13.3 Peripheral Nerve Blocks & Ultrasound Guidance

13.3 Peripheral Nerve Blocks & Ultrasound Guidance

Peripheral nerve blocks (PNBs) provide site-specific surgical anesthesia and targeted postoperative analgesia while avoiding the systemic hemodynamics and airway instrumentation of general anesthesia. Today, point-of-care high-resolution ultrasonography represents the standard of care for regional anesthesia. Anesthesia technologists play a vital clinical role in selecting and preparing appropriate ultrasound transducers, setting up sterile probe sheaths and acoustic coupling gel, operating regional block needles, and monitoring for procedure-specific complications such as pneumothorax, vascular puncture, and diaphragmatic paresis.

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Upper Extremity Blocks: Brachial Plexus Anatomy & Approaches

The brachial plexus is formed by the anterior primary rami of the C5, C6, C7, C8, and T1 spinal nerve roots. As it descends from the neck toward the axilla, it organizes into successive anatomical divisions:

BRACHIAL PLEXUS ANATOMICAL DIVISIONS:

  ROOTS (C5-T1)  -->  TRUNKS (Superior, Middle, Inferior)  -->  DIVISIONS (Anterior, Posterior)
        |
        v
  CORDS (Lateral, Posterior, Medial)  -->  TERMINAL BRANCHES (Musculocutaneous, Axillary, Radial, Median, Ulnar)

  (Clinical Mnemonic: "Robert Taylor Drinks Cold Beer" -> Roots, Trunks, Divisions, Cords, Branches)

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|                   BRACHIAL PLEXUS REGIONAL BLOCK APPROACHES                 |
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  1. INTERSCALENE BLOCK (C5-C7 Roots / Trunks):
     - Location: Between Anterior & Middle Scalene muscles at C6 (cricoid level)
     - Surgical Coverage: Shoulder & Proximal Humerus (Spares Ulnar Nerve / C8-T1)
     - Complications: 100% Phrenic Nerve Palsy, Horner's Syndrome, Hoarseness,
       Vertebral Artery injection risk

  2. SUPRACLAVICULAR BLOCK (Trunks & Divisions - "The Spinal of the Arm"):
     - Location: Posterior-lateral to Subclavian Artery over the First Rib
     - Surgical Coverage: Arm, Elbow, Forearm, Hand (Complete, compact block)
     - Complications: Pneumothorax (~1%), Phrenic Palsy (~50%), Subclavian puncture

  3. INFRACLAVICULAR BLOCK (Cords - Lateral, Posterior, Medial):
     - Location: Surrounding Axillary Artery deep to Pectoralis Minor muscle
     - Surgical Coverage: Elbow, Forearm, Hand (Ideal for continuous indwelling catheters)
     - Advantages: Low pneumothorax risk, highly stable catheter site

  4. AXILLARY BLOCK (Terminal Branches - Median, Radial, Ulnar):
     - Location: Surrounding Axillary Artery within the axillary neurovascular sheath
     - Surgical Coverage: Forearm, Wrist, Hand
     * Special Requirement: Musculocutaneous nerve lies separately in Coracobrachialis
       muscle and MUST be blocked with a separate dedicated local anesthetic deposit!

Interscalene Brachial Plexus Block

• Anatomical Target: Performed at the level of the cricoid cartilage (C6 vertebral level) in the interscalene groove between the anterior scalene and middle scalene muscles. Under ultrasound, the roots/trunks appear as a vertical stack of three or four hypoechoic round structures resembling a "traffic light".
• Indications: Total shoulder arthroplasty, rotator cuff repair, acromioplasty, and proximal humerus fractures.
• Limitations: The block reliably anesthetizes C5, C6, and C7. However, it frequently spares the inferior trunk (C8 and T1), leaving the ulnar nerve unblocked. Therefore, interscalene blockade is poorly suited for surgery on the medial forearm, wrist, or hand.
• High-Risk Complications:
  1. Ipsilateral Phrenic Nerve Palsy (100% Incidence): The phrenic nerve (C3, C4, C5) courses directly across the anterior surface of the anterior scalene muscle beneath the prevertebral fascia, mere millimeters from the interscalene injection site. Standard local anesthetic volumes (15 to 20 mL) cause an intentional or unavoidable 100% incidence of ipsilateral diaphragmatic paralysis. In healthy individuals, this produces a transient 25% to 30% reduction in forced vital capacity (FVC). However, in patients with severe chronic obstructive pulmonary disease (COPD), baseline hypoxemia, morbid obesity-hypoventilation syndrome, or contralateral diaphragmatic paralysis, this reduction precipitates acute respiratory failure. Interscalene block is therefore generally avoided in these patients, or modified (for example, lower volumes or alternative approaches) by the provider.
  2. Horner's Syndrome: Caused by local anesthetic tracking medially to block the cervical sympathetic chain and stellate ganglion. Manifests as ipsilateral ptosis (drooping eyelid), miosis (constricted pupil), anhidrosis (absence of sweating over the ipsilateral face), and conjunctival injection. It is benign and self-limiting, resolving as the block wears off.
  3. Recurrent Laryngeal Nerve Palsy: Causes unilateral vocal cord paresis, resulting in hoarseness and a subjective feeling of throat fullness.
  4. Vertebral Artery Cannulation: The vertebral artery ascends through the transverse foramina of the cervical vertebrae directly medial and deep to the interscalene groove. Inadvertent intra-arterial injection of even a very small volume of local anesthetic delivers a direct bolus to the brain, triggering immediate grand mal seizures, coma, and cardiorespiratory arrest.

Supraclavicular Brachial Plexus Block

• Anatomical Target: Known as the "spinal of the upper extremity". At the supraclavicular level, the brachial plexus trunks and divisions are tightly clustered into a compact "bundle of grapes" sitting immediately posterior-lateral to the pulsating subclavian artery, supported directly on top of the hyperechoic first rib.
• Indications: Entire upper extremity below the shoulder—distal humerus, elbow, forearm, wrist, and hand.
• Pneumothorax Risk (~1%): The cupola of the parietal pleura and apex of the lung lie directly medial and deep to the first rib. If the regional block needle is advanced too deeply or directed too medially past the bony barrier of the first rib, it punctures the pleura, creating a pneumothorax. Technologists must verify that the anesthesia provider clearly identifies the bright, hyperechoic first rib (which casts an acoustic shadow) and the sliding pleural line with "comet-tail" artifacts before needle advancement.

Infraclavicular & Axillary Blocks

• Infraclavicular Block: Targets the three cords (lateral, posterior, and medial) arranged circumferentially around the axillary artery, deep to the pectoralis major and minor muscles. Excellent for elbow, forearm, and hand surgery. Because the needle enters below the clavicle and far lateral to the thoracic cage, pneumothorax risk is near-zero under ultrasound guidance. It provides an exceptionally stable, flat muscular bed for long-term continuous indwelling catheter infusions.
• Axillary Block: Targets the terminal branches within the axillary neurovascular sheath: median, radial, and ulnar nerves arranged around the axillary artery.
• The Musculocutaneous Nerve Caveat: The musculocutaneous nerve exits the lateral cord high in the axilla, leaving the neurovascular sheath early to course independently within the muscular substance of the coracobrachialis muscle. If local anesthetic is only injected around the axillary artery, the musculocutaneous nerve will be missed! The provider must advance the needle separately into the coracobrachialis muscle to deposit 5 to 8 mL of local anesthetic around the musculocutaneous nerve; otherwise, the patient will retain full sensation over the lateral forearm.

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Lower Extremity Blocks: Femoral, Sciatic, Popliteal & Ankle

LOWER EXTREMITY BLOCKS & ANATOMICAL LANDMARKS

  1. FEMORAL NERVE (Lumbar Plexus L2-L4):
     - Landmark: Femoral Triangle beneath Fascia Iliaca (Lateral to Medial: NAVEL)
     - Distribution: Anterior thigh, femur, knee (Causes Quadriceps motor weakness)

  2. SCIATIC NERVE (Sacral Plexus L4-S3):
     - Largest nerve in the body; posterior thigh, knee, and entire lower leg/foot

  3. POPLITEAL SCIATIC BLOCK (Popliteal Fossa):
     - Targets Sciatic bifurcation into Tibial (medial) and Common Peroneal (lateral)
     - Distribution: Foot, ankle, Achilles tendon (Ideal for ankle surgery)

  4. ANKLE BLOCK (5 Distinct Peripheral Nerves):
     - Saphenous Nerve (Femoral)           --> Anterior to Medial Malleolus
     - Posterior Tibial Nerve (Sciatic)    --> Behind Medial Malleolus (sole of foot)
     - Sural Nerve (Sciatic)               --> Behind Lateral Malleolus (lateral foot)
     - Deep Peroneal Nerve (Sciatic)       --> Anterior ankle between tendons (1st web space)
     - Superficial Peroneal Nerve (Sciatic)--> Subcutaneous band over anterior ankle (dorsum)

Femoral Nerve Block

• Anatomy & Orientation: Formed from the posterior divisions of L2-L4 in the lumbar plexus. In the groin, the femoral nerve emerges beneath the inguinal ligament into the femoral triangle, lying deep to both the fascia lata and fascia iliaca.
• The NAVEL Mnemonic: Memorize the lateral-to-medial orientation of structures within the femoral triangle at the inguinal crease: Nerve, Artery, Vein, Empty space, Lymphatics. The femoral nerve lies lateral to the pulsating femoral artery.
• Clinical Effect & Fall Risk: The femoral nerve innervates the quadriceps femoris muscle. Femoral nerve blockade produces profound motor weakness of knee extension, significantly increasing the postoperative fall risk. In modern total knee arthroplasty, the femoral block is frequently replaced by an adductor canal block, which selectively blocks the sensory saphenous nerve while sparing quadriceps motor function.

Popliteal Sciatic Block

• Anatomy: In the upper popliteal fossa, the large sciatic nerve divides into two major divisions: the tibial nerve (which continues straight down medially) and the common peroneal (common fibular) nerve (which deviates laterally around the fibular head).
• Block Location: Local anesthetic (15 to 25 mL) is deposited immediately proximal to this bifurcation point, within the common paraneural sheath enclosing both components.
• Indications: Ankle fractures, calcaneal reconstruction, bunionectomy, and Achilles tendon repair. To achieve total surgical anesthesia of the ankle, the popliteal block must be supplemented with a saphenous nerve block to cover the medial malleolus.

The 5 Nerves of the Ankle Block

The ankle block is a pure peripheral sensory block requiring no motor impairment of the leg. Anesthesia technologists must memorize all five nerves, their origins, and their anatomical trajectories:

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Wrist Block

The ASATT content outline names the wrist block among regional blocks whose landmarks and complications technologists should know. A wrist block anesthetizes the hand by blocking three nerves just proximal to the wrist crease, while leaving forearm muscle function largely intact.

• Complications: Intraneural injection (avoid injecting if the patient reports sharp paresthesia or high resistance), ulnar artery puncture, and hematoma.
• Limitation: A wrist block does not cover an upper-arm tourniquet, so tourniquet pain may limit longer cases.

Digital Block

A digital block anesthetizes a finger or toe by blocking the paired palmar (plantar) and dorsal digital nerves at the base of the digit, either through injections on each side of the proximal phalanx or through a web-space or transthecal approach.

• Volume: Small volumes (a few milliliters per side) are used, because large circumferential volumes can compress the digital vessels.
• Additives: Epinephrine has traditionally been avoided in digital blocks; current evidence suggests low-concentration epinephrine is safe in many patients, but the choice follows the provider's plan and institutional practice.
• Complications: Vascular compromise from excessive volume or tight ring tourniquets, and intraneural or intravascular injection.

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Ultrasound Physics & Transducer Technology

Medical ultrasound operates via the piezoelectric effect. Lead zirconate titanate (PZT) ceramic crystals within the transducer expand and contract when pulsed with alternating electrical voltages, emitting high-frequency mechanical sound waves. Returning sound waves deform the crystals, creating electrical voltages that are digitally processed into cross-sectional grayscale images.

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|                   ULTRASOUND TRANSDUCER SELECTION CRITERIA                  |
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  HIGH-FREQUENCY LINEAR PROBE (10 to 15 MHz):     LOW-FREQUENCY CURVILINEAR PROBE (2 to 5 MHz):
  - Wavelength: Very short                         - Wavelength: Long
  - Resolution: Exceptional high spatial detail    - Resolution: Lower spatial detail
  - Penetration: Shallow (< 3 to 4 cm deep)        - Penetration: Deep acoustic reach (8 to 15+ cm)
  - Beam Shape: Rectangular footprint              - Beam Shape: Diverging fan / sector
  * CLINICAL USE: Superficial blocks               * CLINICAL USE: Deep blocks & spine
    (Interscalene, Supraclavicular, Axillary,        (Deep Sciatic, Lumbar Plexus, Epidural
     Femoral, Vascular access)                        depth in obesity)

Frequency, Resolution & Attenuation Tradeoff

Sound wave frequency determines the fundamental operational parameters of the transducer:

• High-Frequency Transducers (10 to 15 MHz): Produce very short acoustic wavelengths. Because spatial resolution is directly proportional to frequency, high-frequency probes provide exquisite, razor-sharp anatomical visualization of tiny fascial planes, nerve fascicles, and needle tips. However, high-frequency sound waves suffer rapid tissue attenuation (energy loss via heat absorption and scattering), limiting their acoustic penetration to structures less than 3 to 4 cm deep.
• Low-Frequency Transducers (2 to 5 MHz): Produce longer wavelengths that penetrate deeply through dense adipose and muscle tissues (up to 8 to 15 cm or more), but sacrifice fine axial and lateral spatial resolution. Indicated for deep sciatic nerve blocks in obese patients, lumbar plexus blocks, and ultrasound-guided neuraxial (spinal/epidural) pre-procedure scanning.

Acoustic Impedance & Echogenicity Patterns

Structures are visualized based on acoustic impedance (tissue density multiplied by sound velocity) and the reflection of sound waves back to the probe:

• Anechoic (Jet Black): Structures that contain no internal acoustic reflectors; sound waves transmit completely through without reflection. Examples: Blood vessels (arteries, veins), local anesthetic fluid pockets, simple cysts.
• Hypoechoic (Dark Gray): Tissues that reflect small amounts of sound waves back to the transducer. Examples: Skeletal muscle tissue, lymph nodes, internal neural fascicles.
• Hyperechoic (Bright White): Structures with high acoustic impedance that reflect a large percentage of the sound beam. Examples: Dense bone cortex, fascial sheaths, epineurium, tendon sheaths, and regional block needle shafts.
• Acoustic Shadowing: When sound strikes a dense, highly reflective barrier (such as bone cortex or gallstones), virtually 100% of the beam is reflected or absorbed. No sound travels deeper, creating an acoustic shadow (a dark void) directly beneath the hyperechoic surface (e.g., beneath the first rib or transverse processes).

Needle Visualization: In-Plane vs. Out-of-Plane

NEEDLE GUIDANCE APPROACHES:

  1. IN-PLANE APPROACH (Long-Axis View):             2. OUT-OF-PLANE APPROACH (Short-Axis View):

        [Ultrasound Probe]                                [Ultrasound Probe]
        +================+                                +================+
               /                                                  |
              / (Needle advanced parallel                         . (Needle crosses perpendicular;
             /   to sound beam plane)                                appears only as a single
            v                                                        hyperechoic dot)
     Entire needle shaft & tip visualized                High danger: Mistaking shaft for tip!
     in real-time as a bright white line

• In-Plane Technique: The needle is inserted into the skin at one edge of the ultrasound probe and advanced strictly parallel to the ultrasound beam's longitudinal plane. The entire needle shaft and the sharp needle tip are continuously tracked in real-time as a bright, hyperechoic line advancing toward the target nerve. This offers maximum safety by preventing inadvertent arterial puncture or pleural laceration.
• Out-of-Plane Technique: The needle is inserted perpendicular to the transducer face, intersecting the thin acoustic beam slice. The needle appears on the monitor only as a single bright, hyperechoic dot. The primary clinical danger of out-of-plane guidance is mistaking an acoustic reflection from the needle shaft for the true needle tip; the real tip may actually reside several centimeters deeper in an unseen plane, risking undetected nerve impalement or vascular perforation.