Intro to Regional Anesthesia - PM Flashcards
Local Anesthetics are?
Drugs that reversibly bind to voltage-gated sodium channels (NAv)
NAv channels have what three functional states?
resting (closed), open, inactive
Symptoms of toxicity can occur from any route of administration, but what is the most prominent method?
Inadvertent intravascular injection
What is the antidote for local anesthetics?
Time, there is no reversal agent
What are the two intermediate carbon groups?
Esters and amides
What is responsible for the classification of Local Anesthetic?
the intermediate linkage
What are the 5 Esters?
Procaine, Chloroprocain, Tetracaine, Cocaine, Benzocaine
What are the Amides?
Lidocaine, Mepivacaine, Prilocaine, Bupivacaine, Ropivacaine, Articaine. (They all have two i’s)
Lipophilic
means fat loving. Wants to bind to lipids.
Hydrophobic
means fear of water.
In general, duration of action is mostly correlated with?
Protein binding, so the more protein bound a LA is, the longer the duration of action.
In general, what correlates most with potency?
Lipid solubility. A LA’s high lipid solubility equals a high level of potency.
pKa
expresses relationship between uncharged (non-ionized & lipid soluble) and charged (ionized & water soluble) drug concentrations. The drug specific pKa is the pH at which the concentrations are 50/50.
LA’s typically have what kind of base?
Weak (pka’s higher than 7.45) (Lower than 7.35 would be a strong base)
What does adding a vasoconstrictor do to a LA?
hint - 5 things
- Limits spread of agent
- longer duration of effect
- Reduces systemic absorption
- Hemostasis for improved visulization
- Possibility of using a smaller dose of LA overall
What are potential negative effects of using a vasoconstrictor and a LA together?
Tissue ischemia leading to necrosis in end organ
Use of Epi with an LA is contraindicated in/on?
nose, fingers, toes, or penis, and should be avoided in avulsed flaps and areas with tenuous blood supply.
What is the reversal of epi if injected into contraindicated tissue?
phentolamine (1.5-5mg)
LA prepackaged with epi are usually indicated with what?
Red top and red lettering on label
What does buffering an unionized drug with Sodium Bicarb do?
Speeds its penetration to the nerve tissue
What are the LA’s that possess mild vasoconstrictive properties?
Ropivacaine and Lidocaine
Most LA’s produce 1._____ that results in increased 2. _____ which can lead to an increase in LA 3.______ . This in turn shortens the duration of 4._____ and increased the probability of 5. ____ effects.
- relaxation of vascular smooth muscles
- vasodilation and increased blood flow
- absorption
- action
- toxic
Cocaine (in a clinical setting) is only administered in what way?
topically
What tissue receives the greatest amount of LA from distribution?
muscle tissue (acts as a depot due to greater tissue mass)
Primary method of metabolism for Esters?
ester hydrolysis - which occurs through the action of esterase in plasma, RBC’s, and liver. Primarily, plasma cholinesterase rapidly metabolizes esters.
Primary method of metabolism for amides?
In the Liver by microsomal cytochrome p-450 enzymes. (dependent on hepatic blood flow and p-450)
LA’s exert their action by inhibiting what?
The passage of NA ions through nerve membranes.
How do LA’s prevent normal function of afferent neurons?
by stabilizing the excitable tissue in the region of application.
What does LAST stand for?
Local Anesthetic Systemic Toxicity
Most common symptom of LAST (life threatening)
Seizures
Most common cause of LAST
inadvertent vascular injection
Signs of CNS toxicity
mild effects: tongue numbess apprehension restlessness peri-oral tingling tremors slurred speech, drowsiness cardiorespiratory depression of course seizures Coma & Death
What concentration of lidocaine plasma would you see circumoral numbness?
5 mcg/ml (from lecture, possible test question)
Signs of toxicity Cardiovascular
palpitations, HTN, ventricular dysrhythmia, myocardial depression, bradycardia, hypotension, cardiovascular collapse.
Signs of toxicity on Peripheral vascular system (PVS)
hint: Biphasic
Low concentrations, LA produce vasoconstriction and an increase in SVR.
As doses increase, vasodilation begins to occur resulting in significant hypotension
If your injecting a patient with LA’s, how long should you be watching them?
Should be watching them for at least 10 minutes, but most s/s of toxicity will occur within the first 5 minutes of administration.
True allergies are common or rare?
Rare
Which is more likely to cause an allergic reaction, ester or amides?
Why?
Esters
Due to preservative para-aminobenzoic acid (PABA)
Toxicity-Methemoglobinemia
Prilocaine (Citanest)
the metabolite o-toluidine oxidies hemoglobin to methemoglobin.
Results in tachypnea, metabolic acidosis, and brownish-gray cyanosis.
How is Toxicity-Methemoglobinemia reversed?
Spontaneus reversal over 2 to 3 hours, or by administration of methylene blue (1mg/kg)
What is the “vacuum cleaner” drug?
Intrallipid - bolus 20% 1.5ml/kg over 1 minute. Is a reversal agent for signs/symptoms of toxicity.
What effect does LA toxicity have on a patients electrical function of the heart?
Increases PR interval and duration of QRS complex. Progresses to sinus bradycardia, complete AV block, and/or cardiac arrest.
