Dose Calculation - Local Anesthesia
Updated: Jan 22, 2022
In the past I have talked about various medical emergencies occurring at a dental clinic.
One medical emergency that I have not talked about is Local Anesthesia Toxicity (overdosage).
Although rare, it can be life threatening.
So before talking about Local Anesthesia Toxicity, I will first explain Local Anesthesia dose calculation.
To prevent Local Anesthesia Toxicity, we should know the maximum recommended dosage (MRD) of the local anesthetic agent to be given in a patient.
MRD is always given in weight (milligrams, mg). This is because there are multiple concentrations available of different drugs.
For example, Lignocaine (also called as Lidocaine) is available in 0.5%, 1%, 1.5%, 2% and 4% concentrations. 1 ml of different concentrations will contain different weight (mg) of Lignocaine molecules.
Another thing to remember is that we measure local anesthetic in volume (millilitre, ml) as we administer it in liquid form.
If we don’t know the conversion from mg to ml, then chances are that we may administer local anesthetic in the quantity more than the MRD and this may lead to Local Anesthesia Toxicity.
Hence, to prevent LA toxicity, knowledge of dose calculation of LA is essential.
This is something that I struggled with a lot during my training at dental school.
This is my attempt at making the concept as easy as possible to understand.
I will be using Lignocaine as an example to explain the conversion from mg to ml. The method can be applied to any LA agent.
Below given values are the maximum recommended dose of Lignocaine.
Lignocaine With vasoconstrictor – 7 mg/kg
Lignocaine Without vasoconstrictor – 4.4 mg/kg
Absolute maximum dose, Lignocaine with vasoconstrictor – 500 mg
Absolute maximum dose, Lignocaine without vasoconstrictor – 300 mg
7 mg/kg means that 7 mg lignocaine with vasoconstrictor can be given per 1 kg of the patient.
So, if the patient weighs 70 kgs, then we can administer a maximum of 490 mg (7mg x 70kg) lignocaine with vasoconstrictor in that patient.
In the same way, in a 70 kg individual, maximum dose of lignocaine without vasoconstrictor is 308 mg (4.4mg x 70kg).
Let us take another example,
If a healthy patient weighs 80 kgs then the maximum dose of lignocaine with vasoconstrictor as per the above mentioned method will be 560 mg (7mg x 80kg).
BUT, in an 80 kg individual we cannot give 560 mg lignocaine. Keep reading to know why.
Absolute maximum dose means that, whatever be the weight of the patient’s, we cannot administer local anesthetic drug beyond that dose.
Absolute maximum dose of Lignocaine with vasoconstrictor is 500 mg. That means that we cannot give more than 500 mg of lignocaine in any patient, regardless of his body weight.
Hence, in an 80 kg patient maximum we can administer is 500 mg and not 560 mg.
Even if the patient has a body weight of 200 kg, we cannot administer more than 500 mg to that patient.
Also remember, every local anesthetic agent (Lignocaine, articaine, bupivacaine etc.) has a different MRD.
I will be showing you the logic behind the conversion of dosage in mg to ml.
As I mentioned earlier, I will be using the example of the most commonly used local anesthetic agent, Lignocaine.
Lignocaine is available in different concentrations (0.5%, 1%, 1.5%, 2% and 4%).
The most commonly used concentration in dental practice is 2%.
I will be using 2% Lignocaine as an example here.
Lignocaine molecules are measured by weight and mixed in distilled water for delivery of the drug.
Hence we can say that the concentration of 2% is weight/volume concentration (weight of local anesthesia molecules / volume of distilled water).
Weight is measured in Kilograms (kg) and water volume is measured in Litres (L).
Also remember that 1 litre of water has a weight of 1 kilogram.
So, kg and L are equivalent units.
Same way, 1 millilitre of water weighs 1 gram.
This means that millilitres and gram (g) are equivalent units.
When giving the concentrations, always the equivalent units are used.
So, kilogram (kg) will be used with litre (L) and gram (g) will be used with millilitre (ml).
Also, 2% of anything means 2 units out of 100 units of anything.
Hence the 2% concentration means that 2 kg lignocaine in 100 litres of water i.e., 2kg/100L (weight/volume).
We don't administer local anesthesia in kilograms or litres. We administer local anesthesia in milligram or millilitres.
So, we need to convert it into smaller units.
As i mentioned earlier millilitres and gram are equivalent units.
Hence we can say that 2% lignocaine solution contains 2 gram lignocaine in 100 ml distilled water i.e., 2g/100ml.
Now, 1 g = 1000 mg and 2 g = 2000 mg.
Hence, we can say that 2% solution contains 2000mg lignocaine in 100 ml distilled water.
2000 mg lignocaine - 100 ml distilled water
20 mg lignocaine - 1 ml distilled water.
Hence we can say that 1 ml of 2% solution contains 20 mg lignocaine.
Now, in a 70 kg healthy individual, 490 mg Lignocaine with vasoconstrictor can be administered.
We already know that 1 ml solution contains 20 mg lignocaine.
So, for a 70 kg healthy individual, 24.5 ml (490/20) of 2% solution can be administered.
We also have an absolute maximum of 500 mg for lignocaine.
Hence, in any case (regardless of the patient's body weight) a maximum of 25 ml (500/20) of 2% solution can be administered.
In other words, an individual with body weight beyond 71.5 kg can receive a maximum of 25 ml.
In the below given image, you can see that maximum recommended dose of 80kg, 90kg and 100kg individuals are same.
So this was the dose calculation of local anesthetic agents.
Now we will talk about the dose calculation of adrenaline (epinephrine).
Below given is the MRD of adrenaline.
In a healthy patient-0.2mg/appointment
In a cardiovascular disease patient-0.04mg/appointment
Do note, that the doses mentioned above are per appointment. Unlike local anesthetic agent, the dose does not depend on the patient's body weight.
We convert the MRD of adrenaline for a healthy patient (0.2 mg/appointment).
Like other drugs, adrenaline in local anesthetic solution comes in different concentrations like 1:50000, 1:80000, 1:100000 and 1:200000.
This concentration is also given in weight/volume (weight of adrenaline molecules / volume of distilled water).
In india the most commonly used concentration is 1:80000. So we are going to use this as an example for dose calculation.
1:80000 means 1 in 80000.
Gram (g) and millilitre (ml) are equivalent units.
Hence, we can say that 1 gram adrenaline is present in 80000 ml solution.
Converting gram to milligrams, we can say that 1000 milligrams are present in 80000 ml solution.
That translates to 1 mg adrenaline in 80 ml.
So, 1 ml of solution contains 0.0125 mg adrenaline (1/80 mg).
Now in a healthy individual, 0.2 mg can be given in a single appointment.
Hence, in a healthy individual a maximum of 16 ml (0.2 / 0.0125) of 1:80000 solution of adrenaline can be administered.
In the same way, for a patient with a cardiovascular disease a maximum of 3.2 ml of 1:80000 solution of adrenaline can be administered.
You should note that that this 2% lignocaine and 1:80000 adrenaline are both in a single solution.
So the maximum volume of this (local anesthesia + adrenaline) solution will be governed by the lowest of the two MRD values (MRD of local anesthesia and MRD of adrenaline).
For example, in a 70 kg individual,
24.5 ml 2% lignocaine solution can be given.
16 ml 1:80,000 adrenaline can be given.
Lowest of the two values is to be administered.
Hence for a 70 kg individual, a maximum 16 ml of 2 % Lignocaine mixed with 1:80,000 Adrenaline can be given.
If you read the instruction leaflet that comes with the local anesthesia vial, they mention the amount of local anesthesia present in 1 ml of solution.
Click here to read one such information leaflet.
So, if you read carefully, you don't have to calculate the dosage. It is already mentioned in the leaflet.
I hope that you found this article helpful.
Let me know in the comment section, whether you have experienced local anesthesia toxicity and how you managed it.
Also, do let me know what topics would you want me to write about next. The topics can be from local anesthesia, Oral & Maxillofacial Surgery or Medical emergencies.