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 approach is supported by the most recent and highest quality study, which emphasizes the importance of using lean body weight for drug dosing in obese patients to avoid relative overdose 1.

Key Considerations

• Local anesthetics are primarily distributed in lean tissues, not adipose tissue, so morbidly obese patients do not require proportionally higher doses.
• Using total body weight for dosing calculations could lead to toxic levels in the bloodstream, potentially causing serious complications like seizures or cardiac arrhythmias.
• Practitioners should monitor these patients closely for signs of toxicity and consider using ultrasound guidance for more precise administration.
• Lipid emulsion therapy should be available as a rescue measure for local anesthetic systemic toxicity.

Dosing Guidelines

• For lidocaine, the maximum safe dose should be calculated based on lean body weight, with a recommended maximum dose of 4.5 mg/kg LBW.
• When using other local anesthetics like bupivacaine, similar principles apply with appropriate dose adjustments.
• The use of multimodal, opioid-sparing analgesia approaches is recommended to improve postoperative recovery, with techniques such as regional anaesthetic techniques, infiltration of local anesthetics, and nonsteroidal anti-inflammatory drugs (NSAIDs) being effective in reducing opioid requirements 1.

Additional Recommendations

• Anaesthesia induction should preferably be based on lean body weight to avoid hypotension.
• Classic target-controlled infusion (TCI) models have poor predictive ability when used in patients with obesity, and alternative approaches should be considered.
• The decision regarding which inhalational agent to use should be determined based on existing comorbidities and other related factors.

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.