Many anesthesiologists have shied away from performing peripheral nerve blocks on obese patients, because the techniques they have been using are inadequate to the task. Those of us who have been using a nerve stimulator without ultrasound have found it difficult, if not impossible, to palpate helpful landmarks like muscle bellies, bony prominences and arterial pulsations through a thick layer of fatty tissue. Those of us who have been using ultrasound alone also face challenges; ultrasound can be disorienting, since shades of black and white can make it difficult to ensure that what we are looking at really is the nerve we are seeking.
We have found a better way to deliver the benefits of peripheral nerve blocks to obese patients. At our facility, the combination of ultrasound imaging and confirmatory nerve stimulation allows us to deliver peripheral nerve blocks to obese patients with confidence. The ultrasound provides the real-time anatomic information so we can insert the needle in the correct location, while the motor response to nerve stimulation verifies that we are targeting the correct nerve.
Ultrasound is especially useful when the nerve we are seeking is close to structures we must avoid. With a supraclavicular block, for example, the nerves of the brachial plexus are very close to the subclavian artery and the lung. By watching the needle approach the plexus under ultrasound, we can be confident that we will avoid these structures. After confirming the correct location with nerve stimulation, the ultrasound also tells us if the local anesthetic we inject is going where we want it. We can safely move the needle above and below the plexus to make sure all the nerves are surrounded by local anesthetic. This approach has led to a very high success rate.
Another key to safe peripheral nerve blocks in these patients is very careful sedation and monitoring. For all patients, we routinely administer IV midazolam and fentanyl to reduce anxiety and discomfort during the blocking procedure; and we perform full EKG, pulse oximetry, and frequent BP monitoring during the procedure regardless of body weight. We routinely administer oxygen and, during the surgical procedure, we continue the sedation and may infuse IV propofol, as well. Since obese patients are very susceptible to obstruction of their airways under IV sedation, we titrate with extra care and we tend to keep sedation somewhat lighter. Fortunately, sleep apnea due to obstruction develops progressively, so we closely watch the patient's respiratory pattern, which provides sufficient warning of pending obstruction.
Thanks to these techniques, we can offer all patients prolonged post-op pain relief without the need for high-dose narcotics and their side effects, regardless of body weight. We can also offer obese patients a safer surgical experience, because we can avoid the increased risk of respiratory complications that obese patients are subjected to during and after general anesthesia.
Dr. Coombs is Director of Regional Anesthesia with the Department of Anesthesiology at the University of North Carolina at Chapel Hill.