Type of Local Anesthetic: ester
Pharmacokinetics: rapid onset time and short-lived duration of action. Rapid onset time is thought to be secondary NOT to its PKa (remember, PKa is approximately 9 and thus it is more protonated/less likely to cross cell membrane at physiologic pH) but because it can be used in relatively high concentrations due to its low systemic toxicity.
Why its Safe in OB: It gets rapidly metabolized by ester hydrolysis by pseudocholinesterase (half-life, 45 seconds) therefore maternal systemic toxicity is rare and almost no drug crosses the placenta.
Why it’s Sometimes Preferred in OB: No drug crosses the placenta (see above) and it has a very quick speed of onset since it is used in high concentrations. Can establish appropriate epidural anesthesia for emergency C-section when an epidural is already in place.
Disadvantages: 1)Short duration of action (usually needs re-bolusing of epidural catheters every 30 minutes). 2)May interfere with the action of epidurally administered opioids because it antagonizes μ- and κ-opioid receptors. 3)Some reports state it may interfere with the action of subsequent epidural bupivacaine. 4)Previous preparations were considered to be neurotoxic because some patients developed arachnoiditis after accidental subarachnoid injection (thought to be secondary to the chelation of calcium by additives, along with the preservatives used for stabilization). Subsequent removal of EDTA, metabisulfite and methylparaben has eliminated this risk yet caution is still advised because the former preparations are still commercially available. Onset: 6-12 min, peak 10-20 min, duration 30-60 min