Regional Anesthesia and Obesity

by Pedro P. Tanaka, MD, PhD, and Jay B. Brodsky, MD

Both from the Department of Anesthesia, Stanford University Medical Center, Stanford, California.

INTRODUCTION
One consequence of the worldwide obesity epidemic is that every day in operating rooms and ambulatory treatment centers, obese patients present for all types of surgical procedures. Many of the same concerns that the anesthesia team has when anesthetizing a morbidly obese patient for a bariatric procedure are valid for these other operations as well. The challenge of managing a patient with significant medical comorbidities and the potential for airway complications is always present. In addition, many morbidly obese patients have obstructive sleep apnea (OSA) and are extremely sensitive to the respiratory depressant effects of sedatives, opioids, and general anesthetics.

Discussion
A successful regional anesthetic technique can offer distinct advantages over general anesthesia for an obese patient. As illustrated in Table 1, benefits include minimal airway intervention, a reduction or even complete avoidance of cardiopulmonary depressant agents, and the possibility of improved postoperative analgesia. If complications from over-sedation are reduced, post-anesthesia care unit (PACU) and overall hospital stays should also be decreased. Unfortunately there have been very few studies describing regional anesthesia in obese patients, and even fewer reports for extremely obese patients. We do not know the actual risks and benefits of regional anesthesia for these patients. What are the safe and effective doses of local anesthetics in obesity and are they similar or different from those needed for normal weight patients? There are additional considerations that are unique to the obese patient as seen in Table 2. These include the technical challenge of identifying the appropriate nerve, or the epidural or intrathecal space in a patient whose anatomic landmarks may be obscured. Special equipment, such as longer needles, may be required. Obese patients can be difficult to move and position for a regional block, and an inadequate block may necessitate establishing an airway and performing general anesthesia in less than ideal conditions. The most extensive experience with regional anesthetic techniques in obesity is with neuraxial (spinal and epidural) anesthesia. The majority of information we have on neuraxial anesthesia and obesity comes from obstetrics, so any relevance to the obese non-obstetric population must be inferred.[1–2]

Obese patients require less epidural local anesthetic than normal weight patients to achieve a similar level of anesthetic block. Following administration of 20ml of 0.75% bupivacaine in the L3-L4 interspace, there was a direct correlation between cephalad spread of analgesia and body mass index (BMI).[3] Obese parturients (BMI>30kg/m2) in labor have significantly reduced epidural analgesic requirements, and have higher sensory blocks with a similar dose of local anesthetic than normal weight patients.[4] Thus, it would seem prudent to administer local anesthetics in smaller volumes or in divided doses until an appropriate sensory block is achieved for any obese patient undergoing epidural anesthesia. Not surprisingly, location of the epidural space is often technically more difficult in large patients. In one study of obstetrical patients, 74 percent of those weighing greater than 300 pounds (136.4kg) needed more than one attempt to place an epidural, and 14 percent required more than three attempts.[5] In another study, the initial epidural catheter “failed” in 42 percent of morbidly obese parturients, but in only six percent of control patients. Misidentification of appropriate landmarks and anatomic distortion were implicated for the high initial anesthetic failure rate in the obese group.[6] Similar results have been reported by others.[7]

Increasing weight is significantly correlated with the depth of placement of an epidural at all epidural sites (lumbar, lower, and upper thoracic) and with all approaches (midline, paramedian).[8] After the epidural catheter is inserted, usually with the patient in the sitting position, and after the catheter is securely fixed to the skin, catheter position can change and actually be withdrawn from epidural space during repositioning to the lateral decubitus or supine positions. The magnitude of catheter movement is greatest with obese patients. Multi-orificed epidural catheters should be inserted a minimum of 4cm into the epidural space in obese patients since unrecognized catheter migration with position change can result in loss of anesthetic agent and an inadequate block.[9]

The incidence of complications with epidural anesthesia increases with increasing weight. For example, inadvertent epidural venous puncture during placement occurred more frequently in obese parturients, and this resulted in a higher incidence of multiple punctures.[10]

In a retrospective study of post-dural puncture headache in morbidly obese parturients who experienced a “wet tap” during epidural placement, obese patients had a 24-percent of incidence of headache compared to a 45-percent incidence in non-obese patients. Fewer headaches in obese parturients may be explained by their increased abdominal panniculus, which raises abdominal pressure and slows cerebrospinal fluid (CSF) leak.[11] Obese patients may also be less active postoperatively so the symptoms of post-dural puncture headache might have been masked. Ultrasonography (US) has been used to facilitate epidural needle placement in morbidly obese patients for pain treatment, and to improve epidural guidance in obese obstetric patients.[12]

As with epidural anesthesia, obesity is a factor influencing spinal anesthesia. Obese pregnant patients having spinal anesthesia with low doses of hyperbaric bupivacaine (7.5–10mg) experienced higher sensory blocks than non-obese patients.[13] Similarly, patients undergoing urologic procedures given 4mL of isobaric 0.5% bupivacaine at the L3-L4 space demonstrated a positive correlation between the height of blockade and obesity. Other reports have correlated obesity and higher cephalad spread of sensory block with spinal anesthesia.[14–15]

With obesity, the vena cava is compressed from the weight of the abdominal contents. At one time it was postulated that collateral circulation through the distended extradural veins reduced CSF volume.[16] CSF volume was measured in volunteers and a smaller CSF volume was indeed found in obese subjects. However, magnetic resonance imaging (MRI) demonstrated that the increased abdominal pressure probably decreases CSF volume by displacing tissue into the vertebral canal through the intervertebral foramina rather than by changing venous volume.[17] Since CSF volume is less in obese patients, the total volume of local anesthetic needed to achieve the same height of neural blockade should be less than in normal weight patients.

One study found continuous spinal anesthesia of morbidly obese patients undergoing vertical banded gastroplasty to be safe intraoperatively and provided satisfactory conditions in the immediate postoperative period.[18]

Bedside US assessment of the of the lumbar spine may facilitate the performance of spinal anesthesia in morbidly obese patients, particularly if no landmarks can be identified or a landmark-based approach has been unsuccessful.[19] Fluoroscopic imaging has also been used in an extremely obese patient to identify relevant landmarks, to approximate the distance to the intrathecal space, and to confirm proper position of the spinal needle as it was advanced.[20]

Reports of complications of spinal anesthesia in obese patients are once again limited to just a few case reports or small clinical series. For example, transient neurologic symptoms, described as leg or buttock pain, occurred more often in obese (BMI>30kg/m2) than normal weight patients following spinal anesthesia with lidocaine.[21] As little information as we have on neuraxial blocks in obesity, there is even less data on peripheral nerve blocks in this population.

A single large study considered 9,342 peripheral nerve blocks in 7,160 patients undergoing ambulatory surgery. Patients were placed into four groups depending on their BMI (<25kg/m2, 25–29kg/m2, 30–34kg/m2, >35kg/m2). For risk adjustment, the authors categorized regional anesthetic procedures into the following four subsets to combine blocks with similar characteristics: 1) centro-neuraxial blocks; 2) peripheral nerve blocks; 3) continuous peripheral nerve blocks; and 4) paravertebral blocks. All peripheral nerve blocks were performed using a nerve stimulator technique. This study found that high BMI represented an independent risk factor for block failure.[22]

In a subsequent publication by the same group consisting of an almost identical database, the block failure in the different BMI categories was as follows: BMI <25kg/m2=9.5%, BMI 25–29kg/m2=10.7%, and BMI>30kg/m2=12.7%.[23] The risk-adjusted block failure rate for peripheral nerve blocks was statistically significantly higher in obese patients but not in overweight patients. Pain with movement was significantly less in obese and overweight patients. Whether these results are related to increased pain tolerance, higher prevalence of diabetic neuropathy, or less extensive surgery in this subset of patients is unknown. A high level of overall satisfaction in the obese group clearly demonstrated that regional anesthesia techniques were well accepted among patients with increased BMI. A conclusion of this study was that overweight and obese patients should not be excluded from undergoing regional anesthesia in the ambulatory setting. In the only other relevant study, Franco et al[24] investigated the impact of BMI on the success rate of supraclavicular block. Their success rate was 94.3 percent in obese patients, but this was significantly less than in their non-obese population. Once again, the use of US can make an invaluable contribution in performing successful peripheral nerve blocks in obese patients.[25–26]

Conclusion
An extensive review of the medical literature reveals a paucity of studies of regional anesthesia and obesity. Although we can make some general recommendations for regional anesthesia in obese patients (Table 3), with so few studies addressing this topic these recommendations are based on the authors’ biases. We urge anesthesiologists and surgeons who routinely manage obese patients with regional anesthetic techniques to share their experiences with others.

References
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Tags: Anesthesiology Perspective

Category: Anesthetic Aspects of Bariatric Surgery, Past Articles