Let’s begin this article by saying, “You’re right!” Whatever oxygen flow rate you routinely use when anesthetizing your patients, whether it’s determined by careful calculation, by habit, or by setting it where you’re told to set it, “You’re right!” This is one of those wonderful times when there is no universal agreement as to the proper flow rate for the various anesthesia breathing systems. So, the pressure’s off. You’re doing it right. [exhale]
Now that we’ve established there are different right ways to do it, we’re free to consider some new right ways to try. But most of us are reluctant to tamper with the way we run anesthesia because we know anesthesia is part science and part magic. And although we trust the science part, magic can be unpredictable. Before we attempt new right ways to do things, let’s take a look at some of the science behind how to decide the oxygen flow rate for small animal anesthesia.
How much oxygen does the patient need?
The oxygen flowing through an anesthetic gas machine is used to provide the patient with sufficient oxygen and to provide a carrier for the anesthetic agent. Since the oxygen flow meter regulates the flow of oxygen per minute, we need to know how much oxygen our patient uses in a minute. This is called the metabolic oxygen consumption rate per minute. A safe estimate of oxygen consumption for small animal patients is 4 – 7ml/kg/min or 2 – 3ml/lb/min. That means that my 14 pound cat Ellie is consuming around 40ml of oxygen each minute she monitors the dog’s behavior from her perch on the couch.
The kind of breathing circuit makes a difference
In general terms, there are two kinds of anesthesia breathing circuits: rebreathing and non-rebreathing. Rebreathing circuits are most often circle systems that include a carbon dioxide absorber like soda lime. The granular soda lime extracts exhaled carbon dioxide from the circuit which allows the gas to be breathed again or rebreathed. Rebreathing circuits use lower gas flows, which decreases the cost and decreases pollution while it retains moisture and heat for the patient. However the soda lime and unidirectional flow valves of a circle system are significant sources of resistance to breathing. Smaller patients can’t overcome the resistance of a circle system and so you must use a non-rebreathing circuit. Non-rebreathing circuits depend on high oxygen flow to remove exhaled carbon dioxide from the circuit between breaths. High oxygen flow rates are inefficient, expensive, and carry heat and moisture away from the patient. The decision to select a non-rebreathing circuit is often made by the weight of the animal, but it is actually a decision that the patient is too small to overcome the resistance of a rebreathing circuit.
Because of the simplicity of a non-rebreathing circuit, there is little debate over what recommended oxygen flow rate to use. The AAHA recommended flow rate of 200ml/kg/min is generally accepted as appropriate. That flow rate is 33 times more oxygen than is needed to meet a patient’s metabolic oxygen consumption each minute, but that high flow rate assures the patient will not rebreathe any of its exhaled carbon dioxide.
Rebreathing circuits offer a wider selection of flow rates. The same rebreathing circle system can be operated as a closed system, a low-flow system, or a semiclosed system, depending on the selected oxygen flow rate. They do not depend on the position of the pop-off valve. The pop-off valve should always be fully open regardless of the oxygen flow rate.
The closed system flow rate meets the patient’s actual metabolic need for oxygen each minute. It’s a style of anesthesia that demands a thorough understanding of the patient, the anesthetic agent, the gas machine, and the varying degrees of stimulation during the surgical procedure. It’s best suited for anesthesia nerds. Some variation of the semiclosed system is used in most small animal practices. The semiclosed flow rate well exceeds the patient’s metabolic requirement for oxygen, and a significant amount of excess gas is exhausted through the pop-off value. The flow rate traditionally falls within 22 – 44ml/kg/min, most often settling at 30ml/kg/min. Remember that the patient’s metabolic oxygen consumption rate per minute is less than 10ml/kg, so there is little concern that you won’t provide enough oxygen flow.
The vaporizer plays a role in your oxygen flow rate
AAHA also makes recommendations for the oxygen flow rate when using a rebreathing circle system. The AAHA recommendation during the maintenance phase of anesthesia is between 200 and 500ml/min. That flow is not calculated using the weight of the patient; it is the recommended setting of the flow meter. They warn that the gas machine must be leak free for those flows to be effective, and concedes that flows that low may be out of the comfort zone of most of us. However, they may not have considered the tendency for most vaporizers to over deliver at low flow rates. Most vaporizers recommend oxygen flow rates not less than 500ml/min for accurate delivery of anesthetic gas.
Ready to try a new right way?
Whatever oxygen flow rate you routinely use when anesthetizing your patients, whether it’s determined by careful calculation, by habit, or by setting it where you’re told to set it, “You’re right!” Now that we’ve taken a look at some of the science behind how to decide the oxygen flow rate for small animal anesthesia, maybe we’ll try another right way.
For more information, refer to AAHA Anesthesia Guidelines for Dogs and Cats
Ken Crump (kencrump.com) is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists. He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada. Ken retired from Colorado State University in 2008.