From the Guidelines
Local anesthetic dosing in morbidly obese patients should be based on ideal body weight (IBW) rather than total body weight (TBW) to avoid toxicity. For example, when using lidocaine, the maximum safe dose would be calculated as 4.5 mg/kg of IBW, not TBW. This approach applies to all commonly used local anesthetics including bupivacaine, ropivacaine, and mepivacaine. When performing regional anesthesia in these patients, consider using ultrasound guidance to reduce the volume of local anesthetic needed while maintaining efficacy. Morbidly obese patients may have altered pharmacokinetics, including increased volume of distribution for lipophilic drugs and potentially delayed onset of action. They may also experience faster spread of neuraxial anesthetics due to increased intra-abdominal pressure and reduced epidural space.
Key Considerations
- Monitor these patients closely for signs of local anesthetic systemic toxicity (LAST), which include perioral numbness, metallic taste, tinnitus, seizures, and cardiovascular collapse.
- Have lipid emulsion (20%) readily available as rescue therapy for LAST.
- The physiological basis for using IBW is that local anesthetics primarily distribute in lean tissue with good blood supply, not in adipose tissue, making dosing based on total weight potentially dangerous in obesity.
- As recommended by the Association of Anaesthetists of Great Britain and Ireland Society for Obesity and Bariatric Anaesthesia 1, drug dosing should generally be based upon lean body weight and titrated to effect, rather than dosed to total body weight.
Clinical Application
The guidelines for local anesthetic dosing in morbidly obese patients emphasize the importance of using ideal body weight to calculate the maximum safe dose. This approach is supported by studies that demonstrate the risks of toxicity associated with dosing based on total body weight 1. By using ideal body weight and considering the altered pharmacokinetics of morbidly obese patients, clinicians can minimize the risk of toxicity and ensure effective pain management.
Additional Recommendations
- Consider using short-acting agents and multimodal opioid-sparing analgesia to minimize the risk of respiratory depression and other complications 1.
- Ensure that all staff involved in the care of morbidly obese patients receive training in managing these patients, including specific training on moving and positioning 1.
From the Research
Guidelines for Local Anesthetic Dosing in Morbidly Obese Patients
- The optimal dosing scalar for most drugs used in anesthesia, including local anesthetics, is lean body weight, rather than total body weight or ideal body weight 2.
- When administering local anesthetics, the mass of the drug, rather than the volume or concentration, primarily determines the onset, success, and duration of the block 3.
- There is a lack of specific guidelines for dosing local anesthetics in morbidly obese patients, and clinicians must use their judgment and available pharmacokinetic data to derive the best dosing regimens 4.
- The use of local anesthetics in morbidly obese patients requires careful consideration of the potential risks, including local anesthetic systemic toxicity, and strategies to reduce these risks should be employed 3.
Considerations for Anesthesia Care in Morbidly Obese Patients
- Morbidly obese patients are at increased risk for adverse respiratory events secondary to anesthetic agents, and awareness of the pharmacology of commonly used anesthetic agents is necessary for safe and effective care 2, 5.
- A multidisciplinary team approach is key to successful outcomes in morbidly obese patients, and preoperative evaluation and perioperative management should be emphasized 5.
- The choice of local anesthetic and dosing strategy should be individualized based on the patient's specific needs and medical history, and clinicians should be prepared to adjust their approach as needed to ensure safe and effective anesthesia care.