The anaesthesia breathing system is an assembly of components which connects the patient’s airway to the anaesthesia machine during general anaesthesia. It allows spontaneous, controlled and assisted respiration.

The conventional dual limb circuit has an inspiratory and an expiratory limb.

The newer LIMB-O circuit has a partition separating the inspiratory and expiratory lumens. This permits heat transfer from warm exhaled gases to the inspiratory gases through the partition reducing heat loss from the patient.

Presence of septum decreases the compliance of the circuit ehich allows for delivery of a more accurate tidal volume.

After approval from the Institutional Ethics Committee, all patients undergoing elective neurosurgical procedures under general anaesthesia and satisfying the inclusion and exclusion criteria will be considered for the study.

Thirty consenting patients in whom long LIMB-O circuit is being planned to be used will be included as study group.

Similar number of age, gender and type of surgery matched patients will be included where conventional circuit is to be used.

Standard anaesthesia technique protocol as described below will be followed for all cases.

General anaesthesia will be given in all cases.

PREMEDICATION-  Inj. glycopyrrolate 0.2mg iv, Inj. fentanytl 2mcg/kg iv

INDUCTION- Patient will be preoxygenated for 3 minutes. Patient will be induced with Inj. thiopentone 4-6 mg/kg iv followed by Inj. vecuronium 0.1mg/kg iv. Tracheal intubation with appropriate size cuffed endotracheal tube is attempted.

MAINTENANCE- Anaesthesia is maintained with propofol infusion and oxygen, nitrous oxide (50:50) and isoflurane if needed. Maintenance fresh gas flows is kept at 1L per minute.

Common warming technique will be used for each patient using forced air blowers at 39 degree celsius.

All patients will be ventilated with 6-8 ml/kg tidal volume with volume controlled ventilation to maintain ETCO2 (without PEEP) around 35 mm hg.

For the present study, under these conditions, the parameters to be studied will be noted and compared.

Preoperatively, the baseline resistance, compliance and leak of each of the two circuits will be noted in vitro.

A nasopharyngeal and skin temperature probes will be used to note the patient’s core temperature prior to surgery (after induction), every hour and at the end of surgery. Thus, the heat difference (delta t) will be noted.

One hour after the maintenence flows are set, PEEP, peak and plateau pressures will be noted and 3 consecutive readings of the resistance and compliance of the circuit will be noted.