The effect of lipid emulsion infusion on postmortem ropivacaine concentrations in swine: endeavoring to comprehend a soldier's death.

Background: Lipid emulsion (20%) is advocated as a rescue drug for local anesthetic toxicity. No study has measured the impact of lipid emulsion therapy on postmortem local anesthetic serum levels.

Methods: We anesthetized Yorkshire swine (n = 11) and standard monitors were placed. The swine received 1.5 mg/kg/min IV ropivacaine until death (asystole). Blood samples were drawn before infusion (baseline) and at 5-minute intervals during the infusion for measurement of blood gases and free, bound, and total serum ropivacaine concentrations via high-performance liquid chromatography. Five swine received ropivacaine only, and 6 swine received ropivacaine plus a single bolus dose of 20% lipid emulsion (1 mg/kg) when the mean arterial blood pressure reached 50 mm Hg. Ropivacaine infusions were terminated at asystole and no resuscitation was initiated. Total ropivacaine dose and time to death were recorded. The swine were cooled (mean temperature, 25.5°C ± 0.8°C at 6 hours postmortem) to reflect morgue conditions. Serum samples were drawn at asystole, 1, 3, and 6 hours postmortem for analysis. Additionally, a craniotomy and laparotomy were performed at those times to remove 1.5 to 3 g each of brain, lung, liver, kidney, and muscle for analysis.

Results: Analysis of the postmortem serum ropivacaine concentrations in the control and the lipid-treated animals indicated that both the total (bound and not bound to proteins) and free (not bound to proteins) ropivacaine concentrations were significantly higher in the lipid-treated animals (P = 0.0094 and P = 0.0063, respectively). Furthermore, time had a significant effect on increasing the postmortem free ropivacaine concentrations (P = 0.0095). The lipid group had a statistically significant earlier onset of death (asystole) compared with the control group (P = 0.0274). Tissue analysis indicated that the ropivacaine concentration significantly decreased postmortem in the lung, kidney, and brain tissues of the lipid-treated animals (P = 0.0168, P = 0.0073, and P = 0.0018, respectively). Tissue drug concentrations in the control animals remained unchanged after death.

Conclusions: Our data show that postmortem blood samples in swine that experience local anesthetic cardiovascular collapse and are treated with lipid emulsions will result in measurements that cannot be directly extrapolated to premortem drug concentrations.