– 1 L/min for cases scheduled < 2 hours
See notes for references, details

5. Indications for Desflurane
• Rapid return to consciousness required (i.e. post
operative neuro exam)
• Morbidly obese and expected case duration greater
than 2 hours
– Desflurane typically saves 2-4 minutes in long
cases (>4 hours) with normal BMI, vs 10 minutes
in morbidly obese
• Recommended maintenance flow: 0.5 – 1 L/min
– Low flow rate of < 1L/min can decrease cost
advantage of Sevoflurane
• There is no indication for desflurane in an
anesthetic expected to last less than 2 hours

6. Isoflurane
• Patients that will not be extubated
immediately after the surgical procedure
– Ideal for intubated ICU cases
• Recommended maintenance flow:
0.5 – 1 L/min
• Isoflurane offers a dramatic cost savings
– However, may be counterbalanced with
longer early recovery (eye opening,
hand squeezing) and increased PACU
time
90% Recovery from anesthesia relative to
anesthetic duration

7. Low flow anesthesia
• Low-flow anesthesia (LFA): a rebreathing system that results in at least 50%
of the exhaled air being returned to the lungs after CO2 absorption.
• Over 80% of anesthetic gases are wasted when flows of 5 L/min are
used
Metabolic flow = 250ml/min
Minimal flow = 250-500 ml/min
Low flow = 500-1000 ml/min
Medium flow = 1-2 L/min
High flow = 2-4 L/min
Super-high flow = >4 L/min

8. Advantages of Low Flow Anesthesia
• Cost savings
• Preservation of humidity and temperature
of gas mixture
– Reductions in heat loss
– Maintenance of body heat
– Prevention of bronchial drying
– Prevention of postoperative shivering
• Reduction of pollution
– Nitrous oxide has 300x the greenhouse gas
potency as carbon dioxide
– All anesthetic agents are harmful to humans
with prolonged exposure or pregnancy
Esophageal temperature relative to
anesthetic duration and fresh gas
flow rate

9. Low Flow Anesthesia Technique
• After securement of the airway, initial high FGF of 3-4 LPM is
necessary to fill the breathing system with the desired gas
composition and achieve steady state
− This generally requires three time constants
− Time constants vary with flow rate and volume of the circle system
1. Initial HIGH flow
Time constant (min) Brain equilibrium (min)
Sevoflurane 2 6
Desflurane 2 6
Isoflurane 3-4 10-15
Nitrous Oxide 2 6

10. Low Flow Anesthesia Technique
2. Low flow
• After the circle system and patient are saturated with anesthetic agent, the
fresh gas flow can be minimized
• The difference between inspired anesthetic gas concentration (FiAgent) and
fresh gas (dialed) anesthetic gas concentration will be larger with low flow
anesthesia
– Patient uptake requirement of maintenance anesthetic gas is not related to
fresh gas flow
– Delivery of anesthetic gas to the circle system is directly related to fresh gas
flow
– Consequently, lower fresh gas flow (LFA) requires higher anesthetic gas
concentration in the fresh gas (dialed agent concentration) to maintain the
same anesthetic depth and account for maintenance gas uptake

11. Low Flow Anesthesia Technique
• FiO2 will be much lower than fresh gas ratios suggest after prolonged
periods of low flow
– For example: fresh gas flow of 0.5 LPM O2 and 0.2 LPM air, or 0.4 LPM O2 and 0.1
LPM air will equilibrate to approximately 50% FiO2 in an average adult
– Set fresh gas oxygen concentration at least 10% higher than desired FiO2 in order
to obtain target FiO2
– Larger patients will require more O2, while smaller patients will require less due
to metabolism.
• Approximate metabolic Oxygen requirement for an average adult is 0.25 LPM
– BJH does not recommend less than 0.4 LPM of fresh oxygen in order to prevent
hypoxic FiO2 in adults
• Set FiO2 alarm threshold of at least 30% to prevent hypoxic flows
– Ecoflow will assist with prevention of hypoxic flow

12. Disadvantages of Low Flow Anesthesia
• Slower response to changes in fresh gas anesthetic concentration
– FiAgent will take longer to increase if fresh gas agent is increased
– This will lead to a slower increase in EtAgent
– To overcome this, “overcharge” your circle system with a higher than desired
fresh gas agent concentration for a short period of time
• Slower response to changes in fresh gas oxygen percentage
– Increases in FiO2 will take longer at lower fresh gas flowrates
– If higher FiO2 is urgently required, increase the fresh gas flow rate

13. Disadvantages of Low Flow Anesthesia
• Carbon monoxide accumulation?
– CO is produced when some anesthetic agents interact with dry soda lime.
– CO can be difficult to eliminate under low-flow.
– However, even in long-term closed system anesthesia, the increase in CO concentration remains
negligibly low and is of no risk to the patient. Amsorb does not produce carbon monoxide.
• Faster CO2 accumulation in absorbent
– Less CO2 is sent to scavenging resulting in a higher percentage of expired CO2 being stored in
absorbent (Amsorb). This necessitates more frequent absorbent replacement.
• Accumulation of compound A?
– Compound A is derived from the contact of Sevoflurane with some absorbents
• This compound is nephrotoxic in rats but has not been demonstrated to be nephrotoxic in humans
– Our absorbent at BJH Amsorb®, does not contain monovalent bases that promote
formation of Compound A

14. Contraindications to Low Flow Anesthesia
• Toxic substances exhaled by the patient:
– Alcohol
– Acetone
– Carbon monoxide (CO)
– Methane (CH4)
Therefore the use of low flow anesthesia is contraindicated in:
– Patients who are intoxicated (alcohol, cyanide)
– Uncompensated diabetic states (DKA)
– Carbon monoxide poisoning
– Malignant hyperthermia

15. • Ecoflow is coming on new anesthesia machines
• Helps to guide fresh gas oxygen vs air flow rate in low
flow anesthesia to prevent hypoxic FiO2
• https://www.youtube.com/watch?v=Od6gs85wb4c
• More details to follow

16. Conclusions
• BJH wastes hundreds of thousands of dollars on excess scavenged anesthetic gas
• Use recommended lowest flow for maintenance fresh gas flowrate
– Desflurane: 0.5 – 1 L/min
– Sevoflurane: 1 L/min (scheduled < 2hr), 2 L/min (scheduled > 2hr),
• Sevoflurane does not result in production of Compound A or carbon monoxide
when used with Amsorb® (our anesthesia machines)
• There is no indication for Desflurane in cases realistically scheduled for <2 hours
• Set fresh gas oxygen at 0.4 LPM or higher to avoid hypoxia
• There are contraindications for low flow anesthesia
• We anticipate a 20% decrease in anesthetic gas costs is possible with more
judicious anesthetic gas usage and choice ($200,000/year)

17. Appendix

18. Low flow anesthesia - origins
• Inhalational anesthesia and closed system anesthesia are
almost of the same age.
– Almost/closed anesthesia systems have been in use since 1850.
• In the mid 1950’s, when halothane was brought forth, the use
of LFA and closed system anesthesia diminished significantly.
This was largely due to the inherent problem of the first
generation halothane vaporizers, which was the unreliable
delivery of vapor at low fresh gas flow.

19. Low flow anesthesia - origins
• Introduction of isoflurane in the early 1980’s gave way to a renewed interest in LFA
and closed circuit anesthesia.
– It was further enhanced by the fact that anesthetic agents are atmospheric pollutants, especially
N2O, halothane, enflurane, and to some extent isoflurane.
• The introduction of new low solubility agents, like desflurane and sevoflurane
(1990`s), have initiated a renaissance in the use of LFA, in order to contain costs
associated with adapting FGF to patient demand

20. Low Flow Anesthesia Technique
3. Recovery
– The length of anesthetic administration influences the rate at which
concentrations of anesthetics decrease after their discontinuation
Decrement time (min)
≤60mins anesthesia
duration
6 hrs anesthesia
duration
Sevoflurane 7 65
Desflurane 5 14
Isoflurane 47 86
• decrement times - times needed for 80% or 90% decreases in anesthetic concentration