Rethinking the Boundaries of Local Anesthetic Systemic Toxicity in the Era of Sustained-Release Formulations

Abstract

Sustained-release local anesthetic formulations introduce prolonged and dynamic exposure patterns that challenge current frameworks for assessing local anesthetic systemic toxicity (LAST). Recent pharmacovigilance findings, pharmacokinetic evidence, and reports of delayed and recurrent LAST further support the need to re-evaluate LAST across three conceptual boundaries. First, the exposure boundary challenges the use of recommended maximum doses as stand-alone safety thresholds. Co-administration, formulation stability, and the local tissue microenvironment may influence drug release and systemic exposure. Second, the temporal boundary emphasizes that a lower initial peak plasma concentration does not necessarily imply a shorter window of vulnerability, particularly when systemic exposure is prolonged or follows a bimodal pattern. Third, the resuscitation endpoint boundary highlights that return of spontaneous circulation (ROSC) does not necessarily indicate resolution of the underlying toxicological process, as hemodynamic instability may recur after initial ROSC. Assessment of LAST associated with sustained-release formulations should not be limited to administered dose or peak plasma concentration but should consider the entire exposure trajectory. Future guidelines should also define long-term monitoring windows, specific triggers for continuous infusion of lipid emulsion, and safe upper limits for lipid emulsion dosing.

Chen et al recently reported a real-world pharmacovigilance analysis of adverse events associated with liposomal bupivacaine (LB) and bupivacaine using the FDA Adverse Event Reporting System.¹ Their study identified a broad spectrum of adverse event signals associated with these formulations and underscored the importance of continued surveillance after LB administration. These findings raise an important clinical question: in the era of sustained-release local anesthetic formulations, are current frameworks for assessing local anesthetic systemic toxicity (LAST) sufficient to capture evolving drug exposure patterns and temporal characteristics associated with sustained-release formulations?

This question has been further illustrated by Belfar et al, who described an instructive and unsettling case of LAST presenting as a refractory cardiac arrest followed by a protracted, fluctuating course of hemodynamic instability after subcutaneous infiltration of LB.² This unusual clinical scenario required an extraordinarily high total dose of lipid emulsion to manage such recurrent cardiovascular collapse.

Unlike plain bupivacaine, the liposomal formulation utilizes DepoFoam multivesicular liposome technology to gradually release the encapsulated bupivacaine over several days.³ While this design offers prolonged analgesia, it also introduces a distinct exposure profile that may not be fully characterized by traditional models based primarily on administered dose or peak plasma concentration. Importantly, delayed and recurrent LAST have been increasingly recognized as important features of local anesthetic toxicity, rather than isolated or unexpected events. Gitman and Barrington demonstrated in 2018 that approximately 25% of reported LAST cases occurred more than 1 hour after surgery, including cases of mortality and recurrent toxicity.⁴ Macfarlane et al further emphasized the heterogeneous temporal profile of LAST, reporting that delayed presentations beyond 1 hour after surgery accounted for approximately 20% of cases and were associated with severe outcomes, including mortality.⁵ More recently, Fettiplace et al described the changing landscape of local anesthetic-associated morbidity and mortality, indicating that serious adverse outcomes continue to occur despite advances in prevention and treatment strategies.⁶ Given these dynamic exposure patterns, sustained-release formulations challenge us to re-evaluate LAST across three conceptual boundaries.

First, the exposure boundary: the recommended maximum dose should no longer be regarded as a stand-alone safety threshold. As highlighted by Gadsden and Long, clinicians frequently admix or co-administer plain bupivacaine with LB to bridge the analgesic gap, relying on the assumption that total systemic exposure remains predictable and within a safe ceiling.³ However, in the dynamic real-world surgical site, the total local anesthetic burden, formulation physical stability, and patient-specific microenvironment interweave, rendering simple arithmetic dose summation potentially misleading.

Second, the temporal boundary: a lower initial peak plasma concentration (C_max) does not equate to a shorter window of vulnerability. Bungart et al demonstrated that prolonged local anesthetic exposure through truncal catheter techniques may result in delayed systemic accumulation, with reported peak blood concentrations and toxicity occurring 33–60 hours after infusion initiation.⁷ These findings indicate that exposure duration and trajectory, rather than peak concentration alone, are critical determinants of LAST risk. Sustained-release formulations further emphasize the importance of this exposure trajectory. Pharmacokinetic data summarized by Gadsden and Long demonstrate that LB exhibits a distinct bimodal release profile, with an initial minor peak occurring within the first hour and a delayed systemic exposure peak emerging approximately 12–36 hours post-dose.³ Furthermore, this delayed release is heavily accelerated by tissue vascularity; in highly vascularized surgical sites, more than 90% of the total dose is released within the first 24 hours.³ This prolonged and dynamic exposure profile highlights the limitations of risk assessment frameworks based solely on early C_max measurements. In the era of sustained-release formulations, considering the entire exposure trajectory rather than a single pharmacokinetic parameter may provide a more comprehensive framework for interpreting local anesthetic safety.

Third, the resuscitation endpoint boundary: return of spontaneous circulation (ROSC) marks the termination of an acute arrest, yet it potentially obscures the ongoing, underlying toxicological process. This disrupted resuscitation endpoint boundary is particularly treacherous when sustained-release formulations interact with the kinetics of lipid rescue. Belfar et al described recurrent hemodynamic instability 30 minutes after initial ROSC, necessitating escalation of the lipid emulsion infusion rate and a prolonged maintenance infusion.² This clinical phenomenon closely echoes established experimental evidence, which demonstrated that initial successful lipid resuscitation from bupivacaine toxicity was frequently plagued by a high incidence of recurrent asystole and fatal secondary collapse.⁸

From a pharmacokinetic perspective, static pharmacokinetic models often predict that even combined maximum doses of LB and plain bupivacaine yield a theoretical combined C_max (approximately 1300 ng/mL) well below the often-cited toxic threshold of 2000 ng/mL.^3,9,10 Yet, Belfar’s case challenges this mathematical complacency, suggesting that real-world tissue microenvironments can prompt aberrant, massive drug release that drives systemic levels into the toxic zone.² While lipid emulsions act as a temporary “lipid sink” to rapidly sequester bupivacaine from myocardial tissue, the subsequent intravascular metabolism or physical degradation of these lipid droplets prematurely re-releases the trapped local anesthetic back into the circulation.⁸ This endogenous re-release from degraded lipid carriers can catastrophically overlap with either the delayed second peak absorption or a localized, vascularity-driven fast-release surge from the LB depot.³ The heart is subjected to a secondary, wave-like re-toxicification. Consequently, brief circulatory recovery can be misleadingly stable, masking an ongoing cellular threat, and supports the need for a from bolus-and-stop to a prolonged, titratable lipid maintenance strategy.

Therefore, we argue that effectively managing LAST in the era of sustained-release formulations requires more than simply applying traditional algorithms developed for plain local anesthetics. We urgently need robust, consensus-driven guidelines that delineate long-term monitoring windows, define specific triggers for continuous lipid infusions, and establish safe lipid dosage ceilings for sustained-release toxicity. Until then, clinicians should remain vigilant: with liposomal formulations, ROSC is not the finish line—it is merely the intermission.

Disclosure

The authors declare that they have no conflicts of interest.