“Why would my patient wake up just because I reduced my oxygen flow rate?”
We originally posed the question in Part 1 of this series. It’s a common challenge that often leads to abandoning the practice of low flow anesthesia. It’s also a problem overlooked in the AAHA Anesthesia Guidelines for Dogs and Cats, when they talk about oxygen flow rates for circle systems and recommend, “During the maintenance phase, total O2 flow rate should typically be between 200 and 500 mL [per minute].” I agree with that recommendation, but there’s more to the story. The short answer to the question is that the patient wakes up because it’s not getting enough anesthetic gas.
In Part 2 and Part 3 of this four-part series, I filled in some of the background you need for rest of the story. In this final Part 4, we’ll look at how the flow of oxygen through a vaporizer determines the actual dose of anesthetic the patient receives.
Again, stay with me. This gets a little fussy, but I guarantee it’s worth it. And if you want to play along, grab a calculator – math is involved. But I promise it’s no more complicated than calculating a tip at a restaurant. As a bonus, at the end of this post you’ll find a link to the entire series compiled as one printable PDF.
The most important take-home message from Part 3 is that the anesthetic gas coming from a vaporizer is a percentage of the total oxygen flow. That percentage doesn’t actually tell us the dose of inhalant we are presenting to the patient, it just tells us that a known portion of the gas flowing from the vaporizer is inhalant.
Grab your calculator and we’ll break this down to its parts.
Let’s say that you have your oxygen flow set at 1 liter per minute flow, and your vaporizer set at 1.5%. With those settings, how many milliliters of volatilized anesthetic are you presenting to the patient every minute? I’m pretty specific about the wording of that question. Administering anesthetic gas is not like administering any other drug. When you draw up a syringe of a drug and inject it into a patient, you know that the patient is getting all of the drug in the syringe. Therefore, you know how much drug is administered. When administering anesthetic gas, the patient uses significantly less than the total amount of the oxygen flow, and therefore oxygen and inhalant are being exhausted through the pop-off valve. So we can only calculate the amount of inhalant being presented to the patient each minute. To determine that actual dosage of inhalant administered to the patient, you’d need to use an agent monitor. Agent monitors, by the way, are great tools to understand how all of this works. If you have one anywhere near you, play with it. They’re awesome.
Back to the question. You have your oxygen flow set at 1 liter per minute flow, and your vaporizer set at 1.5%. How many milliliters of volatilized anesthetic are you presenting to the patient every minute?
You’re being asked what 1.5% of 1 liter is. The metric system is simple when you remember all of the rules. In this case, it’s helpful to know that 1 liter = 1000 milliliters. Another useful hint when talking about numbers is whenever you hear the word “of”, it means “times”. So the numeric expression is 1.5% X 1000 milliliters. Or 0.015 X 1000 = ?
The answer is 15ml. The entire thought could be stated like this: With the oxygen flow set at 1 liter per minute, and the vaporizer set at 1.5%, you are presenting 15 milliliters of inhalant anesthetic to the patient each minute.
Is the patient using all of that 15ml of inhalant anesthetic presented each minute? No. And remember, we don’t know how much of the 15ml it’s using.
Now we decide to try low flow anesthesia. We turn the oxygen flow rate down from 1 liter per minute to 500ml per minute, and leave our vaporizer dial setting at 1.5%. How many milliliters of volatilized anesthetic are we presenting to the patient every minute now? The numeric expression is now 1.5% X 500 milliliters. Or 0.015 X 500 = ?
The answer is 7.5ml. With the oxygen flow set at 500ml per minute, and the vaporizer set at 1.5%, we are presenting 7.5 milliliters of inhalant anesthetic to the patient each minute. Even though the vaporizer dial setting is the same in each instance, the actual amount of anesthetic gas presented to the patient at 500ml oxygen flow is half of the amount presented at 1 liter flow. Half. It’s not surprising the patient may become more lightly anesthetized.
When we challenge ourselves to step out of our comfort zone by reducing the oxygen flow rate during anesthesia, it’s important that we see the broader picture and anticipate our response to changes in the patient. The patient will often become more lightly anesthetized and may even wake up. This often leads anesthetists to abandon the idea of low flow anesthesia. But now that we understand the relative potency of anesthetic gas, and how a vaporizer’s delivery is tied to the oxygen flow rate, we can anticipate that the patient may get lighter, and we can be prepared to increase the vaporizer dial setting.
There are many great tools for today’s veterinary anesthetist, but the most important monitoring tool is the person sitting at the head of the patient. AAHA summed up their Anesthesia Guidelines with this closing statement, calling out the key to success: “Successful anesthetic management requires trained, observant team members who understand the clinical pharmacology and physiologic adaptations of the patient undergoing anesthetic procedures, as well as the use of anesthetic and monitoring equipment.”
Click on this link for all 4 Parts on a single printable PDF.
Ken Crump AAS, AHT is a writer and animal anesthetist, and writes Making Anesthesia Easier for DarvallVet, a division of Advanced Anesthesia Specialists. He makes dozens of Continuing Education presentations on veterinary anesthesia and oncology across the United States and in Canada. Ken retired from the Veterinary Teaching Hospital at Colorado State University in 2008.