Calculating Volatile Anaesthetic Agent Consumption During General Anaesthesia
The standard equation for calculating volatile anaesthetic consumption is: Agent Consumption (ml) = Fresh Gas Flow (L/min) × Agent Concentration (vol%) × Duration (min) × Agent-Specific Constant, where the constant represents the volume of vapour produced per ml of liquid agent. 1
Agent-Specific Constants
The critical component of this calculation is the agent-specific constant, which represents the volume of vapour (in ml) produced from 1 ml of liquid anaesthetic agent 1:
- Sevoflurane: 184 ml vapour per 1 ml liquid 1
- Desflurane: 210 ml vapour per 1 ml liquid 1
- Isoflurane: 195 ml vapour per 1 ml liquid 1
- Halothane: 229 ml vapour per 1 ml liquid 1
Practical Calculation Method
The formula can be expressed as: Liquid Agent Consumed (ml) = [Fresh Gas Flow (L/min) × Vaporizer Setting (vol%) × Duration (min) × 10] / Agent-Specific Constant 1
The multiplication by 10 converts the percentage concentration and litres to millilitres for dimensional consistency 1.
Alternative Calculation from Inspired and Expired Concentrations
For retrospective analysis when vaporizer settings are unavailable, a more complex model can calculate consumption from monitored parameters 2:
FVAP = [FIN - (dead space fraction × FIN + (1 - dead space fraction) × FET) × (1 - FGF/MV)] / [1-(1 - FGF/MV)]
Where 2:
- FVAP = vaporizer fresh gas flow with agent
- FIN = inspired agent concentration
- FET = end-expired agent concentration
- FGF = fresh gas flow
- MV = minute ventilation
The empirically determined dead space fractions are 2:
- Sevoflurane: 0.49
- Desflurane: 0.66
- Isoflurane: 0.59
Accuracy and Validation
The calculation method overestimates actual consumption by approximately 5-7% when compared to the gold standard of weighing vaporizers 3. Specifically:
- Sevoflurane calculations overestimate by 6.2% (3.0 ± 2.9 ml) 3
- Desflurane calculations overestimate by 5.0% (3.5 ± 6.7 ml) 3
The retrospective model using inspired/expired concentrations demonstrates a median performance error of -1% and median absolute performance error of 8%, which is acceptable for pharmacoeconomic analyses 2.
Clinical Application Considerations
Fresh gas flow significantly impacts both wash-in time and total agent consumption, with an optimal flow rate of approximately 4 L/min balancing speed and economy 4. Higher flows beyond 4-4.8 L/min provide minimal additional benefit for wash-in but substantially increase consumption 4.
Important Caveats
- The calculation assumes steady-state conditions and may be less accurate during rapid changes in fresh gas flow or vaporizer settings 1
- Modern anaesthesia machines with electronic vaporizers (like Dräger Primus™) display consumption directly, with reliability within 6-8% of measured values 5
- The method requires accurate recording of fresh gas flows and agent concentrations throughout the anaesthetic duration 3
- During cardiopulmonary bypass, volatile agents cross the oxygenator membrane, and concentration should be monitored in the oxygenator exhaust line 6
This calculation method enables both prospective monitoring and retrospective analysis of volatile agent consumption from anaesthesia records, making it valuable for pharmacoeconomic assessments and quality improvement initiatives 1, 3.