Out-of-Plane Approach

There are several advantages to the out-of-plane approach to regional block (Table 12-1). This approach is most similar to traditional approaches to regional block guided by nerve stimulation or palpation. Therefore, the out-of-plane approach provides a natural transition from one form of guidance to another. The out-of-plane approach uses a shorter needle path than in-plane approaches. If short-axis views of the nerve are used, an out-of-plane approach results in catheter placement that is guided along the path of the nerve. One disadvantage of the out-of-plane approach is the extent of the unimaged needle path (structures that may lie short of or beyond the scan plane). If the needle tip crosses the scan plane without recognition, it can be advanced beyond the scan plane into undesired tissue.

Table 12-1 Comparison of Out-of-Plane and In-Plane Approaches

In-Plane Approach

There are several advantages to the in-plane approach. It provides the most direct visualization of the needle tip and injection. The amount of unimaged needle path is typically small. The needle tip is visualized before advancement. One disadvantage is the long needle path, which results in more tissue for the needle to cross. Large-bore needles are often used with this approach to facilitate alignment. Partial line-ups (visualization of the needle shaft without visualization of the needle tip in the scan plane) create a false sense of security and therefore compromise safety of the technique.

External marks on the transducer can be used to guide needle placement for in-plane technique. However, the mechanical axis of the transducer and its acoustic axis are not always precisely aligned.¹

Offline Markings

Offline techniques involve external skin markings from ultrasound scans without imaging during needle placement. Changes in patient position, mobility of the skin, and dynamic changes with needle placement and injection limit the utility of this approach. The skin adjacent to the sides of the transducer can be marked. Alternatively, a paper clip or solid metal stylet (preferably with dull ends) can be used to create artifact within the field to mark the position of the object. For this technique, spatial compound imaging should be turned off to enhance the artifact.² The M-mode center line can be used to facilitate offline markings in the center of the field.

Hand-on-needle provides better needle control for in-plane technique. This is important for blocks above the clavicle where the injection hand is stabilized. Hand-on-syringe provides the ability to control needle movement and injection by one operator.

Skill is probably more important than approach alone. There will probably never be a good study comparing the two approaches (out-of-plane versus in-plane) because of strong institutional biases regarding how to perform regional blocks.

By musculoskeletal convention, the long-axis images will be shown with the proximal side on the left and the distal side on the right. Long-axis views are useful for demonstrating longitudinal distributions of local anesthetic along the nerve path in one image. However, in clinical practice, it is usually easier to view the nerve in short axis and slide along the nerve path. Right-handed operators prefer a right-hand screen bias so that they can see their hands and display during the procedure.

FIGURE 12-1 Schematic drawing of the short-axis (SAX) and long-axis (LAX) out-of-plane (OOP) imaging (left panels), and SAX and LAX in-plane (IP) imaging (right panels).

(Adapted from Gray AT. Ultrasound-guided regional anesthesia: current state of the art. Anesthesiology 2006;104:368–73.)

FIGURE 12-2 Set-up for regional block with hand-on-syringe or hand-on-needle approaches.

FIGURE 12-3 Median nerve viewed in short axis (A and B) and in long axis (C and D).