...Each ml of Isoflurane produces approximately 195 ml of Isoflurane vapour, say roughly 200 ml.
A small bottle of Isoflurane contains 30 ml of liquid Isoflurane. Since, 1 ml of liquid Isoflurane produces 200 ml vapour, 30 ml of liquid Isoflurane will produce 6000 ml Isoflurane vapour.
An adult patient on an average consumes 250 ml of Oxygen per minute and produces 200 ml Carbon dioxide.
In order to keep a safe margin, 4 times the requirement of Oxygen = 1000 ml can be delivered. Plus 1000 ml of Nitrous Oxide. That is 50:50 Entonox mixture, for analgesic effect. Therefore, a total of 2000 ml of Oxygen and Nitrous Oxide mixture called Fresh Gas Flow (FGF) can be delivered per minute.
When one sets a vaporiser setting of 1%, it implies that 1% of the FGF of 2000 ml will be Isoflurane vapour = 2000 x 1/100 = 20 ml per minute.
Since, 30 ml liquid Isoflurane produces 6000 ml vapour, at a setting of 1% and FGF of 2000 ml/min (20 ml Isoflurane vapour per minute), it will last for 6000/20 = 300 minutes i.e. 5 hours.
But, with a FGF of 4 L O2 and 4 L N2O = 8 L (1% of 8 L = 80 ml vapour per minute) instead of the reasonably safe FGF of 2 L (if EtCO2 not available) in a closed circuit with functioning Soda Lime, the 30 ml bottle of liquid Isoflurane will last for only 6000/80 = 75 minutes = 1 hour 15 minutes. Maybe, in short cases possible but in long cases like FESS, will have to run for more bottle of liquid Isoflurane in the middle of the operation unnecessarily just because of the wasting by high FGF.
The 4 times safe FGF of Oxygen together with equal flow of Nitrous Oxide is enough to deliver safe amount of Oxygen and also to FLUSH the accumulated Carbon Dioxide, which is also absorbed by Sodalime unless using low flows.
A canister of Sodalime contains about 1000 grams. Each 100 gm absorbs 26 Litres of Carbon Dioxide. An adult patient produces 200 ml Carbon Dioxide per minute after consuming 250 ml Oxygen.
Say for an example, a surgery lasts for 5 hours = 300 minutes. The patient will produce 200 x 300 = 60,000 ml Carbon Dioxide = 60 L, which will be absorbed by 60/26 = 230 gm Sodalime i.e. 1/4th of sodalime canister (if 1 kg capacity). Upon exhaustion, sodalime turns bluish. When fresh it is whitish. It begins to turn bluish from the bottom because the exhaled gas is made to pass through a hollow cylinder in the centre of the canister towards the bottom from where the gas passes upwards through the granules of the sodalime.
In summary, generally 1000 ml Oxygen + 1000 ml Nitrous Oxide mixture is safe enough to deliver Oxygen and to flush out Carbon Dioxide when the remaining sodalime is not fully exhausted = bluish, while monitoring.
Eternal vigilance over the patient and the parameters is essential anyway by a qualified Anaesthesiologist to meet the Standard I criteria of minimum monitoring besides the Standard II (oxygenation, ventilation, circulation and temperature) monitoring.
How sodalime works:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3821269/:
ROUTINE PRACTICE OF LOW-FLOW ISOFLURANE ANESTHESIA:
http://journals.lww.com/anesthesia-analgesia/Fulltext/1998/02001/Routine_Practice_of_Low_Flow_Isoflurane_Anesthesia.36.aspx
Cost comparison of sevoflurane with isoflurane anesthesia in arthroscopic menisectomy surgery:
https://www.ncbi.nlm.nih.gov/m/pubmed/10566918/
Maintenance Agent Cost Calculations:
http://www.vasg.org/spreadsheets/cost_calculators/Maintenance_Agent_Costs_01_22_13.xls
A small bottle of Isoflurane contains 30 ml of liquid Isoflurane. Since, 1 ml of liquid Isoflurane produces 200 ml vapour, 30 ml of liquid Isoflurane will produce 6000 ml Isoflurane vapour.
An adult patient on an average consumes 250 ml of Oxygen per minute and produces 200 ml Carbon dioxide.
In order to keep a safe margin, 4 times the requirement of Oxygen = 1000 ml can be delivered. Plus 1000 ml of Nitrous Oxide. That is 50:50 Entonox mixture, for analgesic effect. Therefore, a total of 2000 ml of Oxygen and Nitrous Oxide mixture called Fresh Gas Flow (FGF) can be delivered per minute.
When one sets a vaporiser setting of 1%, it implies that 1% of the FGF of 2000 ml will be Isoflurane vapour = 2000 x 1/100 = 20 ml per minute.
Since, 30 ml liquid Isoflurane produces 6000 ml vapour, at a setting of 1% and FGF of 2000 ml/min (20 ml Isoflurane vapour per minute), it will last for 6000/20 = 300 minutes i.e. 5 hours.
But, with a FGF of 4 L O2 and 4 L N2O = 8 L (1% of 8 L = 80 ml vapour per minute) instead of the reasonably safe FGF of 2 L (if EtCO2 not available) in a closed circuit with functioning Soda Lime, the 30 ml bottle of liquid Isoflurane will last for only 6000/80 = 75 minutes = 1 hour 15 minutes. Maybe, in short cases possible but in long cases like FESS, will have to run for more bottle of liquid Isoflurane in the middle of the operation unnecessarily just because of the wasting by high FGF.
The 4 times safe FGF of Oxygen together with equal flow of Nitrous Oxide is enough to deliver safe amount of Oxygen and also to FLUSH the accumulated Carbon Dioxide, which is also absorbed by Sodalime unless using low flows.
A canister of Sodalime contains about 1000 grams. Each 100 gm absorbs 26 Litres of Carbon Dioxide. An adult patient produces 200 ml Carbon Dioxide per minute after consuming 250 ml Oxygen.
Say for an example, a surgery lasts for 5 hours = 300 minutes. The patient will produce 200 x 300 = 60,000 ml Carbon Dioxide = 60 L, which will be absorbed by 60/26 = 230 gm Sodalime i.e. 1/4th of sodalime canister (if 1 kg capacity). Upon exhaustion, sodalime turns bluish. When fresh it is whitish. It begins to turn bluish from the bottom because the exhaled gas is made to pass through a hollow cylinder in the centre of the canister towards the bottom from where the gas passes upwards through the granules of the sodalime.
In summary, generally 1000 ml Oxygen + 1000 ml Nitrous Oxide mixture is safe enough to deliver Oxygen and to flush out Carbon Dioxide when the remaining sodalime is not fully exhausted = bluish, while monitoring.
Eternal vigilance over the patient and the parameters is essential anyway by a qualified Anaesthesiologist to meet the Standard I criteria of minimum monitoring besides the Standard II (oxygenation, ventilation, circulation and temperature) monitoring.
How sodalime works:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3821269/:
ROUTINE PRACTICE OF LOW-FLOW ISOFLURANE ANESTHESIA:
http://journals.lww.com/anesthesia-analgesia/Fulltext/1998/02001/Routine_Practice_of_Low_Flow_Isoflurane_Anesthesia.36.aspx
Cost comparison of sevoflurane with isoflurane anesthesia in arthroscopic menisectomy surgery:
https://www.ncbi.nlm.nih.gov/m/pubmed/10566918/
Maintenance Agent Cost Calculations:
http://www.vasg.org/spreadsheets/cost_calculators/Maintenance_Agent_Costs_01_22_13.xls
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