Local Anesthetics have been one of the most important inventions in human history. Many of our patients ask us how they work and are interested in their chemistry. Local anesthetics are the most frequently used drugs in at St. Lawrence Dentistry with many thousands of doses being administered each year. These are drugs used to relieve pain in your mouth without the loss on consciousness and they do this by blocking nerve conduction. More specifically, they block sodium channels which blocks a nerve transmission or ‘action potential’. Consequently no message is sent to the brain.
The cell biology of local anesthetics is interesting. Depolarization is defined as the loss of the difference in charge between the inside and outside of a cell. When a nerve ‘depolarizes’ it opens ‘sodium channel’ and this is what allows a nerve action potential to occur. Local anesthetic molecules sit in the ‘open state’ of a channel and prevent sodium passage. This is why you feel no pain in a dental procedure.
St. Lawrence Dentistry plays close attention the dosages and duration of local anesthetics. The higher the dose of local anesthetics, the more of the molecules will arrive at the area, and hence the faster the onset.
The most important factors which determine how effective a local anesthetic are the ‘pH’ of the tissue and the ‘pKA’ of the local anesthetic. pKA is the relative concentration of the local anesthetic in the non ionized lipid soluble form versus the ionized water soluble form. The ionized form of local anesthetics are the one that can penetrate the cell membrane and this is what makes it effective. However, the non-ionized form is not as effective since it cannot penetrate cell membranes. The pKa of most local anesthetics is around 8-9.
There are two categories of modern local anesthetics: amides and esters. The amide locals are the most commonly used at St. Lawrence Dentistry and have pKa ranging from 7.9-7.6. The other type are the esther locals are which are closer to pKA 9. The closer the pKa is to the tissue pH, the more ionized form available and the more effective it will be.
Understanding this principle becomes important when treating dental infections as the pH of the infection can be more acidic. Local anesthetics closest to the physiological pH will have a higher concentration of non-ionized base form. This means it can more easily pass through nerve cell membranes in give a quicker onset of action. St. Lawrence Dentistry will consider the probable pH of your tissue when selecting the most appropriate anesthetic for you.
For example, if injecting the common local lidocaine (pka of 7.9) into an infected area with a pH of 5.9 will only allow a 1000:1 ionized versus no ionized form. This means only a fraction is available for diffusion into a cell. All in all you would find it would not work well.
Some people ask us what are the components of local anesthetic. A local anesthetic molecule consists of a lipophilic centre (aromatic group), a hydrophilic centre (tertiary amine), and an intermediate group (amide or ester). One interesting fact is Bupivicaine is the longest lasting amide anesthetic which can last 9 hours.
When using local anesthetics we are always take steps to avoid adverse reactions associated with them. Adverse reactions to local anesthetics are psychogenic, allergy, toxicity, and parasthesia. Syncope is the most common psychogenic side effect, it can result from both an increase or decrease in heart rate and blood pressure. It is usually the result of inadvertent epinephrine injection into a vessel. Allergic reactions to local anesthetics are very rare and are usually related to a compound ‘p-aminobenzoic’. Historically there were allergies to methyparabin contained in anesthetics. However, this compound is not contained in the modern solutions. There are no true allergies to epinephrine in anesthetics as this is an ‘endogenous neurotransmitter’ which naturally occurs in your body. Toxicity to local anesthetics are rare but is can be related to excessive dosages or direct injection into a vessel.
A common question we get from patients is inquiring about how we calculate the maximum dose for local anesthetics. Our most common anesthetic used at our office are lidocaine and articaine and they both have a maximum dosage of 7mg/kg. When you see a percentage listed on a local anesthetic carpule this is in the units ‘grams/100ml’. Hence 2% lidocaine has 20mg/ml and each cartridge has 1.8ml. Hence for a 25kg child the max dose would be calculated as follows.
- Max dose is 7mg/kg
- 7mg x 25kg=175mg
- 2% lido has 20mg/ml
- 175mg/ 20mg/ml=8.75ml
- 8.75ml/1.8ml=4.86 cartridges
One rare complication of local anaesthetics is ‘parasthesia’. This is a partial loss of sensation and is usually accompanied by burning, tingling feeling, and usually resolves in 4-6 weeks.
Local anesthetics containing 4% solutions have been found to have a higher percentage of paresthesia. Hence, at St. Lawrence Dentistry we mainly use a 2% solution.
Hematomas are another rare complication for anesthetics and result from leakage of blood from vessels into tissue. Statistics have shown the most likely most likely place for this to happen is when giving a ‘PSA nerve block’ which is an injection for numb your upper teeth. These usually heal with discoloration and most of the time no further treatment is needed.
If a patient has a hematoma we generally will advise the following: Apply ice for next 6 hours 15 mins on 15 off. Avoid aspirin or and non-steroidal anti-inflammatory medications as they may cause bleeding.
Trismus is another rare anesthetic complication and is a motor disturbance of the trigeminal nerve characterized by the inability to open the mouth. The muscle mostly affected is the called the ‘middle pterygoid’.
If you have trismus the treatment we sometimes recommend will be the following: Place a hot moist towels 20mins on/10min off, use of analgesics, and practicing opening and closing. In this instance we have found ibuprofen 600mg strength is helpful.
Epinephrine is contained in many local anesthetics and decreases the rate of absorption of the local, into the cardiovascular system. Hence less chance of toxicity. It also decreases bleeding, increases the duration of action of the local. Epinephrine does react with medications especially beta blockers, tricyclic antidepressants, and amphetamines. Generally this will manifest as an increase in blood pressure and heart rate. If we have any concerns about this happening we will lower the dose of epinephrine used. For our patients with cardiovascular issues we will generally use below 2 carpules of 1:200k epinephrine.
St. Lawrence Dentistry will do everything possible to make your dental appointment comfortable with the help of local anesthetics. If you have any questions about their interesting chemistry please let us know.
References: University of Toronto, Faculty of Dentistry online Lecture series 2018.