Four years ago, Anesthesiology published a clinical concepts and commentary article that reviewed the use of ultrasound guidance for regional anesthesia.1This article described the underlying principles and available literature of this nascent field. General efficacy and safety of these approaches have been borne out in a large number of subsequent clinical trials.2However, a recent letter to the editor has raised the theoretical concern that bioeffects may be harmful to patients undergoing regional anesthesia procedures guided by ultrasound.3 

Although it is clear that there are thermal and mechanical bioeffects of ultrasound, there are no confirmed adverse bioeffects when diagnostic levels of ultrasound are used.*Most bioeffects simply dissipate during the duty cycle of pulse sequence ultrasound and are significantly attenuated by the perfusion of living tissue.4Moreover, when using a handheld probe for imaging during peripheral nerve block, it would be very unlikely for a transducer to be maintained in a fixed position for an extended period. Interestingly, some of the postulated bioeffects of high-intensity ultrasound actually include the promotion of nerve regeneration and conduction block,5,6two effects potentially beneficial to those patients undergoing regional anesthesia procedures. Nonetheless, prudent use of ultrasound means using the lowest levels of exposure to achieve the desired goals (as low as reasonably achievable principle).

When studied in vitro , the threshold for ultrasound producing reduction in peripheral nerve compound action potentials was approximately 100–200 W/cm2(continuous wave, 30-s burst, reported intensity as the spatial peak temporal average).7This reduction correlated with nerve temperature elevation from ultrasound exposure and was more pronounced at low frequencies. Irreversible effects only occurred at more than 400 W/cm2, well above the current Food and Drug Administration imposed limit of 720 mW/cm2(intensity as the spatial peak temporal average) for diagnostic imaging.8Admittedly, the interaction between local anesthetic toxicity and ultrasound has not been experimentally studied by such models, and the concerns that have been raised will hopefully encourage such investigations.

The low incidence of significant adverse neurologic outcomes prevents definitive conclusions regarding these complications after regional anesthesia. The difficulties involved in investigating these low frequency, zero tolerance, events can be appreciated by the fact that the relative safety of using nerve stimulation versus  a paresthetic technique has never been adequately resolved, despite decades of debate. Although the relatively high incidence of transient postoperative neurologic symptoms after regional block can be used to assess the relative risk,9the validity of these symptoms as a surrogate marker of significant injury is speculative. Although substantive data concerning significant injury are lacking, a large retrospective study recently reported five seizures and three nerve injuries in 3,290 patients undergoing peripheral nerve blocks guided by nerve stimulation, but no such events in 2,146 patients undergoing similar blocks guided by both nerve stimulation and ultrasound.10There are obviously substantial limitations to such retrospective reviews. Nonetheless, these data, and the published and unpublished experience with ultrasound in this setting, fail to raise alarm and instead imply greater safety by the addition of ultrasound imaging. Whether this is indeed true is obviously of great interest.

Dr. Cory's letter raises important questions regarding the potential impact of beam intensity on neurologic outcomes after regional anesthesia. Beam intensity is only one of the numerous differences between ultrasound guidance and other approaches to regional blockade that could impact safety, all of which mandate rigorous investigation.

†University of California, San Francisco, San Francisco General Hospital, San Francisco, California.

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