Fast Facts: Is less more? Calculating local anesthesia cartridges

In this episode of  Fast Facts: Ask the Expert , host Andrew Johnston sits down with pharmacology expert Tom Viola to tackle a common clinical question: How many cartridges should we actually be using?

While many clinicians focus on the number of "carps" administered, Tom breaks down the chemistry of why volume can sometimes work against you. By understanding the relationship between concentration (2% vs. 4%) and the pH-lowering effects of acid in the tissue, you can achieve better results with fewer injections.

Key takeaways:

• The molecule match: Why 2 cartridges of 4% lidocaine provide the same anesthetic power as 4 cartridges of 2%.

• The acid issue: How high-volume injections can drop tissue pH and make it harder to maintain profound anesthesia.

• Safety first: Remembering the difference between your "maximum recommended dose" and your "clinical goal."

Episode transcript

Andrew Johnston:

Welcome back everyone into Fast Facts: Ask the Expert. This is February. Tom Viola, thank you again for being with us. I'm really excited talking about local anesthesia again.

Tom Viola:

Thank you, Andrew. It's a pleasure to be a part of this project. You know, you and I have been working together for so long now, and I enjoy every minute of it. And, being a part of Ask the Expert humbles me. Certainly. I don't consider myself an expert in many things, but I do love to talk about anesthesia. So, yeah, let's, let's move forward. And we want your question for today.

Andrew Johnston:

Oh my gosh, it's a good one. It's one that we hear quite a bit. And, you know, for me, I've worked in a lot of offices that do some surgeries. And so this is something that's top of mind. But I've also worked in pediatrics. So again, top of mind. How many cartridges of local anesthesia should we be using? Is less more?

Tom Viola:

Okay, I'm so glad you asked this question because this comes up so often, and it really has to do with molecules. This is a tough one for us to remember. You know, as soon as I hear the word molecules, molar, or general chemistry, I start feeling, you know, pangs of nausea and feelings of vomiting. So I'm going to make it as simple and nausea-free as possible.

Okay. The best way to visualize this in your mind is to say, okay, I've got two options. If I want to look at it this way, I can either use two cartridges of 4% anesthetic or four cartridges of 2% anesthetic. Both of those options are going to deliver the same number of molecules of local anesthesia. So which one should I use?

Okay. Well, obviously since they both provide similar molecules, you could use both. But remember that in every cartridge of local anesthesia there's an extra ingredient: acid. And since every cartridge of anesthesia contains acid, the more cartridges—the number, I'm talking about the number of cartridges you use—if you use more cartridges, you're adding more acid to the tissue, which drives down the pH and therefore ultimately will make it more difficult to establish anesthesia and maintain anesthesia.

So we like to use less cartridges. But if we use less cartridges, doesn't that mean less molecules? No. We'll have to bump up the concentration. So using two cartridges of 4% anesthetic delivers the same number of molecules of anesthetic, but half the amount of acid as four cartridges of 2% anesthetic. And that's why it's more important to always consider the volume and the concentration and realize when in doubt, up the concentration and lower the volume.

But as a matter of fact, keep in mind that every anesthesia we use, every local anesthetic that we use, has a maximum recommended dose that's allowed for a patient of a certain weight. So that's more of an upper limit. It doesn't have to be the amount we use in each procedure.

Andrew Johnston:

Awesome Tom, thanks again so much. Where can people find you?

Tom Viola:

Anywhere you would like to, on social media at Pharmacology Declassified. I'm on all the platforms and reach out to me anytime on my email address at [email protected].