Pain Pathophysiology Basics
· Noxious stimuli cause opening of cation channels on the membrane of neurons, allows sodium and other positive ions into cell.
· Extra positive ions going into the cell depolarize the cell causing nearby voltage gated sodium channels to open, triggering a chain reaction down length of the axon
· Local anesthetics inhibit conduction of action potentials in free nerve endings, can’t travel up the neuron, so patient won’t perceive the pain.
· Greater effect on nerves that are small and myelinated (usually are the nerves that transmit pain signals), whereas at larger doses can lose temperature, touch, pressure, and finally motor function.
Ropivacaine Dosing (UpToDate)
Adult
Dose varies with procedure, onset and depth of anesthesia desired, vascularity of tissues, duration of anesthesia, and condition of patient.
Major nerve block:
35 to 50 mL dose of 0.5% solution
Infiltration/minor nerve block:
1 to 100 mL dose of 0.2% solution
1 to 40 mL dose of 0.5% solution
Max dose of 3 mg/kg
Infants ≥6 months, Children, and Adolescents: Single Injection Peripheral nerve blocks/local anesthesia - Maximum dose of 3 mg/kg/dose based on lean body mass; dosing based on extrapolation from adult experience; additional data necessary to more clearly define the pediatric upper dose limit. Note: For infants <6 months of age, dose reductions (eg, by 30%) have been suggested by experts
Commonly accepted doses:
Head and neck blocks: 0.05 mL/kg.
Maxillary nerve: 0.15 mL/kg
Upper extremity blocks:
Brachial plexus: 0.2 to 0.3 mL/kg.
Digital nerve: ≤0.2% (2 mg/mL) solution: 0.05 mL/kg.
Truncal blocks:
Transversus abdominis plane: 0.2 to 0.5 mL/kg.
Ilioinguinal nerve: 0.075 mL/kg.
Rectus sheath: 0.1 to 0.2 mL/kg.
Lower extremity blocks:
Femoral nerve: 0.2% to 0.5% (2 to 5 mg/mL) solution: 0.2 to 0.4 mL/k
Sciatic nerve: 0.2 to 0.3 mL/kg.
Bupivacaine Dosing (UpToDate)
Adult
Local anesthesia: Infiltration: 0.25% infiltrated locally; maximum: 175 mg.
Peripheral nerve block: 5 mL of 0.25% or 0.5%; maximum: mg/day
Pediatrics
Dose should not exceed 2 mg/kg plain solution, or 3 mg/kg with epinephrine
For infants <6 months, maximum doses should be reduced by 30%
Commonly suggested doses:
Head and neck: 0.05 mL/kg.
Upper extremity:
Brachial plexus: 0.2 to 0.3 mL/kg.
Digital nerve: 0.05 mL/kg.
Truncal blocks:
Transversus abdominis plane: 0.2 to 0.5 mL/kg (Jöhr 2015).
Rectus sheath: 0.1 mL/kg.
Ilioinguinal: 0.075 mL/kg.
Lower extremity blocks:
Femoral nerve: 0.2 to 0.3 mL/kg.
Sciatic nerve: 0.2 to 0.3 mL/kg.
Adolescents: 0.25% or 0.5% solution with or without epinephrine: 5 mL; maximum daily dose: 400 mg/day (manufacturer's labeling).
Ropivacaine vs Bupivacaine
· A strict correlation exists between the lipid solubility of the local anesthetic and its potency and toxicity.
· Ropivacaine is less lipophilic than bupivacaine and is less likely to penetrate large, myelinated motor fibers, resulting in a relatively reduced motor blockade.
· The reduced lipophilicity is also associated with decreased potential for central nervous system toxicity and cardiotoxicity
· CNS symptoms (restlessness, anxiety, tinnitus, dizziness, blurred vision, tremors, depression, or drowsiness) may be early warning signs of CNS toxicity, and generally occur before cardiac symptoms.
· Bupivacaine is the most cardiotoxic of all the local anesthetics agents due to its strong affinity for the cardiac calcium channel and intravascular injection of bupivacaine may result in cardiovascular collapse prior to the onset of CNS toxicity.
· Cardiac function involving contractility, conduction time, and QRS prolongation found to be significantly less with ropivacaine than with bupivacaine.
· According to minimum local anesthetic concentration (MLAC) studies, which are based on effective analgesia in 50% of patients) ropivacaine has similar potency to bupivacaine at higher doses (eg, doses required for peripheral nerve blocks for surgical anesthesia), ropivacaine is less potent than bupivacaine and levobupivacaine at lower doses
· Peripheral nerve block: The long-acting sensory and motor block provided by ropivacaine is 0.5% or 0.75% for axillary, interscalene and subclavian perivascular brachial plexus block for hand or arm surgery compared favorably with bupivacaine 0.5% with a similar quality of regional anesthesia.
· In lower limb surgeries where sciatic or combined femoral and sciatic block was given for knee, ankle, or foot procedures, ropivacaine 0.75% (25 ml) had a significantly faster onset of sensory and motor block than 25 ml bupivacaine 0.5%. Although ropivacaine had a significantly shorter duration of sensory block, the duration of motor block remained similar with both agents
Conclusion
· Ropivacaine is a well-tolerated regional anesthetic.
· The efficacy of ropivacaine is like that of bupivacaine and for peripheral nerve blocks.
· Clinically adequate doses of ropivacaine has a reduced potential for CNS toxicity and cardiotoxicity, which makes this medication a great tool for regional anesthesia.
References
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3106379/
https://europepmc.org/article/med/2679230