Local Anesthetics and pH

[username456789]

So in pharm we learned that the free base form of a local anesthetic (amides or esters) is the form that penetrates membranes better, and the cationic form is the active form that blocks VGNaC's.

But they've been extremely unclear on what kind of environment they're more "potent" in. In an acidic area, more ionized and less free base would exist. In a more basic pH, the free base would predominate.

Yet we've essentially had both situations presented as "less than ideal", it seems. Obviously the free base has to penetrate and the ionized form has to do the job. But since you really can't change the body's pH very well (although you could mix in some NaHCO3 I guess), I'm not really sure I understand which is more "ideal."

For instance, we're told that in an inflammed area, it would have a lower pH, and thus you'd have to inject more of the anesthetic (or inject it more peripherally with regards to the wound) because less free base would exist. Then we're told that an area with a very basic pH (and thus a higher amount of free base) would cause a reversal of a nerve block because of the lower amount of ionized active form.

I guess I'm just confused about how to actually apply the information.

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[Green Chimneys]

I guess I'm just confused about how to actually apply the information.

Do not inject local anesthetic in to an abscess for I&D. It will not work. Also, adding some bicarb to local anesthetic can make the onset a little faster and may increase the effect of the drug.

 

[canjosh]

Do not inject local anesthetic in to an abscess for I&D. It will not work. Also, adding some bicarb to local anesthetic can make the onset a little faster and may increase the effect of the drug.

Seriously?! I understand that local is less effective due to the local environment, but that doesn't equal total ineffectiveness. Sure, I&D of an abscess is painful no matter what. But, I don't think that means you should completely abandon the use of local anesthesia...even if mostly for psychological effects.

 

[Green Chimneys]

You can certainly use a field block to infiltrate the skin around the abscess, and I absolutely agree with doing that. However, sticking a needle directly in to the abscess as you would a trigger point is counter productive. 1 it's an extra needle stick, and 2 you're just cranking more volume in to an already tender abscess. No need to add to the pain with your local.

 

[canjosh]

Aha thanks for clarifying. That absolutely makes sense.

 

[armorshell]

You can certainly use a field block to infiltrate the skin around the abscess, and I absolutely agree with doing that. However, sticking a needle directly in to the abscess as you would a trigger point is counter productive. 1 it's an extra needle stick, and 2 you're just cranking more volume in to an already tender abscess. No need to add to the pain with your local.

Also potentially tracking pus down to deeper tissues if you penetrate through the abcess or reuse the needle for subsequent injections.

 

[bawer234]

Do not inject local anesthetic in to an abscess for I&D. It will not work. Also, adding some bicarb to local anesthetic can make the onset a little faster and may increase the effect of the drug.

I've actually seen an ER doc do just this. The pt was in a lot of pain and I'm not sure how much the local helped.

You're also right about the NaHCO3. Na bicarb and EPI will both help the duration, onset and efficacy of the local, but too much Na bicarb can cause the local to ppt out of solution (Lidocaine is stored in HCL) so it can be a balancing act of how much to give.