Local Anesthetics Flashcards
What is the general effect of all local anesthetics?
Local anesthetics bind to sodium channels in nerves to block nerve transmission
What kinds of nerves are easiest to block?
- Myelinated
- Small
- High firing frequency
What is the function of Aδ nerves?
Pain
Cold temprature
What is the function of Aβ nerves?
Tactile
Proprioception
What is the function of Aα nerves?
Motor
What is the function of B nerves?
Preganglionic Sympathetic
What is the function of C nerves?
Visceral Pain
What is the order in terms of easiest to hardest nerves to block in the type A nerves?
δ > β > α
In terms of the nerves overall, what are the easiest to hardest nerves to block?
B > A > C
What is the onset time of LAs dependent upon?
pH and pKa
High pH will increase the speed of onset.
What is the potency of LAs dependent upon?
Lipid solubility
As lipid solubility increases the potency of the LA will increase.
What is the duration of effect of LAs dependent upon?
Local protein binding NOT plasma binding
Increased protein binding will mean longer duration of action.
What can be added to LAs to increase the speed of onset?
NaHCO3
What can be added to LAs to localize them to the area?
EPI used to vasoconstrict to keep the drugs locally to the injection area
Is EPI useful in localizing LAs that are highly protein bound?
No because vasoconstriction would not be useful for localization when proteins are already functioning in that role
What Na+ gates are open at rest?
h open
m closed
What Na+ gates are open at activation?
h open
m open
What Na+ gates are open at inactivation?
m open
h closed
What is the function of A𝛄 nerves?
Muscle tone
What are the first signs of spinal/epidural anesthesia?
Sympathectomy>sensory>motor
What are the first signs of peripheral anesthesia?
First sign may be lack of proximal muscle coordination - finger to nose
What are the LA esters?
Cocaine
Procaine
Tetracaine
Benzocaine
What are the LA amides?
Lidocaine
Bupivicaine
Where does binding occur on the Na receptor?
Binding occurs on the cytoplasmic (inside) side of the receptor so LA must traverse the membrane. Only the uncharged can cross but the charged form is the active form.
What are some contraindications to LA use?
Bacteremia
Pre-existing neurologic disease
Abnormal coagulation
Where is LA administered for spinal?
Needle inserted at L3 - L5 and the drug is deposited around cauda equina
Where is LA administered for epidural?
Any level is possible and the site of action is the nerve roots
What are the respiratory effects of LA at the thoracic level?
Minimal reduction in vital capacity with abdominal muscle
paralysis but the loss of proprioception can be upsetting to patient as they cannot feel their own breathing
What are the respiratory effects of LA at the high spinal level?
BP will be reduced due to B type nerve block that causes the brainstem to trigger suppression of respiration.
NEVER due to phrenic nerve paralysis
How is respiratory arrest due to LA effects treated?
Vasopressors and supportive ventilation
What are the cardiovascular effects of neuraxial LA?
Arterial dilation and decrease in BP and decrease in HR due to decreased filling.
How are LA esters eliminated?
Plasma cholinesterase
How are LA amides eliminated?
Liver via cytochrome P450
What is the toxicity of LA on neurons?
Can inhibit inhibitory neurons that will lead to increased excitatory neuron action
How are LA induced seizures treated?
Benzodiazepines
Succinylcholine
How is LA toxicity prevention performed?
Fractionated injection with intermittent aspiration to make sure you are not in a blood vessel
Use of epinephrine as intravascular marker
What is a treatment for LA toxicity?
Intralipid. It is lipophilic and creates a “sink” that LA can be sequestered into
What is a common side effect of neuraxial anesthesia?
Postdural puncture headache (PDPH) is a common complication of neuraxial anesthesia its incidence is influenced by many factors
