What is the recommended dose of spinal anesthesia for obese patients?

From the Guidelines

The recommended dose of spinal anesthesia for obese patients should be reduced compared to non-obese patients, typically by 20-25% of the standard dose, and dosing should be based on height rather than weight, as the cerebrospinal fluid volume correlates better with height than with body mass. For example, for cesarean delivery, 9-10 mg of hyperbaric bupivacaine (rather than the standard 12-15 mg) is often appropriate, with the addition of opioids like fentanyl (10-25 mcg) or morphine (100-200 mcg) to enhance analgesia without increasing the block height 1. Obese patients have increased intra-abdominal pressure and engorged epidural veins, which reduce cerebrospinal fluid volume and lead to higher cephalad spread of local anesthetics.

Key Considerations

• Positioning is crucial; a ramped position with elevation of the head and shoulders helps with both airway management and reduces the risk of high spinal block.
• Ultrasound guidance may be beneficial for identifying landmarks in patients where these are difficult to palpate.
• Careful dose adjustment helps prevent high or total spinal anesthesia while still providing adequate surgical anesthesia.
• The use of lean or adjusted body weight as scalars for drug dosing is recommended, rather than total body weight, to avoid relative overdose 1.
• Regional anesthesia is preferred to general anesthesia, although a plan for airway management is still mandatory, and specific equipment such as extra-long spinal or epidural needles should be available 1.

Equipment and Resources

• Specialized equipment for managing obese surgical patients, including bariatric operating tables, gel pads, and wide Velcro strapping, should be available 1.
• A portable ultrasound machine and long spinal and epidural needles may be useful in these cases.
• A neuromuscular blockade monitor and depth of anesthesia monitoring can help minimize residual sedation and ensure patient safety.

From the Research

Dose of Spinal Anesthesia for Obese Patients

• The recommended dose of spinal anesthesia for obese patients is a topic of ongoing research, with various studies providing insights into the optimal dosing strategies 2, 3, 4, 5, 6.
• A study published in the British Journal of Anaesthesia in 1990 found that obese patients required a lower dose of plain 0.5% bupivacaine to achieve the same level of sensory block as non-obese patients, and recommended administering the drug at the L4-5 interspace instead of L3-4 to avoid extensive spread of the block 2.
• Another study published in Anesthesiology in 2011 determined the effective dose (ED(50)/ED(95)) of intrathecal bupivacaine for cesarean delivery in morbidly obese patients, and found that the ED(50) and ED(95) values were similar to those of a non-obese population, suggesting that obese patients do not require significantly different doses of spinal anesthesia 3.
• A review article published in the Journal of Clinical Anesthesia in 2005 discussed the pharmacokinetic considerations for anesthesia in obese patients, and noted that the volume of distribution of anesthetic drugs is increased in obese patients, but clearance is usually normal or increased 4.
• Other studies have investigated the use of low-dose bupivacaine combined with opioids such as fentanyl or sufentanil for spinal anesthesia in various patient populations, including obese patients, and found that these combinations can provide effective anesthesia with reduced side effects 5, 6.

Key Findings

• Obese patients may require lower doses of spinal anesthesia to achieve the same level of sensory block as non-obese patients 2.
• The ED(50) and ED(95) values of intrathecal bupivacaine for cesarean delivery in morbidly obese patients are similar to those of a non-obese population 3.
• The volume of distribution of anesthetic drugs is increased in obese patients, but clearance is usually normal or increased 4.
• Low-dose bupivacaine combined with opioids such as fentanyl or sufentanil can provide effective anesthesia with reduced side effects in various patient populations, including obese patients 5, 6.