What is the lowest effective dose and volume of local anesthetic, such as ropivacaine (generic name) or bupivacaine (generic name), for lower extremity peripheral nerve blocks in obese patients?

From the Guidelines

The lowest effective dose and volume of local anesthetic for lower extremity peripheral nerve blocks in obese patients is 0.2-0.5 ml/kg of 0.25% bupivacaine or 0.2% ropivacaine. When considering the management of postoperative pain, particularly in the context of lower extremity peripheral nerve blocks for obese patients, it's crucial to balance efficacy with safety, minimizing the risk of local anesthetic systemic toxicity (LAST) 1.

Key Considerations

• Dosing should be based on ideal body weight rather than actual body weight to avoid toxicity.
• Ultrasound guidance is recommended to visualize anatomical structures, ensure accurate needle placement, and potentially reduce the required volume of local anesthetic.
• The use of adjuvants like preservative-free clonidine at 1 to 2 micrograms kg-1 can be considered to enhance block duration without necessarily increasing the volume of local anesthetic 1.

Local Anesthetic Options

• Bupivacaine 0.25%: 0.2-0.5 ml/kg
• Ropivacaine 0.2%: 0.2-0.5 ml/kg
• These concentrations and volumes are based on the most recent guidance for postoperative pain management, adapted for the specific context of obese patients undergoing lower extremity peripheral nerve blocks 1.

Safety and Efficacy

The approach to using the lowest effective dose and volume is critical in obese patients due to potential alterations in drug distribution and metabolism, which can increase the risk of systemic toxicity. By using ultrasound guidance and basing dosing on ideal body weight, clinicians can minimize risks while maintaining the efficacy of the nerve block 1.

From the Research

Lowest Effective Dose and Volume for Lower Extremity Peripheral Nerve Blocks in Obese Patients

• The lowest effective dose and volume of local anesthetic for lower extremity peripheral nerve blocks in obese patients can vary depending on the specific nerve block and the local anesthetic used 2, 3, 4, 5.
• For femoral nerve blocks, the minimum effective anaesthetic volume (MEAV50) of ropivacaine 0.5% was found to be 15 ml (95% CI, 7-23 ml) with ultrasound guidance and 26 ml (95% CI, 19-33 ml) with nerve stimulation guidance 2.
• The effective dose in 95% of cases (ED95) for femoral nerve block with ropivacaine 0.5% was found to be 22 ml (95% CI, 13-36 ml) with ultrasound guidance and 41 ml (95% CI, 24-66 ml) with nerve stimulation guidance 2.
• For adductor canal blocks, the median effective volume (EV50) of 0.5% ropivacaine was found to be 10.79 ml (95% CI, 10.10-11.52 ml) 4.
• For sciatic and femoral nerve blocks, a dose of 0.25% bupivacaine was found to be effective for surgical management of diabetic foot syndrome in high-risk patients with diabetic nephropathy 5.
• The use of mixtures of local anesthetics, such as bupivacaine or ropivacaine with lidocaine, can also affect the lowest effective dose and volume required for lower extremity peripheral nerve blocks 3.

Factors Affecting the Lowest Effective Dose and Volume

• The use of ultrasound guidance can reduce the minimum effective anaesthetic volume required for femoral nerve blocks 2.
• The type of local anesthetic used, such as bupivacaine or ropivacaine, can affect the lowest effective dose and volume required for lower extremity peripheral nerve blocks 2, 3, 4, 5.
• The presence of obesity or other comorbidities, such as diabetic nephropathy, can affect the lowest effective dose and volume required for lower extremity peripheral nerve blocks 6, 5.