To the Editor:—
Ultrasound-guided regional anesthesia is an emerging field that potentially provides better block efficacy than other current techniques.1–3In particular, ultrasound-guided axillary block had been described as an excellent technique for brachial plexus anesthesia.4,5With this method, a short axis (transverse cross-sectional) view of the axillary artery and surrounding nerves is obtained with the block needle approaching from the lateral aspect of the arm in the plane of imaging. Although V-shaped redirection of the block needle can be used to place local anesthetic on the superficial and deep sides of the axillary artery, transarterial placement of the needle may occur during the procedure.
During an ultrasound-guided axillary block, we observed a bent echo of the 25-gauge 3.8-cm Quincke tip needle (Becton Dickinson and Company, Franklin Lakes, NJ). The needle shaft echo was bent toward the skin surface when the needle crossed the axillary artery (fig. 1). When examined after the procedure, the injection needle was perfectly straight.
Described as a “bayonet artifact,” this ultrasound artifact causes apparent needle deformity and has been reported during breast biopsy in which a needle traverses a tumor surrounded by fat tissue.6Bayonet artifact occurs when the ultrasound beam passes through tissues with different speeds of sound. Similar speed of sound artifacts have been described in the soft tissues of the kidney.7–9Because all commercial ultrasound machines assume a uniform speed of sound of 1,540 m/s,10actual differences among speeds of sound in tissue change the apparent depth of received echoes. The speeds of sound among soft tissues may actually range from 1,450 m/s in fat to more than 1,600 m/s in muscle.10Thus, clinical scanning can routinely produce speed of sound errors of approximately 5%.
This letter describes a bayonet artifact observed during regional anesthesia. The artifact occurred when the block needle passed through the axillary artery with the ultrasound beam nearly perpendicular to the needle. The speed of sound in whole blood (1,580 m/s) is higher than the average speed of sound in soft tissue (1,540 m/s).11Therefore, bending of the needle echo toward the transducer was seen when the needle passed through the axillary artery.
Ultrasound guidance likely reduces the incidence of vascular puncture during regional block.12However, inadvertent vascular puncture has been reported during peripheral nerve blocks despite use of ultrasound guidance.13,14Therefore, bayonet artifacts from transarterial needle placement may occur during regional block even if vascular puncture was not intended.
The basic premise of the in-plane approach for ultrasound-guided regional blockade is that precise placement of the block needle tip near nerves is possible in real time. However, there are circumstances under which the actual needle position and perceived image do not agree. Here, we describe one of those circumstances that occurred during transarterial axillary block.
* San Francisco General Hospital, University of California, San Francisco, California. firstname.lastname@example.org