Lidocaine hurts when you inject it into a patient. It hurts not just because we have to poke through the skin with a needle (this post won’t go into topical anesthetics, J-Tip etc,.) but mostly because lidocaine is acidic. In fact, according to Frank et al. “lidocaine with epinephrine [is] approximately 1000 times more acidic than subcutaneous tissue.” Per the same study here are some estimates of the mean pH of common injectable anesthetics.
- 1% lidocaine with 1:100,000 epinephrine pH = 4.24±0.42
- 2% lidocaine with 1:100,000 epinephrine pH = 3.93±0.43
- 1% lidocaine pH = 6.09±0.16,
- 2% lidocaine pH = 6.00±0.27
There are some advantages to lido w/ epinephrine, but clearly the solutions are more acidic. And either way, epi or not, the anesthetic is far below physiologic pH (7.35-45). We can increase the pH of our lidocaine solutions, and decrease the pain by adding 8.4% sodium bicarbonate solution to the mix as shown in a randomized trial from Matsumoto et al. A previous study from Denmark, as well as Frank et al’s established that the following ratio of 8.4% bicarbonate to lidocaine to be:
1 mL 8.4% sodium bicarbonate : 10 mL 1% lidocaine with 1:100,000 epinephrine
Their work shows that in general a 1:10 ratio would get lido w/ epi (again, the more acidic solution at ph approximately 4) to a more “physiologic” pH of 7.3-7.6. You won’t excessively buffer lidocaine without epi using this ratio either. In order to accomplish this do the following.
- Order “buffered lidocaine” if your institution has this in the EMR (like mine) or, separately, order 1 mL 8.4% sodium bicarbonate, plus your lidocaine of choice (10 mL)
- Fill a syringe first with the 1mL of bicarbonate, then add the 10 mL of lidocaine
- Flip the syringe over several times to mix (no need to shake vigorously)
- Label according to your local practices
This solution will be stable for many hours – possibly up to 24 – but you shouldn’t plan on storing it in the pharmacy this way, or pre-mixing in the ED. And yes, you can do 0.5 mL bicarb and 5 mL lidocaine as well.