Obesity is a nationwide epidemic, affecting over 30% of Americans. Over 50% of pregnant women in the United States are overweight or obese. Neuraxial anesthesia is often a good choice for this population for appropriate surgeries as well as for labor, avoiding airway instrumentation and complications of general anesthesia that are often exacerbated by obesity.
It remains a topic of debate whether the dosage of local anesthetics for neuraxial blocks needs to be adjusted for obese patients. Traditional teaching based on earlier studies recommends decreasing the dose of local anesthetics for spinal and epidural blocks, citing a positive correlation between cephalad spread of sensory blockade and BMI. However newer data calls this correlation into question.
MRI studies demonstrate a smaller cerebrospinal fluid volume in the lumbar subarachnoid space of obese patients, as well as an inverse relationship between lumbar CSF volume and cephalad block extension. The epidural space in these patients is also smaller. The mechanism is thought to be compression and increased abdominal pressure by abdominal fat, which causes caval compression, epidural vein engorgement, and displacement of soft tissue through the intervertebral foramina.
Despite these confirmed physiological differences between obese and non-obese patients, several recent clinical studies have shown no significant difference in the ED50 and ED95 of hyperbaric spinal bupivacaine in obese parturients and those with normal BMI when undergoing cesarean section. There is little data on super-obese patients (BMI >50 kg/m2). Some argue based on these studies that intrathecal dosing of local anesthetics need not be reduced for obese patients, however there is no expert consensus on the topic.
Less controversy exists in the epidural dosing of obese patients, with most studies showing a decreased requirement for local anesthetics in this population.
The fact that block overdose, block failure and inadequate block duration in obese patients may lead to undesirable emergency airway instrumentation in a population with increased incidence of difficult airway raises the stakes of accurate dosing. Options such as combined spinal epidurals allow for dosage reduction in the initial intrathecal component, with the ability to titrate together with the epidural component. This and intrathecal catheters both guard against the possibility of longer surgical duration in patients with high BMI.
While it remains unclear whether obesity warrants a reduction in local anesthetic dosing for neuraxial anesthesia, a prudent approach may be to start on the side of safety and begin with smaller dosages while employing a catheter-based technique.
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