From the Guidelines
Regional anesthesia for lower extremity procedures in super obese patients should be performed with ultrasound guidance, using longer needles and low-frequency curvilinear probes, and considering a lateral approach for femoral or adductor canal blocks, as well as a subgluteal or popliteal approach for sciatic nerve blocks, as recommended by the most recent guidelines 1. When performing regional anesthesia in super obese patients, it is essential to consider the technical challenges posed by their anatomy.
- Ultrasound guidance is crucial for all blocks due to altered anatomy and increased tissue depth, often requiring low-frequency curvilinear probes (2-5 MHz) to achieve adequate penetration.
- Longer needles (100-150mm) are typically necessary to reach target structures.
- Positioning the patient carefully, using pillows or wedges to optimize access to the target site and improve respiratory mechanics, is vital.
- For femoral or adductor canal blocks, considering a lateral approach rather than the traditional approach can improve needle visualization.
- When performing sciatic nerve blocks, the anterior approach is often difficult in these patients; instead, using a subgluteal or popliteal approach with the patient in lateral position can be more effective.
- Increasing local anesthetic volumes by approximately 20-30% (while staying within maximum dose limits based on actual body weight) can account for larger distribution volumes.
- Using lower concentrations of longer-acting agents like ropivacaine 0.2-0.375% or bupivacaine 0.25% can maximize duration while minimizing motor block.
- Continuous catheter techniques are particularly valuable in these patients to reduce opioid requirements and facilitate early mobilization.
- Procedural sedation should be minimal to avoid respiratory compromise, and supplemental oxygen should be provided during the procedure, as recommended by the Association of Anaesthetists of Great Britain and Ireland 1. These modifications address the technical challenges of performing blocks in super obese patients while maximizing block success and safety, and are supported by the most recent guidelines on perioperative care in bariatric surgery 1.
From the Research
Challenges in Lower Extremity Regional Anesthesia for Super Obese Patients
- Super obese patients present unique challenges for anesthesiologists due to difficulties in achieving peripheral and neuraxial blockade 2
- The increased incidence of failed blocks in obese patients compared to normal weight patients requires an experienced anesthesiologist with knowledge of physiologic and pharmacologic differences in obese patients 2
Use of Ultrasound Guidance
- Ultrasound guidance may aid in performing regional anesthesia procedures in super obese patients by providing direct visualization of underlying anatomic structures and real-time needle direction 3
- Further research is needed to determine optimal regional anesthesia techniques, local anesthetic dosage, and perioperative outcomes in obese patients 3
Local Anesthetic Dosage and Volume
- The minimum effective anesthetic volume of 0.5% ropivacaine for ultrasound-guided popliteal sciatic nerve block has been determined to be 6 ml (ED50) and 16 ml (ED95) 4
- The choice of local anesthetic and concentration can affect the duration of analgesia, with 0.75% ropivacaine having an onset similar to 2% mepivacaine and a duration of postoperative analgesia between that of 0.5% bupivacaine and 2% mepivacaine 5
Comparison of Local Anesthetics
- A comparison of 0.5% bupivacaine with 1:200,000 epinephrine, 0.5% ropivacaine, and 1% ropivacaine for low-volume ultrasound-guided interscalene brachial plexus block found similar analgesic durations among the three groups 6
- Increasing the concentration of ropivacaine from 0.5% to 1% did not prolong the duration of analgesia in the context of single-injection low-volume ultrasound-guided interscalene brachial plexus block 6