Obesity is a continuing epidemic in the United States. Roughly 30% of Americans are obese, and over 50% of pregnant women in the US are overweight or obese. Neuraxial anesthesia is often considered in these patients for labor, cesarean section, lower extremity surgeries and other appropriate procedures, as it avoids airway instrumentation and other complications of general anesthesia associated with obesity.
The need for adjusting the dosage of local anesthetics in neuraxial blocks for obese patients remains a topic of debate. While some studies show a positive correlation between cephalad spread of sensory block and BMI, others have not found this correlation. Magnetic resonance imaging studies have shown that obese patients have smaller effective cerebrospinal fluid space in the lumbar subarachnoid space, as well as an inverse correlation between lumbar CSF volume and cephalad extension of the block. It is thought that this decreased CSF volume is due to caval compression by abdominal fat (and, in the obese parturient, the gravid uterus), causing epidural vein engorgement, and displacement of soft tissue through the intervertebral foramina by increased abdominal pressure. The epidural space is also smaller due to compression.
Despite these physiologic differences between obese and non-obese patients, recent clinical studies have not found a significant difference in the ED50 and ED95 of hyperbaric spinal bupivacaine used for cesarean section in obese and non-obese parturients. Data on super-obese patients (BMI >50 kg/m2) remains limited. Some argue that many of the studies showing a need for reduced intrathecal dosing of local anesthetics were conducted decades ago and with higher concentrations of medications. Nevertheless there remains no consensus on whether intrathecal dosage should be reduced.
A review of the literature reveals less controversy on epidural dosing in obese patients, with most studies showing a decreased requirement for local anesthetics.
The debate on dosing of neuraxial blocks in obese patients is further complicated by the difficult placement of these blocks in this population, and the relatively more severe consequences of block failure or inadequate block duration. One technique often employed is to use a combined spinal epidural, in which case a dosage reduction in the intrathecal component may be employed to decrease the risk of high spinal and subsequent emergent airway manipulation, but with the ability to fall back on the epidural component to guard against inadequate sensory block in the face of often longer surgical times in obese patients.
It remains unclear whether obese patients truly require less local anesthetic in neuraxial blocks when compared to non-obese patients, but a prudent approach may be to begin with lower doses if a catheter is in place. If spinal vs epidural anesthesia is desired, either an intrathecal catheter or a combined spinal epidural technique may provide more room to titrate than a single shot.
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