How is liquid agent consumption calculated?

The vaporiser adds vapour equal to the fresh gas flow times the dial percentage. Each millilitre of liquid agent expands into a fixed volume of vapour at room temperature, about 183 mL for sevoflurane and 195 for isoflurane, so dividing vapour output by that expansion gives the liquid used per hour.

Why does fresh gas flow matter so much for cost?

For a calibrated vaporiser the delivered concentration equals the dial setting regardless of flow, but the amount of agent carried away in the fresh gas scales directly with flow. Running 4 L/min uses twice the agent of 2 L/min at the same dial, which is the whole economic case for low-flow anaesthesia.

What is MAC and how is it shown here?

MAC, the minimum alveolar concentration, is the agent concentration at which half of patients do not move to a surgical stimulus. The tool divides your delivered vol% by the standard 1-MAC value for the agent to express depth as a MAC multiple.

Does this account for nitrous oxide or patient age?

No. The MAC figure here uses a standard 40-year-old breathing oxygen only. Nitrous oxide is additive and MAC falls with age, so use a dedicated MAC adjustment tool when those factors apply.

Is delivered concentration the same as end-tidal?

Not exactly. The dial sets the inspired concentration leaving the vaporiser, but uptake by the patient and the breathing circuit means end-tidal concentration lags behind, especially early in a case and at low flows. Always titrate to the measured end-tidal value on your monitor.

What is the Anaesthetic Gas Flow Rate Calculator?

Converts vaporiser dial setting, fresh gas flow, and agent identity into delivered concentration in vol%, MAC multiples, vapour output, and liquid agent consumed per hour for sevoflurane, isoflurane, desflurane, and halothane. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Anaesthetic Gas Flow Rate Calculator

Name: Anaesthetic Gas Flow Rate Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

A variable-bypass vaporiser splits fresh gas so that a precise fraction picks up anaesthetic vapour, delivering the dialled volume-percent of agent to the patient. This calculator turns the dial setting and flow into the practical numbers an anaesthetist cares about: how deep the anaesthesia is in MAC terms and how fast the liquid agent is being used.

How it works

For a calibrated vaporiser the delivered concentration simply equals the dial setting. The volume of agent vapour added each minute is the fresh gas flow scaled by that fraction, and the liquid used follows from the agent’s expansion ratio:

vapour mL/min = FGF_mL/min * (dial% / 100)
liquid mL/h   = (vapour mL/min * 60) / expansion

Each agent has a characteristic liquid-to-vapour expansion at 20 degrees Celsius: roughly 183 mL of vapour per mL of liquid for sevoflurane, 195 for isoflurane, 210 for desflurane, and 227 for halothane. Depth is reported as a MAC multiple by dividing the delivered vol% by the agent’s standard 1-MAC value.

Worked example: sevoflurane at low flow

Sevoflurane at 2% on 2 L/min:

Vapour added: 2,000 mL/min x (2/100) = 40 mL/min
Liquid used per hour: (40 x 60) / 183 = 13.1 mL/hr
Depth: 2% / 2.0% (1-MAC sevo) = 1.0 MAC

Drop the flow to 1 L/min while keeping the dial at 2%:

Vapour added: 1,000 x 0.02 = 20 mL/min
Liquid used per hour: (20 x 60) / 183 = 6.6 mL/hr
Depth: still 1.0 MAC (delivered concentration unchanged)

The flow halved the agent consumption while maintaining the same anaesthetic depth. This is the core economic and environmental argument for low-flow technique.

Agent comparison: what changes between agents

The choice of volatile agent affects cost, environmental footprint, and clinical profile. A few points of comparison relevant to this calculator:

Desflurane has the lowest blood-gas partition coefficient, meaning the end-tidal concentration equilibrates with the inspired concentration fastest. But it has a high vapour pressure and requires a heated, pressurised Tec 6 vaporiser. Its global warming potential is also significantly higher than the other agents — a growing factor in clinical decisions.

Sevoflurane is the most widely used agent globally. Its pleasant induction odour makes it the agent of choice for gaseous induction in children and anxious adults. The expansion ratio (183 mL vapour/mL liquid) and moderate cost make it efficient at low flows.

Isoflurane is cheaper than sevoflurane in most markets and has a slightly higher expansion ratio, but its pungent odour makes gaseous induction less well tolerated and it is now less commonly used for maintenance in many centres.

Halothane is rarely used in high-income countries (withdrawn in most) due to halothane hepatitis risk, but it remains the agent in some lower-resource settings. The calculator includes it for completeness.

Low-flow anaesthesia: why flow matters for cost and the environment

At flows below 1 L/min, most of the gas is recirculated through the breathing circuit after carbon dioxide is absorbed. The fraction of fresh gas (and therefore agent) actually used by the patient increases substantially. Low-flow technique can reduce agent consumption by 50–70% compared to high fresh-gas-flow techniques. Over a 4-hour case, the difference in liquid agent used is clinically significant in both cost and in total halogenated compound released to the atmosphere.

The limiting factor at very low flows is that uptake-driven changes in end-tidal concentration lag behind dial changes — the circuit takes longer to equilibrate. This is why low-flow anaesthesia requires careful end-tidal monitoring and slower adjustments to depth. Always titrate to the measured end-tidal value, not just the dial setting.