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.

Anaesthetic Gas Flow Rate Calculator

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 x (dial% / 100)
liquid mL/h   = (vapour mL/min x 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.

Example and notes

Sevoflurane at 2 percent on 2 L/min produces 40 mL/min of vapour, which is about 13 mL of liquid per hour and roughly 1 MAC. Drop the flow to 1 L/min and the liquid consumption halves while the delivered concentration is unchanged, which is the core economy of low-flow technique. The MAC value here assumes a 40-year-old on oxygen only and ignores nitrous oxide and age, so pair it with a dedicated MAC tool, and always titrate to the measured end-tidal concentration on your monitor.