Ensure that O₂ levels in the E-tank or hospital supply tank are >200 psi or that the oxygen generator is functioning properly. Fill the vaporizer with liquid inhalant. If the patient will be breathing through an RC, change the CO₂ absorbent after 8 hr of use, which may be roughly daily or weekly, depending on the anesthesia case load.

Connect the breathing circuit to the machine. When choosing a breathing circuit, note that NRCs are commonly used for cats and small dogs (patients <3–5 kg [6.6–11 lbs]) because, compared with RCs, they cause less resistance to breathing and have low equipment dead space, both of which are important considerations in small patients. Excessive equipment dead space leads to rebreathing of CO₂ and subsequent hypercarbia/hypercapnia. Dead space should be kept to a minimum and should be ≤2–3 mL/kg, which is ≤20% of total tidal volume. The patient end of breathing circuits (where mixing of inspired and expired gases occurs) is a common source of dead space. Pediatric circuits, which have low dead space, are recommended for use in smaller patients. Rebreathing of CO₂ in the NRC is prevented by high O₂ flow rates, which also allows for a faster turnover in the change of anesthetic depth when adjusting the vaporizer setting. Thus, the O₂ flow should be ~200–400 mL/kg/min when using NRCs.¹⁹ Flow adequacy should be monitored with a capnograph and adjusted to keep the inspired CO₂ to <5 mm Hg. If any occlusion occurs in the expiratory limb of the circuit, the relatively high oxygen flow rates used in NRCs will cause rapid pressurization of the entire system and can lead to airway damage and/ or circulatory collapse within minutes. In addition, the oxygen flush valve should never be used in a patient breathing through an NRC because it directs a high-pressure flow directly into the patient’s airway, potentiating barotrauma. The risk of high airway pressure damage is increased in cats and small dogs because of their small lung capacity (~400 mL per average cat). For safety, a high-pressure alarm can be inserted in the expiratory limb of the breathing circuit. This does not allow escape of gas but emits a loud noise if the pressure in the circuit rises. 

Rebreathing circuits (i.e., circle systems) with lightweight plastic rebreathing hoses and minimal dead space are typically used in patients >3–5 kg (6.6–11 lbs). Pediatric RCs typically have lower equipment dead space compared with hoses for adult patients and can be used for patients <3–5 kg (6.6–11 lbs) if NRCs are not available. RCs depend on functional one-way valves to ensure unidirectional gas flow and on CO₂ absorbent to prevent rebreathing of CO₂. Proper valve function can be assessed by breathing through the circuit while visualizing movement of the correct valve on inspiration and on expiration. During the maintenance phase, total O₂ flow rate should typically be 20–40 mL/kg/min, with a minimum 500 mL/min to ensure accurate vaporizer output. The benefits of lower flow rates include decreased environmental contamination and decreased consumption of O₂ and volatile anesthetic gases (i.e., inhalant anesthetic not inhaled by the patient). Lower flow rates also conserve moisture and heat. When using RCs, a minor disadvantage of lower flow rates is increased time to change anesthetic depth after changing the vaporizer setting.

Leak test the machine and circuit before anesthetizing each patient and after changing any breathing circuit components. This safety measure ensures that oxygen is flowing to the patient and that there is minimal risk of inhalant leakage. For this step, the adjustable pressure limiting, also called the pop-off valve, must be closed. Occlude the end of the breathing circuit and increase the O₂ flow to raise the machine circuit pressure to 20–30 cm H₂0 on the manometer. The oxygen should be turned off, and the pressure manometer should remain steady. If no leak is detected, open the pop-off valve, and then release the breathing circuit occlusion. If a leak is detected, pull the machine from use and initiate troubleshooting procedures.

Ensure proper setup of the scavenging system to limit or eliminate personnel exposure to inhalant gases. The Occupational Safety and Health Administration provides advisory guidelines, although some US states have specific regulations regarding control of waste anesthetic gases. Active scavenging systems are far more effective than passive scavenging systems (e.g., activated charcoal canisters). More information on waste anesthetic gas is available through the American College of Veterinary Anesthesia and Analgesia.²⁰

Prepare for airway management by choosing ETTs of appropriate sizes and intubation tools (e.g., stylets, laryngoscope), along with a face mask for preoxygenation prior to induction. Veterinarians have several options for intubation, including clear cuffed polyvinylchloride, silicone, and self-sealing baffled tubes. Supraglottic airway devices, which do not require intubation, are available for airway management in cats and rabbits. ETT elbow and capnograph adapters are a source of dead space. To decrease dead space breathing without increasing airway resistance, the diameter of the capnograph and elbow adapters should always exceed the internal diameter of the ETT. A single elbow adapter may add up to 8 mL of dead space, which can be excessive in small patients. Low dead space adapters are recommended.

Total Airway Control Devices