Table 3.

Comparison of non-rebreathing and rebreathing anesthesia circuits

Functional consideration	Non-rebreathing circuit (NRC)	Rebreathing circuit (circle system)
Rebreathing potential	No rebreathing of exhaled gases	Rebreathing exhaled oxygen and inhalant
Mechanism for preventing rebreathing of CO2	High oxygen flow rates move exhaled CO2 along the expiratory limb away from the patient	Functional CO2 absorbent and one-way valves
Conservation of body heat	High flow rates of oxygen may lead to increased loss of body heat	Lower flow rates of oxygen and rebreathing of exhaled gases may help to conserve body heat
Resistance to breathing	Minimal	Resistance is caused by CO2 absorbent and one-way valves
Equipment dead space	Minimal	Minimized by using a pediatric Y circuit
Time taken for a change in anesthetic depth	Short, due to small circuit volume and higher oxygen flow rates	Longer, due to larger circuit volume and lower oxygen flow rates

Optimal circuit based on patient size:

✜ Very small cat (<3 kg; 6.6 lb): NRC

✜ Small to medium cat (3–6 kg; 6.6–13.2 lb): NRC or rebreathing circuit

✜ Large cat (>6 kg; 13.2 lb): NRC or rebreathing circuit