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Ultrasound Technique

The following pages focus on providing clinically significant information on untrasound exam technique. The information on our website and specifically on this page is meant to be an adjunct to the PNBschool block class. The suggested PNBschool Process is described in the image below. The PNBschool block class will provide you with the foundation of learning needed to provide your patients with consistent ultrasound

                                                                  

examinations and post operative pain relief. After you have attended your PNBschool block class you need to continue the process by reviewing periperal nerve block images and videos. This begins the practice phase of the process. When you are comfortable with the proper ultrasound technique and clinically significant anatomy you may choose to begin performing ultrasound guided peripheral nerve blocks. After performing the nerve block it is recommended to revisit the ultrasound images and video on this website to compare with the procedure you performed. From this point you need to improve your technique. Ask yourself what can I do better next time? Did I see all pertinent anatomy while performing the block? Did I see my nerve block needle? Did I witness spread of local anesthetic on the ultrasound? Was my block successful as defined by acceptable pain scores post operatively? After answering these questions you must decide how to improve your technique.

Ultrasound Basics

The physics behind current ultrasound technology is in depth. That being said practitioners should have a basic knowledge of ultrasound technology in order to perform ultrasound guided peripheral nerve blocks. This informational website concentrates on clinical relevance. The following information will provide the most basic summary of ultrasound principles.

Ultrasound uses high frequency sound and depending on the frequency used the sound is either reflected, absorbed or allowed to pass through the patients tissues. Each type of tissue has a specific density which reflects the "sound". This sound is sent through the tissue by a piezoelectric crystal in the

ultrasound probe. The sound that is reflected by the tissue is detected by the same crystal that sent it in the probe. The computer processor portion of the ultrasound machine processes the reflected sound waves and digitizes a picture on the ultrasound screen. Anatomical structures, such as muscle or vascular structures, that absorb sound well (reflecting less sound back to the probe) usually have a higher water content. These structures appear dark on the ultrasound screen. Comparatively, structures that reflect more of the sound waves, such as bone or fascia, contain less water and appear very bright on the ultrasound screen. The contrast between different anatomical structures are consistent and reproducible. The frequency of the sound used allows for varying degrees of penetration into the patient. Lower frequencies will provide deeper penetration while higher frequencies are useful in scanning anatomy that is more superficial or shallow. 

Practitioners will commonly encounter artifact while performing ultrasound examination. Comprehension of artifacts such as shadowing, edge shadowing, reverberation (needle echo) and mirror images could be useful but would be better explained in ultrasound textbooks or other websites.

Those of you that would like a more in depth explanation of ultrasound technology will be dealing with terms such as acoustic velocity, impedence, attenuation coefficient, axial resolution, dampening, scatter and refraction. The manufacturers of today's ultrasound units produce user friendly machines which make it possible to perform peripheral nerve blocks safely and consistently with minimal understanding of how these terms apply to ultrasound examination clinically.

Ultrasound Probe and Needle Manipulation

The ability to consistently perform ultrasound guided peripheral nerve blocks successfully is dependent on a practitioner's skill in manipulation of the ultrasound probe and block needle. The process of keeping the target and needle in the same plane enabling ultrasound visualization of both simultaneously is a learned skill. The following sections describe some of the key elements in becoming an expert at performing ultrasound guided peripheral nerve blocks.

Approach

The "approach" when referring to ultrasound guided peripheral nerve blocks describes the orientation of the block needle to the ultrasound probe. There are two approaches to performing ultrasound guided peripheral nerve blocks, the in plane approach and the out of plane approach. Most any nerve block can be performed using either approach. With the in plane approach the needle is oriented so that it enters the skin on the side of the ultrasound probe so that the entire needle can be visualized on the ultrasound screen. With the out of plane approach the needle enters the skin in front or behind the ultrasound probe and is aimed at the plane of the ultrasound waves. The entire needle is not visualized using this approach in fact only a small section is visualized which is about the width of a credit card.There is a benefit to 

understanding and at times implementing

both approach techniques. For example, a target surrounded by vascular structures may dictate the use of one approach over the other. When reasonable it is the opinion of PNBSchool that the in plane approach is superior to the out of plane approach. The in plane approach when performed correctly provides the benefit of visualizing the needle, the target nerve and the initial injection of local anesthetic which we believe will contribute to more consistently successful nerve blocks. 

       A.                                                 B.

A. In plane vs B. Out of plane ultrasound/needling technique.

Source: InternationalShoulderJournal.org

Angle of Approach

 

The angle at which your needle approaches your target will affect your overall ultrasound picture immensely. Remember it is the opinion of PNBschool that the safest way to perform peripheral nerve blocks guided by ultrasound is the in plane approach when possible. Basically the idea is to visualize your needle tip on ultrasound at all times during the procedure. Keeping your needle tip in view during the procedure will greatly increase your chances of avoiding an intraneural or intravascular injection. 

 

The angle of the nerve block needle during the block procedure should be as parallel to the ultrasound probe as possible. The more parallel the needle to the probe, the more ultrasound waves reflected back to the probe and the better the ultrasound picture of the block needle. Too steep of an angle of the block needle will result in a poor resolution picture of the block needle due to fewer ultrasound waves being reflected back to the ultrasound probe. Variables can be manipulated to help keep your block needle  

Ultrasound needle angle of approach. Source: RCELearning.co.uk, Ultrasound in ER medicine. Atkinson et al.

parallel to the ultrasound probe. 

Probe Manipulation

A consistent ultrasound examination for each individual periperal nerve block is imperative to ensure a successful nerve block and to mimimize the rate of complications and morbidity from nerve blocks. In order to perform a thorough ultrasound examination an understanding of how to manipulate the ultrasound probe must be be attained. Proper manipulation of the ultrasound probe will allow a practitioner to optimize the ultrasound picture and easily identify potential pitfalls during the procedure. Probe technique will 

also keep the needle and target in view during the procedure making delivery of local anesthetic consistent and effective. There are several pnemonics used to refer to ultrasound probe technique. The pnemonic used by PNBschool is TRAPS which stands for Tilt, Rotation, Angulation, Pressure and Sink. The figure below explains the first four maneuvers. Again, all of these maneuvers are used to gain better resolution of the ultrasound picture, to help identify important structures on the ultrasound and to help get your block needle in parallel with your target while keeping both visualized on the ultrasound screen.

Manipulation of the ultrasound probe as described in the image to the right may seem common sense to some practitioners. Other practitioners will rely on the images provided to guide them through the examination process. 

The amount of downward PRESSURE used on the ultrasound probe will affect the examination in multiple ways. For an awake patient manipulation of the ultrasound probe can be uncomfortable. The practitioner should be aware that the amount of pressure or force being applied to the probe translates to the patients comfort during the examination. The amount of pressure will also affect the distance between the probe and the target. The higher the pressure the greater the compression of the adipose in the picure. An examiner can use pressure to decrease the distance to target which may help increase the resolution of the ultasound image.

Source: Int Journal of Shoulder Surgery. 2010. July-Sept; 4(3) 55-62.

ANGULATION, ROTATION AND TILT are also used to define movement of the ultrasound probe during examination and execution of a peripheral nerve block. There is really no science behing these maneuvers. They are all used to gain an ultrasound view the examiner desires. This could be a true cross section view or long axis view or any other view of anatomical structures needed to perform the procedure needed. For the purposes of this website the image above is descriptive enough for these maneuvers.

SINK refers to the "sinking" of one end of the ultrasound probe, using the in plane technique, into the patients soft tissue allowing for a more parallel orientation of the needle to the ultrasound probe. This maneuver will allow for better visualization of the needle on ultrasound helping to ensure precise delivery of local anesthetic to the target. Sink combined with a well placed needle entry point will facilitate any

Sink, applying pressure to one end of the ultrasound probe allowing for a more parallel scan of the ultrasound needle.

block utilizing the in plane approach. Keep in mind patients will tolerate different amounts of pressure applied to the ultrasound probe. This will depend on the anatomical location of the ultrasound probe. For example, pressure applied to the probe on or around any bony structure will be tolerated much less by the patient than pressure applied to the probe where there is more adipose tissue. It is important to keep the target visualized while manipulating the ultrasound probe. Utilization of the Sink maneuver will be especially helpful when the angle of approach to the target is steep ie. the morbidly obese patient

Insertion Point of the Needle

Using the in plane approach, the insertion point of the needle in relation to the ultrasound probe and the target may allow for a more parallel orientation of the needle to the probe. The distance between the ultrasound probe and the block needle will affect the angle of approach. Many times a practitioner may insert the needle adjacent to the ultrasound probe in an attempt to keep the needle in the same plane of the ultrasound probe. When the needle is 

inserted closely to the probe in this manner the angle of the needle may be unnecessarily steep to the ultrasound probe and will be difficult to visualize on ultrasound because very few ultrasound waves are returning to the probe. Remember the goal is to keep the needle visualized at all times during the procedure which is facilitated by keeping the needle parallel to the ultrasound probe. Choosing an appropriate entry point for the block needle may allow for a shallow angle of approach as depicted in the image to the right showing how different insertion points to the same target directly affects how parallel orientation of the needle and ultrasound probe. Multiple needle insertion points may not be available for some procedures. 

Source: International Journal of Shoulder Surgery

Peripheral Nerve Block Safety

Continuous visualization of the block needle on ultrasound is paramount to ensuring consistent success from peripheral nerve blocks. Visualization of the block needle will also minimize the incidence of complications such as a prolonged nerve block and long term nerve damage.

 

The use of nerve stimulators and insulated stimulator needles has been a common practice to minimize the incidence of intraneural injection of local anesthetic during peripheral nerve blocks. This practice of utilizing nerve block stimulators was introduced long before ultrasound was introduced to these procedures. Simply stated, a needle introduced into a nerve while being stimulated would produce a large paresthesia which was perceived as pain and would be communicated by the patient. The needle could then be withdrawn until the paresthesia was eliminated thereby ensuring an intraneural injection did not occur. The stimulation of the needle would produce a motor response of the targeted muscle groups (a twitch) when the needle was in proximity to the nerve. The idea was to observe the motor response ilicited by the electrical current before the needle was intraneural.

The decision to use a nerve stimulator concomitantly with ultrasound guidance is practitioner dependent. PNBschool believes and teaches that proper use of ultrasound guidance to continuously visualize the nerve block needle may allow for a PNB to be performed without the use of a nerve stimulator. Consistent visualization using ultrasound guidance of the target nerve, the block needle and the spread of local anesthetic will minimize the risk of an intraneural injection. Also of importance is the feedback from the "block nurse" or assistant injecting the local anesthetic. Injection of the local should be easy. Any difficulty during injection of the medication should be communicated to the practitioner performing the block and the injection should never be forced. The needle should be repositioned to allow for easy injection and the spread of the local anesthetic should be visualized by ultrasound.

During initial ultrasound examination of a patients anatomy undergoing peripheral nerve block the practitioner should decide on the optimal needle path to the target nerve. This path is commonly determined by the depth of the target nerve and any vasculature that may be in view. When first learning ultrasound guided peripheral nerve block technique practitioners commonly "aim" the needle directly at the target. A better choice may be to a select a tangential approach to the target nerve. This will minimize the risk of inserting the needle directly into the nerve (Image A.)

Image A. Nerve anatomy and possible needle placement

Image A. above shows basic nerve anatomy and multiple needle positions. Needle A is positioned tangentially to the target nerve and local anesthetic is deposited near the epineurium. Assuming the local anesthetic continues to be deposited in this manner the nerve will be properly blocked with predictable results and there will be no complications from this block. Needle B  is intraneural depositing local anesthetic just inside the epineurium. The block nurse may feel the local solution is somewhat difficult to inject but may perceive this as a normal variable and not communicate this to the practitioner performing the block. This is very true of an inexperienced block nurse. The patient may perceive a paresthesia upon injection. This "pain" on injection may subside allowing for more local to be injected. The epineurium will expand to some degree which limits any pressure increase intraneurally. A peripheral nerve block with a needle in this position may lead to the patient experiencing a prolonged block lasting approximately 2-4 days but may be longer. There is usually not any long term complications. The patient may be delayed in postoperative physical therapy and ultimately may have a delayed discharge from the hospital. Needle C is intraneural depositing local anesthetic into the fascicle. The patient will experience a large paresthesia. The fascicle will not expand when the volume of local anesthetic is injection. The pressure in the fascicle will increase even with small volumes of local anesthetic. This injection may result in long term nerve damage which may or may not be reversible over time. Depending on the nerve being targeted a permanent sensory and/or motor deficit may occur.

Every effort has been made to ensure the accuracy of the information presented within this website. The information presented does not constitute medical advice.