CONDUCTING LOW‑FLOW ANAESTHESIA Premedication, preoxygenation and induction of sleep are performed according to the usual practice. Initiation of low‑flow anaesthesia The objective is to achieve an alveolar concentration of the anaesthetic agent that is adequate for producing surgical anaesthesia. There are different methods of achieving this objective. Use of high flows during initial phase The time constant is reduced, bringing the circuit concentration to the desired concentration rapidly. Often, an FG flow of 10 L of the desired gas concentration and 2 MAC agent concentration is used. By the end of 3 min (i.e., 3 time constants), the circuit would be brought to the desired concentration. This facilitates better denitrogenation and rapid achievement of desired concentration by counterbalancing the large uptake encountered at the start of the anaesthesia. Use of prefilled circuits Here, we use a different circuit like Magill’s for preoxygenation. Simultaneously, the circle system is fitted with a test lung and the entire circuit is filled with the gas mixture of the desired concentration. After tracheal intubation, the patient is connected to the circle system and rapid achievement of the desired concentration in the circuit occurs. Injection of volatile agent into the breathing circuit The usual requirement of anaesthetic agent is approximately 400–500 ml of vapour in the first 10 min (i.e., 40–50 ml/min). At 20°C, 1 ml liquid halothane yields 226 ml of vapour and 1 ml isoflurane yields 196 ml. About 2 ml of the liquid agent is injected in small increments into the expiratory limb of the circuit. The intermittent injections are often made in 0.2–0.5 ml aliquots manually. Alternatively, continuous infusion may be used with the added advantage of doing away with the peaks and troughs associated with intermittent injections. The accurate dose requirement is given by the formula: Priming dose (ml vapour) = Desired concentration × ([FRC + circuit volume] + [cardiac output × blood gas coefficient]) Maintenance of low‑flow anaesthesia During this phase, we need to maintain steady‑state concentration of the anaesthetic agents. Although the oxygen uptake remains constant at 200–250 ml/min,