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Keeping it Local Part 2: Anesthetic Details

Welcome to Part Two of our discussion about anesthetics, where we’re keeping it local. Last week we talked about the history behind local anesthetics, and today we’re going to discuss the important details about local anesthetics. Let’s get to it with a question and answer style review.

How Do Local Anesthetics Work?

They block voltage-gated sodium channels in peripheral nerves, preventing sodium influx, stopping depolarization with numbness as a result.1

How Are Local Anesthetics Metabolized?

Esters are metabolized by plasma cholinesterases in the blood to PABA. Amides are metabolized in the liver by oxidation reactions.1

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What Properties Affect How Local Anesthetics Work?1

  1. pKa = speed of onset of anesthetic effect - Lower pKa indicates more drug in the uncharged state with more able to cross over the lipophilic cell membrane and faster effect. pKa is inversely proportional to anesthetic effect. 
  2. Lipophilicity = potency - The more lipophilic the drug means the more that can pass across the cell membrane and a more profound anesthetic effect. Lipophilicity is directly proportional to anesthetic potency.  
  3. Protein binding ability = duration of effect - The higher the protein binding affinity, the more anesthetic is bound up into the body’s proteins leading to less anesthetic available to do its job and decreased duration. Protein binding is inversely proportional to anesthetic duration. 

How Can I Speed Up the Anesthetic Effect?

  1. Alkalinize the anesthetic - by adding sodium bicarbonate. Sodium bicarb 8.4% in a 1 cc bicarb to 10 cc anesthetic ratio will raise the pH to the physiologic range.  
  2. Use anesthetic with epinephrine - 1:100,000 is the most common available concentration. 
  3. Use a higher concentration - (but remember this changes the potential toxic dose). 
  4. Choose an anesthetic with a more rapid onset - For example, lidocaine’s onset is faster than bupivacaine’s.
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How Can I Remember Which Anesthetics Are Amides Versus Esters?

Amides have “i” in the first half of the name while esters do not. For example, lidocaine and bupivacaine have “i’s” while procaine does not.

Can I Use the Percent of the Anesthetic to Determine How Much There Actually Is?

Yes, you can! Let’s say you have 1% lidocaine plain that you’re about to inject. The 1% equals 1 gram/100 cc. If you simply move the decimal one place to the right, you will know how many milligrams per 1 cc you have. Thus, 1% lidocaine = 10mg/cc.

Where Can You NOT Inject Anesthetic + Epinephrine?

The answer to this common question is “end-organs”: fingers, toes, nose, penis, earlobes. In reality many physicians have injected these areas with no vasoconstriction-related side effects.

Can Inflammation Make It Harder to Obtain Successful Anesthesia?

Yes. Tissue inflammation decreases the pH below physiologic pH, increasing the acidity of the tissue. Lower acidity increases ionization of the anesthetic (decreasing its lipophilicity), decreasing its ability to penetrate the nerve to inhibit depolarization, making it less effective.

Should I Worry About Anesthetic Allergies?

True allergies are rare but more common with ester anesthetics due to their metabolism to PABA.

Don’t forget that some local anesthetics have preservatives such as methylparabens which can cause allergic reactions

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How Does One Calculate the Maximal Amount of Local Anesthetic Allowable?

This can be accomplished by either weight-based dosing or using the maximum dose methods.2 Let’s use 1% lidocaine without epinephrine as an example and start with the weight-based method.

Weight-Based Method:

Step 1: Change the percent to milligrams by moving the decimal one place to the right. 1% = 10mg.

Step 2: Multiply the patient’s weight in kg (in this case 70kg) by the maximum dose (4.5mg/kg) = 315mg.

Step 3: Determine the maximum injectable volume by dividing the amount found in Step 2 (315mg in this case) by the amount of anesthetic/cc (10mg/cc in the case of lidocaine) = 31.5 cc allowed.

OR, using the

Maximal Dose Method:

Step 1: Change the percent to milligrams by moving the decimal one place to the right. 1% = 10mg.

Step 2: Divide the maximum dose number you’ve memorized (in this case 300mg) by the milligrams of anesthetic (10mg) = 30 cc allowed.

High-yield reference table for common anesthetics.3,4

Type Anesthetic Onset Duration Max Dose with/without epi*
Ester Chloroprocaine 6-12 min 30-60 min Max 800 mg/1000 mg w/epi
Procaine   20-30 min Max 500mg (no epi)
Tetracaine (Pentocaine) 3-8 min 2-3 hours 1.5 mg/kg
Max 120mg/180mg with epi
Amide 2% Lidocaine (Xylocaine) 1-3 min 30 min-2 hours 4.5/7 mg/kg
Max 300mg/500mg with epi
1% Lidocaine 1-3 min 30 min-2 hours 4.5/7 mg/kg
Max 300mg/500mg with epi
0.25% Bupivacaine (Marcaine / Sensorcaine) 2-10 min 2 – 3 hours 2.5/3 mg/kg
Max 175mg/225 mg w/epi
0.5% Ropivacaine (Naropin) 3-15 min 4-12 hours 3/3.5 mg/kg
Max 225mg +/- epi
1.5% Mepivacaine (Carbocaine) 3-20 min 2-5 hours 5/7 mg/kg
Max 400mg/500 mg w/epi

*Note: maximum dosages are highly variable and not based on strong evidence.4

Do I Really Need to Worry About Local Anesthetic Toxicity?

Yes. Systemic toxicity occurs in a dose-dependent manner with CNS excitability (seen as seizures) the most common life-threatening situation. This is due to selective depression of excitatory pathways in the brain.1 Peripherally, anesthetic toxicity can decrease vasoconstriction leading to hypotension and catastrophic cardiac collapse.

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Let’s end our discussion on that happy note with one final comment. Given that maximum doses are not well supported in the medical literature, and it’s only logical that anesthetic effects will differ based on patient weight, using the weight-based dosing system seems to be the safest for best patient outcomes. Hopefully I haven’t numbed your mind with this discussion about local anesthetics. Get it? “Numbed your mind?” “Local anesthetics?” Ok, ok. Bad pun…

Best wishes.

Jarrod Shapiro, DPM
PRESENT Practice Perfect Editor
jarrod@podiatry.com
References
  1. Becker DE, Reed KL. Essentials of local anesthetic pharmacology. Anesth Prog. Fall 2006;53(3):98-109.
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  2. Frank SG, Lalonde DH. How acidic is the lidocaine we are injecting, and how much bicarbonate should we add? Can J Plast Surg. Summer 2012;20(2):71-73.
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  3. Maximum Recommended Doses and Duration of Local Anesthetics. University of Iowa Health Care. https://medicine.uiowa.edu/iowaprotocols/maximum-recommended-doses-and-duration-local-anesthetics. Last updated 10/13/2019. Last accessed 11/1/2020.
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  4. Rosenberg PH, Veering BT, Urmey WF. Maximum recommended doses of local anesthetics: a multifactorial concept. Reg Anesth Pain Med. Nov-Dec 2004;29(6):564-575.
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