Oxygen Flow Rates
The oxygen flow rate is determined by the type of circuit that you are using and the patient's lean body weight (kg). It is typically quoted as a mL/kg/min rate and should be calculated for every patient, every time!
For rebreathing circuits, the induction oxygen flow rate is based on how quickly you want to establish equilibrium. How fast equilibrium is established is based on the time constant equation. The higher the oxygen flow rate the quicker equilibrium will be established. Therefore, right after induction and intubation the flow meter is usually turned a value above 2L/min to help establish equilibrium is a reasonable time frame. Please see the blog titled “Understanding Inhalant Equilibrium and Time Constants” for more information on this topic.
Once equilibrium has been established, a big bonus to a rebreathing circuit is that you can turn the oxygen flow rate down to maintenance rates.
Here comes the confusing terminology………At one point in history rebreathing circuits were classified using the terms closed, low flow or semi-closed.
These terms ONLY refer to the fresh gas flow rate compared to the metabolic oxygen demands of the patient.
• Semi-closed oxygen flow rates are quoted at 22-44 mL/kg/min. This rate is 3-5 times a patient’s metabolic oxygen consumption and is the most popular maintenance oxygen flow rate used in veterinary medicine.
• Low flow is quoted at 10-22 mL/kg/min.
• Closed flow is quoted at 6-10 mL/kg/min. This oxygen flow rate is equal to the uptake and metabolism of oxygen by the patient. Often referred to as a complete rebreathing circuit because it is completely dependent on the CO2 absorbent to extract the CO2 from the exhaled gases before being rebreathed by the patient.
These terms DO NOT relate any information about the state of the APL valve (open or closed) & they do not denote any structural differences in the breathing system.
You can utilize a closed oxygen flow rate with a completely open APL valve!
The limiting factor with low flow and closed flow oxygen flow rates is the vaporizer. The vaporizer is calibrated at a flow rate between 500 mL/min and 10L/min. Some newer vaporizers can go as low as 250 mL/min. If the flow meter is set lower than the calibration range then vaporizer output (dial setting) cannot be guaranteed.
If the calculated maintenance oxygen flow rate for a patient is lower than 250-500 mL/min then it is recommended not to go below the calibration range unless a gas analyzer is used to indicate the inspired and expired inhalant concentration. If a change is made in the vaporizer dial setting then it is imperative that the oxygen flow rate is increased to a reasonable value (above 2 L/min) to allow this new concentration to establish equilibrium within the breathing circuit and then the maintenance oxygen flow rates can resume.
Non-rebreathing Circuits
The oxygen flow rate is the same for induction and maintenance for non-rebreathing circuits. These circuits rely on the high fresh gas flow rates to flush the exhaled CO2 out of the circuit before the next inhalation occurs.
• Bain Coaxial circuit requires 100-200 mL/kg/min
• Jackson Rees circuit requires 300-400 mL/kg/min
If oxygen flow rates are used below these recommended ranges, then the patient will experience significant rebreathing of CO2. If capnography is not used as a monitoring tool then this can easily go undetected until elevated CO2 levels become life threatening.
