19.2 Upper and Lower Limb Peripheral Nerve and Fascial Plane Blocks

19.2 Upper and Lower Limb Peripheral Nerve and Fascial Plane Blocks

Ultrasonography has revolutionized peripheral nerve and plane blockade by enabling real-time direct visualization of neural targets, needle trajectory, and local anaesthetic spread around fascicles and fascial sheets. Maximizing patient safety and block success requires an intimate synthesis of transducer physics, neural electrophysiology, topographic anatomy, and precise injection ergonomics.

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1. Physics and Technology: Ultrasound Guidance and Nerve Stimulation

Ultrasound Principles and Transducer Selection

Ultrasound imaging relies on the piezoelectric effect: alternating electrical currents applied across synthetic ceramic crystals (lead zirconate titanate) induce rapid mechanical expansion and contraction, emitting high-frequency acoustic waves. Returning echoes deform the crystals, generating electrical potentials processed into real-time grayscale B-mode images.

$\text{Velocity } (c) = f \times \lambda$

In human soft tissues, acoustic velocity is assumed to be constant at approximately $1540\text{ m/s}$. The choice of transducer frequency represents a fundamental trade-off between axial resolution and tissue penetration:

• High-Frequency Linear Array Transducers ($10 - 15\text{ MHz}$): Short wavelength yields exceptional axial and lateral resolution ($< 1\text{ mm}$), but the acoustic wave undergoes severe attenuation (due to absorption, scattering, and reflection). Ideal for superficial neural structures located within $< 4 - 5\text{ cm}$ of the skin surface (interscalene, supraclavicular, axillary, forearm nerves, femoral nerve, saphenous nerve).
• Low-Frequency Curved (Curvilinear) Transducers ($2 - 5\text{ MHz}$): Longer wavelengths penetrate deeply into tissues (up to $> 10 - 15\text{ cm}$) at the expense of lower spatial resolution. Indispensable for deep anatomical targets in large or obese patients (gluteal or subgluteal sciatic nerve, lumbar plexus, deep erector spinae plane, pudendal nerve).
• Acoustic Impedance & Doppler Modes: Acoustic reflection occurs at interfaces between tissues with differing acoustic impedance ($Z = \rho \times c$, where $\rho$ is tissue density). Bone and air reflect almost $100\%$ of incident waves, casting dense acoustic shadows beneath the hyperechoic cortex. Color Doppler and Power Doppler exploit the frequency shift of echoes reflected from moving red blood cells to identify arterial and venous structures, preventing inadvertent vascular puncture.

Peripheral Nerve Stimulation (PNS) Electrophysiology

Nerve stimulators deliver constant-current, square-wave pulses ($0.1\text{ ms}$ pulse width at a frequency of $2\text{ Hz}$) to elicit muscle twitches. The relationship between the electrical current required to depolarize a nerve and the needle-to-nerve distance is governed by Coulomb's Law:

$E = \frac{K \times Q}{r^2}$

Where $E$ is the electrical field intensity, $Q$ is current, and $r$ is the distance from needle tip to axon.

+------------------------------------------------------------------------------------------------+
|                       PERIPHERAL NERVE STIMULATION THRESHOLDS                                  |
|                                                                                                |
|  Current Range     Clinical Interpretation & Required Action                                   |
|  ----------------  --------------------------------------------------------------------------  |
|  > 1.0 mA          Needle is distant from nerve; advance cautiously under ultrasound           |
|  0.5 - 1.0 mA      Approaching target nerve sheath; acceptable entry window                    |
|  0.2 - 0.5 mA      Optimal perineural needle tip position outside epineurium; safe to inject   |
|  < 0.2 mA          [ INTRANEURAL WARNING ]: Needle tip may lie inside the epineurium or        |
|                    intrafascicular; injection here risks nerve injury.                         |
|                    -> STOP INJECTION AND WITHDRAW NEEDLE IMMEDIATELY!                          |
+------------------------------------------------------------------------------------------------+

Injection Pressure Monitoring

Forceful injection against resistance is a primary cause of severe permanent nerve injury. Real-time injection pressure monitoring distinguishes safe perineural injection from lethal intrafascicular injection:

• Safe Perineural Space: Opening injection pressure is typically $< 15\text{ psi}$ ($< 103\text{ kPa}$). Local anaesthetic flows freely, surrounding the nerve circumference without tissue distortion ("doughnut sign").
• Intrafascicular or Subperineurial Placement: Opening injection pressure $\ge 15 - 20\text{ psi}$. High injection pressure indicates that the needle tip is wedged within dense perineurial connective tissue or directly inside a nerve fascicle. Injecting at $\ge 20\text{ psi}$ ruptures the protective perineurium, generating sustained endoneurial hydrostatic pressures exceeding capillary perfusion pressure ($30 - 40\text{ mmHg}$), inducing immediate ischemic axonal necrosis and permanent sensory-motor deficit. Injection must be halted immediately whenever opening pressure reaches $\ge 15\text{ psi}$.

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2. Upper Limb Peripheral Nerve Blocks: The Brachial Plexus

The brachial plexus is formed by the ventral rami of C5, C6, C7, C8, and T1, traversing in sequence from Roots $\rightarrow$ Trunks $\rightarrow$ Divisions $\rightarrow$ Cords $\rightarrow$ Terminal Branches ("Rugby Teams Drink Cold Beer").

                                  [ BRACHIAL PLEXUS ANATOMY ]
                                               |
       [ ROOTS ]                     C5, C6, C7, C8, T1
                                               |
       [ TRUNKS ]             Superior (C5-C6), Middle (C7), Inferior (C8-T1)
       (Interscalene Groove)                   |
                                               v
       [ DIVISIONS ]          Anterior & Posterior Divisions (over First Rib)
       (Supraclavicular)                       |
                                               v
       [ CORDS ]              Lateral, Posterior, Medial (around Axillary Artery)
       (Infraclavicular)                       |
                                               v
       [ BRANCHES ]           Musculocutaneous, Axillary, Radial, Median, Ulnar
       (Axilla / Arm)

Interscalene Block

• Anatomy & Approach: Targets the superior and middle trunks (C5, C6, C7) within the interscalene groove, sandwiched between the anterior scalene and middle scalene muscles at the level of the cricoid cartilage ($C6$, Chassaignac's tubercle). Under ultrasound, the roots/trunks appear as a vertical hypoechoic "traffic light" stack of three to four circular structures.
• Indications: Surgery of the shoulder, clavicle, and proximal humerus.
• Limitations: Spares the inferior trunk ($C8 - T1$); therefore, it does not provide anaesthesia to the ulnar nerve distribution (medial forearm, fifth digit, and ulnar half of fourth digit). Unsuitable for hand or forearm procedures!
• Complications & Adverse Effects:
  • Ipsilateral Phrenic Nerve Palsy: Occurs in $\approx 100\%$ of standard-volume ($> 15\text{ mL}$) interscalene blocks due to local anaesthetic tracking anteriorly across the prevertebral fascia over the anterior scalene muscle, where the phrenic nerve ($C3, C4, C5$) courses downward. Produces immediate ipsilateral hemidiaphragmatic paralysis, dropping forced vital capacity ($FVC$) and forced expiratory volume in 1 second ($FEV_1$) by $25 - 30\%$. Strictly contraindicated in patients with severe baseline respiratory impairment (severe COPD, baseline contralateral phrenic nerve palsy, morbid obesity with severe hypoventilation).
  • Horner's Syndrome: Block of the sympathetic stellate ganglion ($C7 - T1$ level): manifested by ptosis, miosis, anhidrosis, enophthalmos, and nasal congestion. Reassure the patient; self-resolving.
  • Recurrent Laryngeal Nerve Palsy: Causes hoarseness and vocal cord paralysis ($10 - 20\%$).
  • Vertebral Artery Injection: The vertebral artery lies immediately medial to the groove within the transverse foramen of C6. Accidental injection of as little as $1 - 2\text{ mL}$ of local anaesthetic directly into the vertebral artery produces immediate, violent grand mal convulsions and cardiac collapse.

Supraclavicular Block

• Anatomy & Approach: Known as the "spinal of the arm." Targets the compact cluster of divisions and trunks located postero-lateral to the pulsating subclavian artery, sitting directly above the hyperechoic first rib ("corner pocket").
• Indications: Extensive surgery of the arm, elbow, forearm, and hand.
• Key Hazard: Pneumothorax. The visceral and parietal pleura lie immediately medial and deep to the first rib. Ultrasound visualization of the hyperechoic first rib with its acoustic shadow acting as a backstop is mandatory to avoid pleural puncture and pneumothorax.

Infraclavicular Block

• Anatomy & Approach: Targets the three cords (Lateral, Posterior, and Medial) clustered circumferentially around the second part of the axillary artery, deep to the pectoralis major and pectoralis minor muscles at the coracoid process level.
• Indications: Forearm, wrist, and hand surgery. Ideal for indwelling catheter placement because the catheter is secured beneath the thick pectoral musculature, drastically reducing dislodgement and motion-related failure compared to the mobile neck.
• Advantages: Provides reliable coverage of the axillary nerve and musculocutaneous nerve, and usually the medial brachial cutaneous nerve (medial cord); the intercostobrachial nerve (T2) is not part of the brachial plexus, so tourniquet pain needs a separate subcutaneous injection across the medial upper arm.

Axillary Block

• Anatomy & Approach: Targets the terminal branches surrounding the axillary artery within the axillary neurovascular fascial sheath: Median nerve (anterolateral/superior to artery), Ulnar nerve (anteromedial/inferior), and Radial nerve (posterior/deep).
• Critical Anatomical Caveat: The Musculocutaneous Nerve: The musculocutaneous nerve leaves the lateral cord high in the infraclavicular fossa and pierces into the belly of the coracobrachialis muscle, lying outside the axillary neurovascular sheath. To achieve complete forearm anaesthesia (motor to biceps and sensory to the lateral forearm via the lateral antebrachial cutaneous nerve), a separate targeted injection of $3 - 5\text{ mL}$ into the coracobrachialis muscle is strictly required!

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3. Lower Limb Peripheral Nerve Blocks

+-------------------------------------------------------------------------------------------------------+
|                                 LOWER LIMB NERVE BLOCKS COMPARISON                                    |
|                                                                                                       |
|  Block Type       Target Nerve(s)         Motor Effects             Ideal Clinical Indication         |
|  ---------------  ----------------------  ------------------------  --------------------------------  |
|  Femoral Block    Femoral (L2-L4) under   Profound quadriceps       Femur fracture, anterior thigh    |
|                   fascia iliaca           weakness; knee buckling   grafting (High fall risk!)        |
|                                                                                                       |
|  Adductor Canal   Saphenous nerve &       Quadriceps motor-sparing  Total knee arthroplasty (TKA);    |
|  Block (ACB)      n. to vastus medialis   (spares rectus femoris)   early physical therapy ambulation |
|                                                                                                       |
|  Popliteal        Sciatic nerve (L4-S3)   Foot drop / loss of       Foot and ankle surgery;           |
|  Sciatic Block    at bifurcation          plantar/dorsiflexion      requires saphenous block for      |
|                                                                     medial ankle / foot coverage      |
+-------------------------------------------------------------------------------------------------------+

Femoral Nerve Block

• Anatomy: Originates from the lumbar plexus (L2, L3, L4). Traverses the pelvis behind the psoas major, emerging beneath the inguinal ligament into the femoral triangle. Lies immediately lateral to the femoral artery, deep to both the fascia lata and fascia iliaca ("NAVL" from lateral to medial: Nerve, Artery, Vein, Lymphatics).
• Indications: Femoral shaft fractures, patella surgery, anterior thigh skin grafting.
• Major Limitation: Causes profound quadriceps femoris motor paralysis. Patients cannot actively extend the knee or bear weight, resulting in knee buckling and a markedly increased incidence of catastrophic postoperative falls. This has led to its replacement by the adductor canal block for elective total knee replacement.

Adductor Canal Block (ACB)

• Anatomy: Located in the middle third of the medial thigh within the subsartorial (Hunter's) canal. Bounded anterolaterally by the vastus medialis, posteromedially by the adductor longus (proximally) and adductor magnus (distally), and roofed anteriorly by the sartorius muscle. The canal contains the superficial femoral artery, femoral vein, the saphenous nerve (terminal sensory branch of the femoral nerve), and the nerve to the vastus medialis.
• Clinical Mechanism & Efficacy: Local anaesthetic ($10 - 15\text{ mL}$) deposited anterolateral to the femoral artery deep to the sartorius blocks the saphenous nerve and articular branches to the anterior and medial knee joint capsule. Crucially, the motor branches to the rectus femoris and vastus lateralis/intermedius arise proximally outside the canal and are entirely spared. ACB delivers equivalent analgesia to a femoral nerve block for total knee arthroplasty (TKA) while preserving quadriceps motor strength, facilitating immediate same-day mobilization and drastically reducing inpatient fall risk.

Sciatic Nerve Block: Popliteal Approach

• Anatomy: The sciatic nerve (L4 through S3) descends through the posterior thigh and enters the popliteal fossa. In the upper popliteal fossa ($5 - 10\text{ cm}$ above the popliteal crease), it bifurcates into the tibial nerve (larger medial branch providing plantarflexion and sole sensation) and the common peroneal (fibular) nerve (lateral branch wrapping around the fibular neck, providing dorsiflexion, eversion, and dorsal foot sensation).
• Technique: Injecting local anaesthetic ($15 - 20\text{ mL}$) within the common paraneural sheath at the point of bifurcation envelopes both divisions.
• Indications: Major surgery of the lower leg, ankle, and foot (Achilles tendon repair, ankle fusion, calcaneal fracture osteosynthesis).
• Clinical Pearl: The sciatic nerve innervates the entire lower leg and foot except for the medial skin strip of the tibia, medial malleolus, and medial arch of the foot, which is innervated by the saphenous nerve (femoral branch). For comprehensive surgical anaesthesia below the knee, a popliteal sciatic block must be combined with a saphenous nerve block.

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4. Fascial Plane Blocks of the Trunk

Fascial plane blocks involve the injection of large volumes of dilute local anaesthetic into intermuscular connective tissue planes, relying on widespread passive hydraulic distribution to block traversing cutaneous and intercostal nerves.

Transversus Abdominis Plane (TAP) Block

• Anatomy: The anterior abdominal wall comprises three muscular layers: external oblique, internal oblique, and transversus abdominis. The anterior primary rami of the lower six thoracic nerves (T7 - T11), the subcostal nerve (T12), and the iliohypogastric/ilioinguinal nerves (L1) run within the neurovascular fascial plane between the internal oblique and transversus abdominis muscles.
• Subcostal vs. Lateral TAP:
  • Lateral TAP Block: Needle placed at the mid-axillary line between the iliac crest and costal margin. Spreads local anaesthetic across T10 - L1; covers somatic pain for lower abdominal surgery (infracolic laparotomy, appendectomy, Caesarean delivery, inguinal hernia).
  • Subcostal TAP Block: Needle placed parallel and immediately inferior to the costal margin. Distributes local anaesthetic across T7 - T9; covers somatic pain for upper abdominal surgery (open cholecystectomy, gastrectomy, hepatic resections).
• High-Yield Clinical Principle: TAP blocks provide purely somatic analgesia to the parietal peritoneum, abdominal wall muscles, and skin. They provide zero visceral analgesia (which is mediated by sympathetic splanchnic afferents). Visceral pain from bowel traction or distension must be addressed with multimodal systemic analgesia.

Erector Spinae Plane (ESP) Block

• Anatomy & Mechanism: The needle is advanced under ultrasound in a parasagittal plane onto the posterior acoustic shadow of the vertebral transverse process (commonly at $T4 - T5$ for thoracic/breast surgery, or $T7 - T9$ for abdominal surgery). Local anaesthetic ($20 - 30\text{ mL}$) is deposited deep to the erector spinae muscle group (iliocostalis, longissimus, spinalis) directly over the bone.
• Mechanism of Spread: Local anaesthetic reliably reaches the dorsal rami; spread anteriorly through the intertransverse connective tissue into the costotransverse and paravertebral spaces, where it can reach the ventral rami and sympathetic chain, is variable between patients and studies.
• Clinical Versatility: Because paravertebral spread is inconsistent, any visceral analgesia from an ESP block is variable and its mechanism is debated. It is extensively utilized for thoracotomy, rib fractures, mastectomy, laparoscopic and open abdominal surgeries, and lumbar spine operations.

Chest Wall Fascial Blocks: PECS I, PECS II, and Serratus Anterior Plane (SAP)