Blocking Strategies

A successful block is the result of multiple scanning principles working in concert.

  • Image orientation
  • Image optimization
  • Needle localization

Image Orientation

Proper orientation cannot be over-emphasized.  It verifies the proper anatomical location of structures and allows you to anticipate where needle insertion will occur on the image.  Additionally, all needle movements will appear as intended on the screen, rather than the opposite of the desired movement (mirror-image).  The images below show a femoral block with proper orientation.  When performing a right femoral block (below left), needle insertion is most often performed using a cross-section, or short axis image with an in-plane approach from lateral to medial.  This allows for the greatest visualization of the needle during the procedure and is oriented so the nerve is the most lateral structure on the image, decreasing the potential for vascular puncture of the femoral artery and vein which are located medial to the nerve.   In the image below right, the orientation icon, indicated by the grey circle in the upper left of the image denotes the lateral position, with the nerve being most lateral.  Using proper orientation, needle insertion should be anticipated from the left (lateral).

In this video, a right femoral nerve block is performed using a combination of ultrasound and nerve stimulation.  The needle is inserted from the left (lateral) side of the screen and moved medial toward the femoral nerve.  The femoral artery can be seen pulsating medial to the nerve on the right side of the screen.  Initial nerve stimulation is consistent with the Sartorius muscle, so the needle is redirected more lateral and inferior.  “Patellar Snap” is then elicited, and once needle position is confirmed, the video ends with the beginning of local anesthetic injection.  Because the insertion angle of the needle is fairly significant, needle visualization is not optimal.

Image Optimization

A high-quality image will improve the success of any ultrasound-guided procedure.  There are several things to consider before starting.  Select the appropriate frequency transducer.  As previously discussed, high-frequency transducers (>7MHz) are ideal for most ultrasound-guided regional anesthesia procedures due to the shallow nature (< 3cm) of most nerves.  Prior to scanning, imagine how the image should appear.  Apply the principles of body ergonomics by adjusting the height of the patient, aligning the ultrasound system, and using an assistant if available.  Apply sufficient gel to the transducer.  It acts as a coupler between the transducer and the skin allowing sound to pass through, and improves the image quality.  Ensure your transducer is initially perpendicular and flat against the skin.  Once obtaining the image, optimize the depth so the structures you wish to image are in the center of the screen.  Adjust the gain to make picture look uniform.  Take a moment to scan proximal and distal to ensure the level you selected is the best location to perform the procedure, as well as to determine if there are any superficial vessels which may complicate the block.  Compare the two images of the brachial plexus below.  While both images have good gain adjustment, and the nerves are in the center of the image, the depth is set better on the right image, improving the chance of a successful block.

Anistropy

Anistropy implies dependency on an angle.  In ultrasound it is used to describe a change in amplitude of received echoes when the angle of insonation has changed.  When initially obtaining an image, it is desirable to place the transducer perpendicular to the skin.  However, nerves do not always travel parallel to the skin, which can make them difficult to see.  Rocking the transducer so that the ultrasound waves become perpendicular to the nerves will increase the amount of returning echoes, improving the visualization of the nerves. The two images below are of the popliteal fossa.  The image on the left is taken with the transducer perpendicular to the skin.  Note how the fascia appears bright while the nerves (yellow arrows) are not so obvious.  By rocking the transducer caudal, the ultrasound now strikes the nerves at an angle closer to 90 degrees increasing the reflection and making the nerves appear brighter.  Also note how the fascia above the nerves does not appear as bright when compared to the image on the left.

Needle Visualization

Needle visualization throughout the procedure is one of the more difficult aspects of ultrasound-guided regional anesthesia. Needles can be visualized either in-plane or out-of-plane during procedures.

 In-plane imaging, as the name implies, allows for complete visualization of the needle under ultrasound.  When performing a procedure using this technique, complete needle visualization is paramount to ensure proper distribution of local anesthetic, as well as preventing potential complications of inadvertent vascular puncture, or intraneural injection.

 

The image below shows the desired path of the needle in relation to the transducer.  This results in complete visualization of the needle (below right).

However, if the needle is not completely in-line with the transducer, as in the image below left, the entire needle will not be visualized and it may actually further in the tissue than imaged (below right).

Consistent and accurate out-of-plane needle insertion is often more difficult to master as only a small portion of the needle is visualized at any one time.  Because only a small portion of the needle can be seen, it is imperative that the needle is inserted at an angle in which the tip passes through the ultrasound beam at the location of the nerve to be blocked or the vessel to be punctured.  If the insertion angle is not correct the shaft, and not the tip of the needle is visualized meaning that the needle is actually deeper in the tissue than it appears.  This may result unintended complications.

The video below shows an out-of-plane needle insertion into a phantom with the needle inserted at angle that shows the needle penetrating a “vessel”.

When performing the out-of-plane technique, two methods can be used to confirm which part of the needle is actually seen on the ultrasound image.  Needle tip vibration and hydrolocation can be helpful in verifying actual needle location.  When using tip vibration, the practitioner makes small, quick movements with the needle.  It the tip of the needle is what is being visualized on ultrasound, the movements will be translated to the tip and easily seen.  If the shaft is what is being visualized, little or no movement will be detected, however movements may be seen in the tissue below the needle.  Hydrolocation uses a small volume of fluid injected through the regional needle to detect the actual level of the needle tip.


References

Pollard BA, Chan VW. An introductory curriculum for ultrasound-guided regional anesthesia: a learner’s guide. Toronto. University of Toronto Press Inc.; 2009:15-18.

Sites B D, Brull R, Chan V W, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med. 2007;32(5):412-418.

Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

Marhofer P, Chan VW. Ultrasound-guided regional anesthesia: current concepts and future trends. Anesth Analg. 2007; 104(5):1265-1269.

Gray AT. Ultrasound-guided regional anesthesia; current state of the art.  Anesthesiology. 2006; 104:368-373.

 

 

 

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