Time constant definition

For a volatile anesthetic agent, the time constant (τ) is volume divided by flow. From the standpoint of the anesthesia circuit, the time constant is the volume of the circuit divided by the fresh gas flow rate (units of L divided by L/min results in min). From the standpoint of the lungs, the time constant is the volume of the lungs (FRC) divided by the minute ventilation. It takes three time constants for 95% of a concentration change to be achieved.

Time Constant (τ)

• Definition (circuit): volume of the circuit divided by the fresh gas flow rate
• Definition (lungs): volume of the lungs (FRC) divided by minute ventilation
• Meaning: it takes three time constants for 95% of a concentration change to be achieved