Local Anesthetic Systemic Toxicity in Obstetrics (LAST)

The obstetric mechanism is the catheter - Inadvertent intravascular injection is the most common cause of LAST, and in obstetrics that means an epidural catheter. LAST is rare from epidural blockade at appropriate doses; the danger is the top-up given without aspirating a catheter that has migrated.

Aspirate and test dose every time - A heart rate rise of about 10 bpm, perioral numbness, tinnitus, metallic taste, or dysarthria points intravascular; rapid dense motor block points intrathecal. If you would rather not give epinephrine to a parturient, the double test dose avoids it — two 5 mL boluses of plain 2% lidocaine 3 to 5 minutes apart, checking for motor block after the first and a sensory level after the second, with a neurologic check after each.

Dilute regimens lower the load - Labor infusions of dilute bupivacaine or ropivacaine with a low-dose lipophilic opioid cut total local anesthetic exposure and reduce the risk of both LAST and a high or total spinal, while minimizing placental transfer.

Bupivacaine concentration limits - Concentrations above 0.5% are contraindicated for anything other than spinal anesthesia — those are the ones tied to toxicity and cardiac arrest. Bupivacaine paracervical block is contraindicated in obstetrics: it causes uterine vasoconstriction and reduces fetal blood flow, an effect not seen with epidural or spinal.

Watch the window - Half of LAST events present within 50 seconds of injection and 75% within 5 minutes. After a potentially toxic dose, observe at least 30 minutes. Do not walk out after a bolus.

First moves are airway and oxygen - Ventilation and oxygenation come before anything else; hypoxia and acidosis both accelerate the toxicity, and the parturient desaturates fast. Benzodiazepines for seizures. If convulsions will not stop, small intermittent doses of succinylcholine halt the muscle activity and the acidosis it generates. Propofol or thiopental are fallbacks but will worsen hypotension and cardiac depression.

Modified ACLS - Epinephrine in small doses, 1 mcg/kg or less, per the 2020 ASRA guidance. Skip vasopressin — it has caused pulmonary hemorrhage. No calcium channel blockers or beta blockers. Amiodarone for ventricular arrhythmias; lidocaine and procainamide add to the toxicity.

Lipid emulsion, early - 20% lipid emulsion 1.5 mL/kg bolus, then 0.25 mL/kg/min, continued 10 minutes past hemodynamic stability. Repeat the bolus and raise the infusion to 0.5 mL/kg/min if she is still unstable; initial maximum is roughly 10 mL/kg over 30 minutes. Give it as soon as prolonged seizure activity or local anesthetic arrhythmias are suspected, and before abandoning resuscitation — it has been used successfully in term pregnancy.

Know where your lipid is - Lipid emulsion is stocked on the unit in about 88% of US obstetric units, which means it is somewhere else in the rest. Find yours before you need it; access times of more than 30 minutes have been reported.

Left uterine displacement through the whole resuscitation - Aortocaval compression will defeat CPR. Manual left uterine displacement throughout, and if the fundus is at or above the umbilicus with no ROSC, perimortem cesarean delivery at 4 minutes.

Put the checklist on the wall - CRNAs with immediate access to written or electronic LAST guidelines score significantly higher on management knowledge, and the most common gap is the epinephrine dose — over half did not know it. Post the current ASRA checklist where the epidural carts live and assign someone to read it aloud when it happens.

Local anesthetic systemic toxicity (LAST) in the parturient begins the same way it does anywhere else — a plasma level spike that blocks voltage-gated sodium channels in brain and heart — but pregnancy stacks the deck. Increased cardiac output speeds systemic absorption and drives higher plasma concentrations from the same dose, and pregnancy is listed alongside hepatic dysfunction, cardiac disease, and the extremes of age as a comorbidity that raises the risk. Engorged epidural veins raise the odds of intravascular catheter placement. Acidosis, hypoxia, and hypercarbia — all readily available in labor — worsen toxicity.

Bupivacaine, the obstetric workhorse, binds inactivated cardiac sodium channels fast and releases slowly, accumulating across heart rates of 60 to 150. With the more potent agents, cardiovascular collapse can accompany or precede the seizure rather than follow it.