Bariatric Times - January 2009 - (Page 25)

Bariatric Times • January 2009 Anesthesiology Perspective 25 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 postdural 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 postdural 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 nonobese 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

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Bariatric Times - January 2009

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