VOLUME 36, ISSUE 1

As peripheral nerve blocks are increasingly being used to manage perioperative pain, it is important that anesthesiologists exercise persistent vigilance when dosing local anesthetics. This is especially true in patients with extreme low body weight and muscle mass. The failure to appreciate this risk factor and adjust the local anesthetic dose accordingly can easily lead to delay in recognition and treatment of local anesthetic systemic toxicity (LAST). The 2017 Executive Summary of the Third American Society of Regional Anesthesia and Pain Medicine Practice Advisory on Local Anesthetic Systemic Toxicity states that “Prevention is the primary and preferred mechanism for reducing the frequency and severity of LAST and meticulous attention to detail is the most important aspect of prevention.”1

Prevention involves three key facets, the first two of which tend to receive the most attention: prevention or identification of direct intravascular injection and reducing systemic uptake from tissues. The third facet is often overlooked, namely identification of patients at higher risk for LAST, with the two highest risk patient categories being those at extremes of age and those with low muscle mass. Low muscle mass is an independent risk factor for LAST because muscle is usually an important reservoir for local anesthetic storage when high volumes are absorbed.2 In the absence of this reservoir (as seen in patients with severely low muscle mass), patients are much more susceptible to LAST. Anecdotally, anesthesiologists who manage only adult patients may overlook adjusting doses for extreme low body weight, failing to recognize that some adults may be closer in weight to many of our pediatric patients.

The safety margin of local anesthetics is determined by the combination of the concentration and volume that yields the maximum safe dose. When neurologic symptoms occur following administration of large doses of local anesthetics, LAST should be the suspected diagnosis until proven otherwise. The importance of lipid emulsion in the emergency treatment algorithm for LAST does not override the fact that prevention is the primary goal. When weight-based local anesthetic dosing is used and the increased risk of LAST is fully appreciated in this unique patient population, safe administration is much more likely.

Similarly, it is important for us to also be mindful of our non-anesthesiology colleagues who may incorporate the use of local anesthetic in the surgical field and inform them of the need for weight-based dosing in our patients who are severely low weight (less than 60kg). A recent publication in a cardiology journal described a 39 kg adult patient undergoing a procedure. When she continued to have discomfort, additional local anesthetic in the surgical field was administered by the proceduralist and, eventually, the cumulative dose was 3.5 times the toxic dose, leading to LAST with cardiac and neurologic manifestations.3

Consideration of weight-based dosing when the patient is less than 60kg should be the first step in prevention since it is the only modifiable factor for which the anesthesiologist has complete control. Underlying severe cardiac comorbidities are commonly present and may prevent the use of an intravascular marker to serve as a harbinger of impending toxicity, thus reducing the ability to immediately identify intravascular injection. There are multiple peripheral nerve blocks performed by anesthesiologists that facilitate intraoperative and postoperative analgesia. Many of these blocks require large volumes of concentrated local anesthetic medication injected near or within tissue that has high uptake of local anesthetic medication. For these reasons, recognition of patients at high risk for LAST and prevention of excessive local anesthetic administration is paramount for patient safety.

In summary, it is important to recognize that prevention of LAST is the most important method to reduce the incidence of this potentially devastating complication. Properly identifying patients with extremely low weight and muscle mass is an important concept that we need to remember for ourselves. However, it is an area that we must also bring to the attention of our colleagues in other specialties who may have less familiarity with the dangers of LAST.