USRA Introduction - The use of ultrasound for regional anesthesia

General Comments

The use of ultrasound for regional anesthesia is relatively new, however interest in this application is growing rapidly. Ultrasound guided nerve blocks were first described as early as 1978, but it was not until the advent of advanced ultrasound technology in the 1990's that interest in this field grew. Published reports of ultrasound guided regional anesthesia have largely focused on brachial plexus blockade in the interscalene, supraclavicular, infraclavicular and axillary regions. Recent studies examining the efficacy of ultrasound guidance for femoral, sciatic, psoas compartment, celiac plexus and stellate ganglion blocks are promising, while ultrasound visualization of the epidural space can facilitate neuraxial blockade in children, adults and parturients.

The materials on this website describe both the techniques for single shot and continuous nerve blocks of the brachial plexus and lumbosacral plexus, as well as techniques for neuraxial blockade. Each technique is described in an easy step-by-step manner and is accompanied by a list of selected references. The goal of the materials on this website is to impart a greater understanding of ultrasound imaging for regional anesthesia to anesthesia practitioners and pain management clinicians.

Conventional peripheral nerve block techniques that are performed without visual guidance are highly dependent on surface anatomical landmarks for localization of the target nerve. It is therefore not surprising that regional anesthetic techniques are associated with a reported failure rate of up to 20% presumably because of incorrect needle and/or local anesthetic placement. Multiple trial-and-error attempts to locate the target nerve can lead to operator frustration, unwarranted patient pain, and time delay in the operating room, especially in patients with difficult anatomical landmarks.

Imaging technology such as MRI and CT scan can successfully localize neural structures. However, ultrasound is the most practical imaging tool for regional anesthesia as it is portable, relatively easy to learn, moderately priced, and does not pose any radiation risk. Ultrasound provides real time imaging guidance during a nerve block procedure.

Advantages of Ultrasound

Reveals the nerve location and the surrounding vascular, muscular, bony, and visceral structures.
Provides real-time imaging guidance during needle advancement allowing for purposeful needle movement and proper adjustments in direction and depth.
Images the local anesthetic spread pattern during injection.
Improves the quality of sensory block, the onset time, and the success rate compared to nerve stimulator techniques (as shown in some clinical studies).
Reduces the number of needle attempts for nerve localization which may prove to reduce the risk of nerve injury.
Differentiates extravascular injection from unintentional intravascular injection.
Differentiates extraneural injection from unintentional intraneural injection.

Limitations of Peripheral Nerve Stimulation Techniques

Peripheral nerve stimulation (PNS) guidance is useful only when a motor response is elicited.
NS provides objective but indirect evidence of nerve location.
Evidence of proper needle placement (i.e. motor response) disappears after injection of 1-2 mL of local anesthetic.
Motor response achieved at less than 0.5 mA does not guarantee a successful or complete block.
PNS does not prevent intravascular, intraneural or pleural puncture.

Disclaimer:

Although reasonable steps have been taken to ensure the accuracy of the information contained on this website, the technique of ultrasound guided nerve blocks may be a challenge to master. As with any new skill that is highly user-dependent, the reader is advised to use discretion and judgment when applying this technique in the care of patients. The authors do not assume any responsibility, express or implied, for any physician who may rely upon the information contained on this website.

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Terminology

Absorption
the loss of ultrasound energy as a result of its conversion to another form of energy such as heat or intracellular mechanical vibration

Acoustic impedance
the resistance to sound transmission through a medium

Acoustic power
the amount of acoustic energy generated per unit time

Amplitude
the strength of a sound signal

Artifacts
display distortions, additions or errors that can adversely affect ultrasound image acquisition or interpretation

Attenuation
the loss of ultrasound energy when the wave travels deep into the tissues due to absorption, reflection and scattering of sound energy

Axial resolution
the ability to distinguish two structures as separate when the structures are lying close to each other along the same axis as the ultrasound path

Cycle
the combination of one rarefaction and one compression equals one cycle

Diffuse reflection
the reflection that comes off a reflector with an irregular surface

Doppler effect
a change in the frequency of sound as a result of motion between the sound source and the receiver; a positive shift occurs when the source and receiver are approaching each other and a negative shift occurs when they are moving away from each other

Dynamic range
the ratio of the maximum level of a given parameter to its minimum level; in ultrasound, the dynamic range defines a range of echo intensities that are displayed as a gradient of grey values (minimum value in black and maximum value in white pixels in the final image)

Echogenicity
the degree of brightness of a structure displayed on ultrasound; this is influenced by the amount of beam returning to the transducer (reflection) after encountering the target structure

Frequency
the number of cycles per second; frequency is the inverse of wavelength; the higher the frequency, the shorter the wavelength

Hyperechoic
the image characteristic of a structure that is highly reflective resulting in a brighter displayed image compared to the surrounding structures; bone and pleura are examples of hyperechoic structures

Hypoechoic
the image characteristic of a structure that is less reflective than the surrounding structure resulting in a darker displayed image compared to the surrounding structures; fluid filled structures e.g., vessels and cyst are hypoechoic

Interface
the boundary between two tissue media with different acoustic impedances

Lateral resolution
the ability of the system to distinguish two structures as separate when the structures are lying side by side

Longitudinal wave
movement of particles in the same direction as the direction of the wave propagation

Period
the amount of time required to complete one cycle

Pulse repetition frequency (PRF)
the number of pulses occurring in a given time interval; for example, 1 Hz (Hertz) is one cycle per second, 10 Hz is 10 cycles per second; a lower PRF is required for unambiguous discrimination of structures at deeper imaging depths

Pulse repetition period
time from the start of one pulse to the start of the next pulse

Pulse duration
the time measured from the start of one pulse to the end of the same pulse

Rayleigh scattering
scattering of the wave in all directions when the reflector is much smaller than the ultrasound wavelength

Reflection
mirror-like redirection and return of a propagating sound wave towards the transducer that follows a standard law of reflection; for example, specular reflection results in the reflected angle being equal to the incident angle of the energy propagation

Refraction
a change in the direction of wave propagation when traveling from one medium to another with different propagation speeds according to the Snell's Law of refraction

Resolution
the ability to distinguish between two structures that lie close to one another

Scattering
a process by which the ultrasound is forced to deviate from a straight-line reflection and trajectory due to small, localized non-uniformities in the tissue

Specular reflection
the reflection that comes off a smooth reflector (e.g., a mirror)

Transverse wave
movement of particles perpendicular to the direction of the wave propagation

Velocity
the sound speed and direction of motion

Wavelength
the distance traveled between two consecutive peaks or troughs of a wave

Regional Anesthesia - Introduction

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