Local Anesthetics part II Flashcards
Calculation of % solution Local with Epinephrine
Ratio strengths are used primarily for solutions: drugs in liquid form.
1: 1,000 is a ratio that represents the parts of solute (Drug) per parts of solvent (Diluent).
1: 1000 strength solution represents 1 part drug to 1000 parts diluent.
Understanding Percent solution to mg/mL (or cc)
Technically, mL is used for volume, but cc is exactly equal to mL so it is used interchangeably.
% concentration is based on g/100mL.
1g = 1000mg This means 1000mg/100mg = 10mg/mL
This is why you can either multiply by ten or move the decimal to the right to convert 1% to 10mg/mL (multiply 10 x 1 = 10).
convert 1% solution to mg/mL.
A 1% solution is the same as 1000 milligrams in 100 mL or 10mg/mL. (1000/100=10)
A 2% solution is 2g/100mL, or 2000mg/100mL. This reduces to 20mg/mL. (2000/100=20)
Percent solutions all are 1000mg/100mL. For example: 2% = 20mg/mL 5% = 50mg/mL 5.5% = 55mg/mL
Epinephrine Concentration
Epinephrine vials are labeled by concentration of a ratio of medication per mL. For example, a solution may be labeled as 1:100,000. This concentration represents 1000mg/100,000mL or 0.01mg/mL.
CONCENTRATION DOSAGE EQUIVALENCE %
1: 1,000 1mg/mL 0.1%
1: 10,000 0.1mg/mL 0.01%
1: 100,000 0.01mg/mL 0.001%
1: 200,000 0.005mg/mL 0.0005%
Convert 1:10,000 into mg/cc.
1:10,000 = 1000mg/10,000cc = 0.1mg/cc
Epinephrine: How many mg/mL in 1:1,000?
1mg/mL
Epinephrine: How many mg/mL in 1:100,000?
0.01mg/mL
How many micrograms (mcg)?
0.01 x 1000 = 10 mcg/mL
Lidocaine 1% Vial, how many mL do you need to get 40mg?
1% = 10mg/mL
40mg / 10mg/mL = 4mL
Bupivacaine 0.25% Vial, need 7.5mg dose to give, how many mL?
- 25% = 2.5mg/mL
7. 5mg / 2.5mg/mL = 3mL
Need Lidocaine 80mg for induction. Have 2% Lido Vial. How many mL do we need?
Lido 2% = 20mg/mL
80mg / 20mg/mL = 4mL
Calculate maximum dose of Bupivacaine for your patient Local Anesthetic. The patient is 80kg
Max Dose Bupivacaine:
Bupivacaine 2 mg/kg
Bupivacaine with epi 3 mg/kg
80kg x 2mg/kg = 160mg Maximum Dose Bupivacaine
Calculate maximum dose of Bupivacaine for your patient Local Anesthetic. The patient is 80kg and we will be adding epinephrine to the local.
Max Dose Bupivacaine:
Bupivacaine 2 mg/kg
Bupivacaine with epi 3 mg/kg
80kg x 3mg/kg = 240mg Maximum Dose Bupivacaine w epinephrine
Calculate maximum dose of Lidocaine for your patient Local Anesthetic. The patient is 70kg
Maximum Dose Lidocaine
Lidocaine 4-5 mg/kg (this is a range up to 5mg)
Lidocaine with epi 7 mg/kg
70kg x 4mg/kg = 280mg Maximum Dose Lidocaine
70kg x 5mg/kg = 350mg Maximum Dose Lidocaine
Calculate maximum dose of Lidocaine for your patient Local Anesthetic. The patient is 70kg and we will be adding epinephrine to the local.
Maximum Dose Lidocaine
Lidocaine 4-5 mg/kg
Lidocaine with epi 7 mg/kg
70kg x 7mg/kg = 490mg Maximum Dose Lidocaine w epinephrine
Lidocaine Max Dose
The 70 KG patient can safely receive: Lido Plain = 350 mg (5mg/kg) 0.5% = 70ml 1% = 35ml 2% = 17.5 ml Lido with EPI = 490 mg (7mg/Kg) 0.5% = 98ml 1% = 49ml 2% = 24.5ml
How many cc of 2% Lidocaine can I give to a 80KG Pt?
2% solution = 20mg/cc
Max dose Lido Plain = 5mg/kg
5mg/kg X 80kg pt. = 400mg max for this pt.
400mg divided by 20mg/kg = 20cc Lido 2%
Simple? be prepared to figure this out in the clinical area
Bupivacaine Maximum Dose
The 70 KG patient can safely receive: Bupivacaine Plain = 140mg (2mg/Kg) 0.25% = 56ml 0.5% = 28ml Bupivacaine with EPI = 210mg (3mg/Kg) 0.25% = 84ml 0.5% = 42ml
What is the concentration of a solution in % if 500mg is added to 100 ml of water
500 mg/100 ml=
5 mg/ml=
0.5%
What is pKa?
The pKa of a drug is the pH at which half of a drug is ionized and the other half is not ionized.
pKa of Thiopental is 7.4 which is exactly the pH of blood (which means half of drug is ionized and the other half is unionized).
Not all drugs have pKa of 7.4 and not all patients are healthy with a physiological pH of 7.4 (some are acidic or alkalotic).
This value has no relationship to the pH of the drug itself.
pKa is a chemical measure of the ratio between water and fat soluble phases of a molecule.
The charged or ionized form of a drug does not penetrate membranes well (the more ionized a local anesthetic, the slower it will penetrate a nerve).
Speed of onset = amount of Local Anesthetic in un-ionized form
Agents with lower pKa are more un-ionized at PH=7.4
The un-ionized form penetrates the lipid bilayer of the nerve axon membrane.
The more the LA is in the un-ionized form, the faster the molecules will penetrate the lipid bilayer of the nerve axon and the more quickly the conduction block will be established.
Un-ionized=Faster=low pKa
How is a known pKa relevant in practice?
When a local anesthetic is mixed in a solution of sodium bicarbonate (high pH), the onset of action will be faster because more of the injected drug is in un-ionized form
Injection of LA at the site of an infection, where pH is low, results in delayed onset of action (weak base+acid=more ionized)
Onset of Action of Local Anesthetics and ionization
Local anesthetics must diffuse through the axolemma before they can interact with receptors
The more ionized a local anesthetic, the slower it will penetrate a nerve.
Almost All Local anesthetics are bases.
Unionized Locals able to enter cell easier.
Physiologic pH & pKa
If a drug with a pKa is 7.4=50% of the drug will be unionized and 50% will be ionized
The unionized form crosses the membranes easier than ionized
Unionized is where the pharmacological activity comes from
What are two reasons it is important to determine the degree of ionization of a drug?
Ionized=water soluble=non-active
Unionized=lipid soluble=active (membrane allows drug to pass through cell membrane)
1. Ionized drugs are hydrophilic and are unable to cross the placental or BBB and are left in circulation
2. Non ionized drugs are lipophilic and are able to diffuse across the membranes (this is where all the pharmacological activity comes from)
What three things do you need to figure out whether or not a drug is ionized or non-ionized?
- Is the drug a weak acid or weak base?
- The pKa of the drug (nothing to do with acid or base)
- pH of the target solution (stomach/blood)
Weak Bases
-ions Chloride, Sulfate Lidocaine hydrochloride, locals Morphine Sulfate All opiods, benzos, ketamine
pH < pKa Ionized > Unionized
pH > pKa Ionized < Unionized
Weak Base
1.0——————————-7.4————————–12
Ionized Unionized
pH < pKa Ionized > Unionized
pH > pKa Ionized < Unionized
If you have a base with pKa of 7.2 at the physiological pH (7.4) is it more Unionized or Ionized?
Weak Base
1.0————–7.2—7.4——————————12
Ionized unionized
MORE UNIONIZED
pH > pKa Ionized < Unionized
If you have a base with a pKa 7.8 at pH 7.4, is it more ionized or unionized?
Weak base
1.0————————-7.4—-7.8——————12
ionized unionized
MORE IONIZED
pH < pKa Ionized > Unionized
Weak Acids
Join with + ions Na, Ca, Mg Sodium Pentobarbital Barbiturates Propofol pH < pKa Ionized < Unionized pH > pKa Ionized > Unionized (Acids are more ionized above the pH)
Weak Acid
1.0——————————-7.4————————–12
Unionized Ionized
pH < pKa Ionized < Unionized
pH > pKa Ionized > Unionized
Sodium Steve has a pKa of 4.5. Will this drug be more than 50% or less than 50% ionized at physiologicalph?
1.0————————-4.5—–7.4——————————-12
Unionized ionized
MORE IONIZED
Sodium is a positive molecule so it is a weak acid
A weak acid means that we have fewer hydrogen ions
The ionized form will dominate
pH > pKa Ionized > Unionized
