From the Guidelines
For obese patients receiving spinal or epidural anesthesia, dosing of local anesthetics like bupivacaine and ropivacaine should be based on lean body weight rather than total body weight, as recommended by the Association of Anaesthetists of Great Britain and Ireland 1. This approach is crucial in minimizing the risk of toxicity while ensuring adequate analgesia. The recommendation to use lean body weight for drug dosing in obese patients is supported by the principle that the volume of distribution and clearance of drugs are more closely related to lean body mass than to total body weight.
Key Considerations for Dosing
- For spinal anesthesia, a standard dose of 10-15 mg of bupivacaine or 15-20 mg of ropivacaine is typically appropriate, regardless of obesity status, as these doses are generally effective for most patients when considering the spread of the anesthetic in the spinal canal.
- For epidural anesthesia, concentration is more important than volume, with typical concentrations being 0.125-0.25% bupivacaine or 0.1-0.2% ropivacaine, as the efficacy of epidural anesthesia is more dependent on the concentration of the local anesthetic rather than the volume administered.
- The physiological basis for these recommendations is that cerebrospinal fluid volume and epidural space anatomy correlate better with height and lean body weight than with total body weight, which is a critical factor in determining the spread and efficacy of local anesthetics in the spinal and epidural spaces.
Administration and Monitoring
When administering local anesthetics to obese patients, careful positioning, slower injection rates, and close monitoring for hemodynamic changes are particularly important due to their increased risk of complications, such as respiratory and cardiovascular instability. The use of a robust airway strategy and caution with long-acting opioids and sedatives, as outlined in the guidelines for the peri-operative management of obese surgical patients 1, is also essential in minimizing risks.
Evidence Basis
While specific dose-finding studies for intrathecal catheter placement, such as those discussed in the guidelines from the Obstetric Anaesthetists' Association 1, provide valuable insights into the potency and efficacy of different local anesthetics and opioid combinations, the fundamental principle of basing drug dosing on lean body weight in obese patients remains a cornerstone of safe and effective anesthesia practice. This approach, combined with careful patient selection, meticulous technique, and vigilant monitoring, helps to optimize outcomes for obese patients undergoing spinal or epidural anesthesia.
From the Research
Recommended Dosing Scalar for Spinal and Epidural Local Anesthetic in Obese Patients
- The recommended dosing scalar for spinal and epidural local anesthetic in obese patients is not explicitly stated in the provided studies, but some studies suggest that the dose requirement for intrathecal bupivacaine may be similar in obese and normal weight women 2.
- A study found that the estimated ED95 for intrathecal bupivacaine for elective cesarean delivery was similar in normal weight and obese women, at 12.78 mg and 11.86 mg, respectively 2.
- Another study suggested that plain bupivacaine be administered at L4-5 instead of L3-4 in obese patients when extensive spread of the block is to be avoided 3.
- The dose-response relationship of ropivacaine 0.75% given epidurally was tested in a study, which found that 15 ml, 20 ml, and 25 ml of ropivacaine 0.75% brought about adequate analgesia in the sacral and lumbar regions in all patients 4.
- A comparison of bupivacaine 0.75%, bupivacaine 0.5%, and ropivacaine 0.75% for epidural anesthesia found that the latency period of analgesia was similar for bupivacaine 0.75% and ropivacaine 0.75%, but shorter than for bupivacaine 0.5% 4.
- A study on the use of ropivacaine in regional anesthesia found that epidural ropivacaine 0.2% was effective for the initiation and maintenance of labor analgesia, and provided pain relief after abdominal or orthopedic surgery 5.
- The study also found that ropivacaine had a lower propensity for motor block and less systemic toxicity compared to bupivacaine 5.
- Another study compared the efficacy of ropivacaine and bupivacaine for postoperative analgesia, and found that 0.2% ropivacaine and 0.125% bupivacaine provided sufficient postoperative analgesia when the spread of anesthesia covered the operated area 6.