Low Flow Anesthesia & Patients Waking Up – Part 2

For Part 1 in this series, click on this link.

“Why would my patient wake up just because I reduced my oxygen flow rate?”

The short answer is that your patient isn’t getting enough anesthetic gas. We’ve established that even at a lower oxygen flow rate, the patient gets more than enough oxygen (see the post Go With The Flow ). If the patient gets enough oxygen, and the anesthetic gas is mixed in the oxygen, why does it not get enough anesthetic gas?

Over the next three posts in this series we will dive deeply into answering that question. We’ll approach the answers from three directions:

We’ll look at the anesthetic gas itself
We’ll look at how a vaporizer works
We’ll look at the flow of oxygen through a vaporizer

In this post we’ll explore how to calculate an appropriate dosage of an anesthetic gas. We’ll look at anesthetic gas as a drug. Stay with me. This gets a little fussy.

A Brand New Gas

We all have our routines we follow when we’re using an anesthetic gas we’re familiar with. We set the vaporizer on a number that’s comfortable. And whether your starting point is determined by careful calculation, habit, or doing what you’re told, it usually works out just fine. They’re all OK methods when we’ve had some experience with the gas we’re using. However, it falls apart when we make changes to the any parts of our routine (like changing the oxygen flow rate) or when we use a gas for the first time.

For the sake of example, let’s look at a gas that I just made up: C3PO. It’s so new, it doesn’t even have a name – just a bunch of letters and numbers. This revolutionary new anesthetic gas has countless advantages, no disadvantages, it’s safe for all patients, and costs less than water. It’s the kind of gas I would make up if I was making up an anesthetic gas (which I just did). The only drawback is that you will need a new vaporizer. Luckily, the distributor gave you the vaporizer.

What do you need to know about this gas before you are ready to use C3PO?

On your list will be how much to use. You are comfortable administering anesthetic gas through a vaporizer. And you’re comfortable turning the dial up a little or down a little according to your patient’s responses during a procedure, but you may have forgotten why your vaporizer dial settings start out higher with Sevoflurane than with Isoflurane, or why the settings for Isoflurane start out higher than for Halothane.

MAC – The Great Equalizer

To jog your memory, let’s revisit the way relative potency is established for different anesthetic gasses. Relative potency is comparing the effectiveness of one gas to another. That’s where MAC comes in. MAC stands for Minimum Alveolar Concentration, but that’s only the first three words of the definition. The full definition is the “Minimum Alveolar Concentration required to prevent purposeful movement from a noxious stimulus.”

I’ve always liked how concise that definition is; just a handful of important words. To start, it tells you that it wants to determine the least amount of gas needed at the smallest part of the lungs, the alveoli, where gas exchange occurs in the blood. Next it describes the response it’s looking for. It doesn’t look for subtle responses like an elevated heart rate or increased respiration rate, it defines the response as movement. And more specifically, purposeful movement. And finally it describes the stimulation as noxious. On a scale of degrees of stimulus, noxious ranks pretty low. More like annoying than painful. So, another way to write the definition of MAC might be “the least amount of gas required to keep a subject from pulling its foot back when you pinch its toe.” As a matter of fact, that exactly describes a MAC study. The noxious stimulus is usually a toe pinch. The purposeful movement is usually the subject pulling its foot back.

It’s a concise definition of a somewhat non-specific comparison. To make it even more non-specific, MAC is an ED50. That means that it establishes an effective dose (ED) for only 50% of patients – therefore ED50. And there are several things that can affect a patient’s requirement for the MAC of anesthetic gas. Those things include the patient’s physical status (a very sick animal won’t need as much anesthetic gas as a bouncy, healthy one), sedatives and analgesics that the patient may have received, the degree of surgical stimulation, and of course the patient may just be in the other 50% group. Knowing the MAC of a gas is as useful as knowing what neighborhood your friend lives in: it might not get you to their front door, but it helps narrow the search.

To apply what we know about MAC to the operating room, we have to start with logic, then adjust to the individual patient and procedure. Once we know the MAC of an anesthetic gas, we know how much gas is required to prevent a patient from reacting to a toe pinch. Since the stimulation in surgery is likely to be greater than pinching a toe, logic dictates that the patient will need a higher dosage of anesthetic gas. In other words, the patient will need more than [1 X MAC] of the anesthetic gas you’re using. But how much more?

From Theory to Practice

It is generally accepted that [1.25 to 1.5 X MAC] is required for surgery – also known as surgical MAC. The question may be asked like this: “What factor of MAC is required for surgery?” And the answer is 1.25 – 1.5 MAC.

The MAC of commonly used anesthetic gases are published. For reference in this example, the MAC of isoflurane is ~1.4%, and the MAC of sevoflurane is ~2.1%. The gas that I made up, C3PO, has a MAC of ~9%. So, if we are to calculate the surgical MAC of each of these three gases, it would look like this:

Isoflurane = surgical MAC of about 1.75% to 2.1%
Sevoflurane = surgical MAC of about 2.6% to 3.2%
C3PO = surgical MAC of about 11.25% to 13.5%

Of course, we have to remember the limitations of MAC. The patient’s physical status, the other drugs it has on board, the degree of surgical stimulus, or that it may just fall into the other 50% group who needs more or less anesthetic gas to achieve the same effect, all impact the actual amount of gas needed. Bearing all of those variables in mind, we know that isoflurane delivered at 1.75% should achieve the same effect as sevoflurane delivered at 2.6%. And if we are presented with a gas we’ve never used, and we know the MAC of that gas, we know the neighborhood where we can start looking for the dosage we need to achieve the effect we want.

In this post, we’ve broken down some of the mysteries behind administering an effective dosage of anesthetic gases. The big question we’re answering in this series is why a patient might wake up simply because you turn down your oxygen flow rate. The short answer is that your patient wakes up because it doesn’t get enough anesthetic gas. This section sheds light on how much anesthetic gas is enough, and how to calculate enough gas, regardless of the gas you’re using. Part three of this four-part series looks at your vaporizer and how oxygen flow rate affects anesthetic gas delivery.

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.