Anesthesia Pharmacology: Local Anesthesics
Vasocontrictors added to local anesthetic solutions
Purpose of adding epinephrine to a local anesthetic solution: Increased duration: addition of epinephrine (1:2000,000 or 5 μg/ml) to cause vasoconstriction
Reduced systemic absorption
Higher anesthetic concentration near nerve fibers
Prolongation of conduction blockade (about 50% longer)
Reduced systemic absorption (by about 33%)
Factors influencing the effectiveness of epinephrine on local anesthesia:
Which anesthetic is used:
Reduced effect of epinephrine on duration of conduction blockade and systemic absorption comparing mepivacaine (Carbocaine) and etidocaine (Duranest) [these are more lipophilic] with lidocaine (Xylocaine), which is less lipophilic
More lipophilic local anesthetics will tend on their own to associate strongly with tissues
Level of sensory blockade needed for spinal or epidural anesthesia
Duration of lower extremity sensory anesthesia extended by epinephrine or phenylephrine (Neo-Synephrine); this effect is not observed for abdominal region anesthesia
Conditions: epinephrine (0.2 mg) or phenylephrine (Neo-Synephrine) (2 mg) added to mepivacaine (Carbocaine) or lidocaine (Xylocaine) -- placed in subarachnoid spaces
Spinal anesthesia e.g. tetracaine (pontocaine). The duration of action is extended by vasoconstrictors
Condition 1: epinephrine plus tetracaine (pontocaine) (6 mg, low-dose)- result: increase in spinal anesthesia success rate
Condition 2: epinephrine plus tetracaine (pontocaine) (10 mg, higher dose)- result: no increase in spinal anesthesia success rate
Mechanism of vasoconstrictor mediated prolongation of local anesthetic action (no effect by epinephrine on time-to-onset):
Decreased systemic absorption
Increased local anesthetic concentration in the vicinity of sensory nerves
Increased uptake of the local anesthetic
Advantages of reduced local anesthetic systemic absorption:
Local anesthetic absorption rate more likely to match metabolic rate resulting in less local anesthetic systemic toxicity.
Toxicities associated with systemic epinephrine absorption:
Local anesthetic + epinephrine: increased cardiac irritability which may cause an increased risk of cardiac arrhythmias.
Increased possibility of hypertensive response in susceptible patients.
Other agents which may be added to local anesthetics that affect duration of action.
Dextran (low-molecular-weight) when added to local anesthetic solutions result in increased peripheral nerve block anesthesia duration, probably due to a reduction in local anesthetic systemic absorption.