Why Bicarbonate is Used with Lignocaine
Sodium bicarbonate is added to lidocaine (lignocaine) primarily to reduce injection pain by raising the pH of the acidic anesthetic solution, and secondarily to accelerate the onset of anesthesia.
Primary Mechanism: Pain Reduction
Buffering lidocaine with sodium bicarbonate raises the pH of the solution, which decreases the burning sensation during injection 1. The mechanism is straightforward:
- Commercial lidocaine solutions are acidic (typically pH 3-5), and this acidity causes significant pain when injected into tissues 1
- Adding sodium bicarbonate neutralizes this acidity, dramatically reducing injection discomfort 2, 1
- Clinical trials demonstrate that approximately 2 out of 3 patients experience reduced pain with buffered lidocaine 2
- In one randomized trial of 100 patients receiving dental nerve blocks, 0% of patients given buffered lidocaine complained of pain compared to 78% (39/50) who received unbuffered lidocaine (p<0.0001) 3
Secondary Benefit: Faster Onset of Anesthesia
Buffering accelerates the onset of local anesthesia by increasing the proportion of non-ionized (active) lidocaine molecules 1, 3:
- The mean time to onset of anesthesia with buffered lidocaine was 34.4 seconds compared to 109.8 seconds with unbuffered lidocaine in dental blocks (p<0.001) 3
- This represents approximately a 3-fold reduction in time to effective anesthesia 3
- For epidural blockade, bicarbonate addition resulted in faster onset (p=0.009) and enhanced depth of blockade 4
Recommended Mixing Ratios
The American Academy of Dermatology recommends specific ratios for optimal clinical effect 2:
- Standard ratio: 10:1 (lidocaine to 8.4% sodium bicarbonate) 2, 1
- Alternative acceptable ratio: 9:1 (9 mL lidocaine/epinephrine to 1 mL bicarbonate) 2, 1
- Both ratios reduce injection pain by 20-40% compared to unbuffered lidocaine with epinephrine 2
Clinical Application Guidelines
The American Academy of Pediatrics recommends buffering lidocaine with bicarbonate, warming the solution before injection, and injecting slowly with a small-gauge needle for almost painless injection 5:
- Buffered lidocaine can be prepared in advance and stocked in emergency departments, remaining stable for up to 30 days 5
- This technique is particularly valuable in pediatric settings where minimizing pain and anxiety is paramount 5
Critical Safety Considerations and Pitfalls
Epinephrine Degradation
Epinephrine concentration decreases by approximately 25% per week after buffering with sodium bicarbonate 2, 6:
- Freshly prepared solutions provide optimal vasoconstriction 2
- Solutions stored for 1 week still maintain nearly equal areas of anesthesia and vasoconstriction compared to fresh solutions 6
Do NOT Buffer Bupivacaine
The American Academy of Dermatology explicitly advises against buffering bupivacaine with sodium bicarbonate, as this causes precipitation and decreased efficacy 2, 1:
- This is a critical safety consideration that distinguishes lidocaine from other local anesthetics 2, 1
- Buffering is specific to lidocaine and should not be extrapolated to all local anesthetics 1
Anatomic Safety
The American Academy of Dermatology considers buffered lidocaine with epinephrine safe for use on all body sites, including digits, ears, nose, hands, and feet 2:
- Historical concerns about avoiding epinephrine in end-arterial territories have been definitively refuted by multiple systematic reviews showing no cases of necrosis 2
Optimizing the Technique
Combining buffering with other pain-reduction techniques provides additional comfort 1: