Lidocaine, Bupivacaine and Ropivacaine

Mechanism

⚡ Lidocaine

• pKa: 7.9 (fast acting latency of 2 - 5 minutes).
• Power: Moderate.
• It has antiarrhythmic (Class Ib) and anti-inflammatory properties when administered as a systemic intravenous infusion.

💉 Bupivacaine

• pKa: 8.1 (slow-acting latency of 15 - 20 minutes).
• Power: High analgesic power and long duration (180 - 400 minutes).
• High intrinsic myocardial cardiotoxicity due to blockade of slow conduction channels.

Structural and Power Differentiation

Ropivacaine is a pure pure enantiomer (S-enantiomer) specifically designed to offer similar analgesic potency and duration as conventional racemic bupivacaine, but with superior differential sensory-motor blockade selectivity and a much higher cardiotoxicity threshold, minimizing unwanted motor blockade effects in obstetric labor analgesia.

Security

Local Anesthetic Systemic Toxicity (LAST)

Excessive systemic absorption or inadvertent intravascular injection of an amine local anesthetic (especially bupivacaine) can trigger the potentially fatal condition of Local Anesthetic Systemic Toxicity (LAST).

Symptoms of initial neurotoxicity: metallic taste in the mouth, perioral paresthesias, tinnitus, dysarthria, progressive dizziness that progresses to generalized tonic-clonic seizures and refractory coma. Symptoms of severe cardiotoxicity: extreme bradycardia, severe AV block, widening of the QRS complex, prolongation of PR conduction, intractable ventricular arrhythmias and asystolic circulatory collapse. Bupivacaine slowly dissociates from cardiac sodium channels in systole, facilitating refractory cardiodepression.

Lipid Rescue Protocol with 20% Intravenous Emulsion

In the event of clinical suspicion of LAST with severe hemodynamic compromise or refractory malignant arrhythmias, the Lipid Rescue Therapy) protocol should be activated as a priorityimmediately:

1. Intravenous bolus of 20% lipid emulsion of 1.5 mL/kg administered in 1 minute.
2. Start a continuous intravenous infusion of 20% lipid emulsion at a rate of 0.25 mL/kg/min.
3. If hemodynamic collapse persists, the intravenous bolus of lipid emulsion can be repeated up to a maximum of two additional times every 3-5 minutes and the infusion rate increased to 0.5 mL/kg/min.
4. The lipid emulsion acts as an intravascular "lipid sink" that extracts lipophilic local anesthetics from the target tissues (heart and brain), accelerating their inert plasmatic redistribution.