What is the recommended approach for dosing local anesthetics, such as lidocaine, in morbidly obese patients?

From the Guidelines

The recommended approach for dosing local anesthetics in morbidly obese patients is to calculate the dose based on lean body weight (LBW) rather than total body weight (TBW). This is supported by the most recent and highest quality study, which emphasizes the importance of using lean or adjusted body weight as the scalar for drug dosing in obese patients 1. The study highlights that using total body weight for dosing calculations can increase the risk of relative overdose, and that most anaesthetic agents should be dosed to effect rather than to total body weight.

When administering local anesthetics like lidocaine, the maximum safe dose should be calculated based on lean body weight, with a recommended dose of 4.5 mg/kg LBW, not exceeding 300 mg total for plain lidocaine or 500 mg for lidocaine with epinephrine in a single administration. Similar principles apply to other local anesthetics like bupivacaine, with appropriate dose adjustments (maximum 2 mg/kg LBW, not exceeding 175 mg total).

Some key points to consider when dosing local anesthetics in morbidly obese patients include:

• Using lean body weight as the scalar for drug dosing to avoid relative overdose
• Dosing to effect rather than to total body weight
• Monitoring for signs of toxicity and having lipid emulsion therapy available as a rescue measure
• Considering the use of ultrasound guidance for more precise administration
• Using multimodal, opioid-sparing analgesia approaches to improve postoperative recovery, as recommended by the ERAS society 1

It's also important to note that regional anaesthetic techniques, such as epidural analgesia and transversus abdominis plane block, can be highly efficient in reducing opioid requirements and improving postoperative recovery in morbidly obese patients 1. Overall, a careful and individualized approach to dosing local anesthetics in morbidly obese patients is necessary to minimize the risk of complications and optimize outcomes.

From the Research

Dosing of Local Anesthetics in Morbidly Obese Patients

• The dosing of local anesthetics, such as lidocaine, in morbidly obese patients requires careful consideration of pharmacokinetic and pharmacodynamic differences compared to non-obese individuals 2.
• Using total body weight to calculate drug doses can result in overdose, while using ideal body weight may lead to sub-therapeutic doses 2.
• Lean body weight is considered the optimal dosing scalar for most anesthetic drugs, including opioids and anesthetic induction agents 2.

Specific Dosing Recommendations for Lidocaine

• A study on dosing optimization of intravenous lidocaine in patients with class 1-3 obesity recommended a loading dose of 2 mg/kg over 20 minutes, followed by an infusion of 3 mg/kg/h, based on lean body weight 3.
• The infusion rate should be reduced to 2 mg/kg/h after 80 minutes to achieve therapeutic plasma concentrations 3.
• Another study on airway topicalization in morbidly obese patients using atomized lidocaine found that a 2% dose provided acceptable intubating conditions with lower plasma lidocaine levels compared to a 4% dose 4.

General Principles for Dosing in Morbidly Obese Patients

• A comprehensive literature review on weight-based antimicrobial dosing in morbidly obese patients highlighted the importance of considering pharmacokinetic changes, such as increased volume of distribution and altered hepatic metabolism 5.
• The review recommended using ideal body weight, adjusted body weight, or actual body weight when calculating antimicrobial doses, depending on the specific drug and patient characteristics 5.
• A randomized controlled pilot study on the effect of intraperitoneal local anesthesia on enhanced recovery outcomes after bariatric surgery found no significant differences in pain scores or analgesic consumption between patients receiving local anesthesia and those receiving normal saline 6.