Intercoastal nerve block. Clinical anatomy

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Intercostal Nerve Block - Clinical Anatomy

Overview of the Intercostal Nerves

The intercostal nerves are the anterior primary rami (ventral rami) of thoracic spinal nerves T1-T11. The 12th thoracic anterior ramus runs below the 12th rib as the subcostal nerve. Each nerve exits the spine through the intervertebral foramen, which lies midway between two adjacent ribs.

Upon exiting, each nerve immediately gives off:

Posterior (dorsal) cutaneous branch - supplies the skin and muscles of the paraspinal area
Lateral cutaneous branch - arises at the midaxillary line; supplies the anterior and posterior lateral chest wall
Anterior cutaneous branch - the terminal branch

Gray's Anatomy for Students, p. 197
Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 675

Neurovascular Bundle - The Key Anatomical Relationship

Each intercostal nerve travels with its corresponding intercostal artery and vein within the subcostal groove - a channel on the ventral (inferior) surface of each rib. The arrangement within the groove, from superior to inferior, is:

Position	Structure
Superior (most protected)	Intercostal Vein
Middle	Intercostal Artery
Inferior	Intercostal Nerve

The mnemonic VAN (Vein, Artery, Nerve) describes this top-to-bottom order. The vein and artery lie above the nerve and are therefore somewhat protected by the rib during a nerve block - but the nerve itself is the most inferiorly exposed structure.

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 675

Muscular Layers of the Intercostal Space

The intercostal space is bound by three muscular layers:

External intercostal muscle - outermost; fibers run inferomedially
Internal intercostal muscle - middle layer
Innermost intercostal muscle - deepest; separated from the internal intercostal by the neurovascular bundle

The neurovascular bundle runs between the internal intercostal and innermost intercostal muscles, deep to the subcostal groove. The fasciae of the internal and external intercostal muscles form the walls of the intercostal groove.

From Miller's Anesthesia (10th ed.), the intercostal space is described as a triangular space with:

Medial border: posterior intercostal and innermost intercostal muscles, endothoracic fascia, and parietal pleura
Lateral border: internal and external intercostal muscles and intercostal membrane
Base: the lower rib

Deep to the innermost intercostal muscle lies the endothoracic fascia and then the parietal pleura - this is the critical structure to avoid.

Miller's Anesthesia, 10th ed., p. 11085
Barash Clinical Anesthesia, 9th ed., p. 2920

Why Block Position Matters

Intercostal nerve block - area of anesthesia (anterior and posterior views, T3-T12)

Cutaneous distribution of intercostal nerves T3-T12 - the shaded area shows the zone of anesthesia achieved (Roberts and Hedges)

The position of the block along the chest wall determines which branches are anesthetized:

Block Location	Branches Blocked	Clinical Effect
Paravertebral / angle of rib (5-7 cm from midline)	All branches (posterior cutaneous + lateral cutaneous + anterior cutaneous)	Full dermatomal anesthesia
Posterior axillary line	Lateral cutaneous + anterior cutaneous	Adequate for rib fractures; most practical with buffer from internal intercostals
Midaxillary line	Anterior cutaneous only (lateral branch already given off)	Incomplete - misses lateral chest wall
Anterior chest wall	Anterior cutaneous only	Very limited coverage

The nerve is blocked before the lateral cutaneous branch arises (i.e., proximal to the midaxillary line) to achieve complete dermatomal coverage. The posterior axillary line is the preferred site because the internal intercostal muscle here forms a buffer between the nerve and the pleura, reducing pneumothorax risk.

Roberts and Hedges', p. 675
Barash Clinical Anesthesia, 9th ed., p. 2990

Anatomical Changes Along the Rib

Posteriorly (near angle of rib, 5-7 cm from midline): The subcostal groove is well-defined and deep. The nerve is separated from the pleura only by the thin intercostal fascia here - careful technique required.
Anteriorly (beyond midaxillary line): The intercostal groove becomes progressively less well-defined. The nerves begin to move away from their protected position under the rib. This is why blocks are best performed posteriorly.
T1-T5 levels: These upper levels may be better approached via paravertebral block because the overlying scapula and bulky paraspinal musculature make intercostal access difficult.
Subcostal nerve (T12): Less closely associated with its rib, making it harder to identify and block with classic intercostal technique.
Barash Clinical Anesthesia, 9th ed., pp. 2990-2991

Ultrasound Anatomy

Ultrasound-guided intercostal nerve block - in-plane needle approach showing rib, pleura, and needle position

USG view: (1) needle contacts rib; (2) needle walks caudally off rib into the intercostal groove above the pleura (Miller's Anesthesia)

Under ultrasound, the key landmarks are:

The rib (hyperechoic curved structure with posterior acoustic shadow)
The parietal pleura (bright hyperechoic line below the rib, with sliding lung sign)
The neurovascular bundle in the subcostal groove just inferior to the rib

Clinical Applications

Indication	Levels Blocked
Rib fractures	Level of fractured rib ± 1-2 adjacent levels (for overlap)
Thoracotomy analgesia	Multiple levels covering incision
Cholecystectomy / upper abdominal surgery	T5-T12 (5-6 levels unilaterally)
Mastectomy	T2-T6
Chest tube insertion	Level of tube insertion
Percutaneous biliary drainage	T6-T10

Because adjacent intercostal nerves overlap significantly, the rib of interest plus the two ribs above and below must be blocked for reliable analgesia.

Complications (Anatomically Explained)

Pneumothorax (most serious): Needle penetrates the parietal pleura, especially when blocking posteriorly where only thin intercostal fascia separates nerve from pleura. True incidence is low (<0.1% per Moore; ~1.4% per Shanti et al.)
Intravascular injection: The intercostal artery and vein lie immediately above the nerve in the groove. Intercostal vessels have high systemic uptake - highest of any peripheral block site - making local anesthetic toxicity a real concern
Hemothorax: Laceration of intercostal vessels
Incomplete block: Overlapping dermatomal supply from adjacent nerves, or block placed too anteriorly (distal to lateral cutaneous branch)
Roberts and Hedges', p. 675-676
Gray's Anatomy for Students, p. 197

Summary: The intercostal nerve block targets the anterior primary ramus of each thoracic spinal nerve as it travels in the costal groove between the internal and innermost intercostal muscles, accompanied by the intercostal artery (above) and vein (most superior). The ideal injection site is at the posterior angle of the rib (5-7 cm lateral to midline) or at the posterior axillary line, where the nerve has already given off the posterior cutaneous branch but not yet the lateral cutaneous branch, and where the internal intercostal muscle provides a safety buffer between needle and pleura.

anesthesia area and procedure

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Intercostal Nerve Block - Area of Anesthesia & Procedure

Area of Anesthesia

Blocking an intercostal nerve produces anesthesia in a band-like distribution around the thorax and upper abdomen corresponding to the dermatomal level blocked.

Cutaneous distribution of intercostal nerves T3-T12, showing the anterior (left) and posterior (right) areas of anesthesia

Area of anesthesia (shaded): Anterior and posterior views showing dermatomal coverage T3-T12 (Roberts and Hedges)

Dermatomal Map - Relevant Levels

Level	Area Covered
T2-T3	Upper chest, axilla, medial arm
T4	Nipple line
T6	Xiphoid
T10	Umbilicus
T12	Groin / inguinal ligament
T5-T12 (multiple)	Full abdominal wall from xiphoid to pubis - both sensory and motor

Each block covers one dermatomal stripe. Because adjacent intercostal nerves overlap significantly, the target rib plus at least one rib above and one below must be blocked for reliable analgesia. For rib fractures, block the fractured level plus two adjacent ribs on each side.

Duration of Anesthesia

Anesthetic Agent	Duration
Short-acting (lidocaine)	1-3 hours
Long-acting (bupivacaine) alone	6-8 hours
Long-acting + epinephrine	8-12 hours (standard)
Partial/residual analgesia	Up to 3 days (possibly via reduced muscle spasm)

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 675-676

Patient Positioning

Three positions are acceptable depending on context:

Position	Best For
Prone (pillow under abdomen)	Operative anesthesia; allows access to both sides; arms draped over edge so scapula falls laterally
Lateral decubitus	Single-sided block; post-trauma patients who cannot lie flat
Sitting upright (arm raised, wrist rested on head)	Emergency department; rib fracture management; raises ipsilateral scapula off upper ribs

Barash Clinical Anesthesia, 9th ed., p. 2990
Tintinalli's Emergency Medicine, p. 287

Landmark-Based Technique (Standard)

Intercostal block - cross-sectional anatomy, probe position, and USG target (yellow circle = injection target between IIM and pleura)

Left: Cross-sectional cadaveric anatomy showing intercostal spaces (IS) with neurovascular bundles (coloured dots) at rib inferior borders. Right: USG view - target zone (yellow circle) is between the internal intercostal muscle (IIM) and pleura (P), between two ribs (R). (Barash Clinical Anesthesia)

Step-by-Step

1. Mark the injection site

Identify the target rib(s) at their angle of greatest posterior curvature, approximately 6-7 cm lateral to the midline (just lateral to the paraspinous muscles)
For T1-T5: this region is obscured by the scapula; consider paravertebral block or use the posterior axillary line instead
Mark the inferior border of each target rib

2. Set up

Needle: 22-25 gauge, 3-4 cm (3.75 cm standard)
Syringe: 5-10 mL
Drug: 0.25-0.5% bupivacaine ± epinephrine, or 1-2% lidocaine
Volume: 3-5 mL per level
Aseptic technique; optional skin wheal of local anesthetic at each mark

3. Skin retraction (key step)

Place the index finger of the non-injecting hand on the inferior border of the target rib
Retract the overlying skin cephalad (upward) over the rib

4. Initial needle contact with rib

Insert needle bevel-up at approximately 10-15° cephalad angulation
Advance until the needle tip contacts the periosteum of the rib
Rest the hand holding the syringe firmly on the chest wall for control

5. Walk the needle caudally off the rib

Slowly release the skin retraction - this causes the shaft to shift and the tip moves toward the inferior rib margin
Using the non-dominant hand to brace, gently "walk" the needle caudally until it slips off the inferior edge of the rib into the subcostal groove
Advance only 3-4 mm beyond the rib edge - this places the tip between the internal intercostal and innermost intercostal muscles, at the neurovascular bundle

6. Aspiration and injection

Aspirate carefully - if blood returns, withdraw and reposition; if air returns, stop immediately (pleural entry)
If aspiration is negative: inject 3-5 mL of local anesthetic while gently moving the needle in and out 1 mm (to ensure compartment penetration and reduce risk of intravascular bolus)
Repeat for each level required
Roberts and Hedges', p. 675-676
Tintinalli's Emergency Medicine, p. 287
Barash Clinical Anesthesia, 9th ed., pp. 2990-2991

Ultrasound-Guided Technique

USG-guided intercostal nerve block - needle contacts rib (1), then walks caudally off the rib into the intercostal groove above the pleura (2)

In-plane needle approach: (1) needle on rib, (2) needle walks into subcostal groove; pleura visible below (Miller's Anesthesia)

Parameter	Value
Probe	High-frequency linear, 5-15 MHz
Orientation	Longitudinal (parasagittal) along the rib
Probe position	Posterior/lateral chest wall
Needle approach	In-plane (allows full needle visualization)

USG landmarks to identify:

Rib: hyperechoic curved line with posterior acoustic shadow (dark cone below)
Pleura: bright hyperechoic line deep to the rib; shows dynamic sliding sign with respiration ("glittering")
Internal intercostal membrane / muscle (IIM): hyperechoic layer immediately above the pleura
Target injection zone: the space between the IIM and the pleura, just below the inferior rib border

Needle steps under USG:

Advance in-plane until tip contacts the rib
Tilt and redirect needle caudally, walking off the inferior rib border
Stop when tip is in the intercostal groove, above the pleural line
Confirm: local anesthetic spread appears as a hypoechoic fluid pocket pushing the pleura downward (away from the rib)

Barash Clinical Anesthesia, 9th ed., p. 2992

Local Anesthetic Dosing Summary

Agent	Concentration	Volume/Level	Onset	Duration
Lidocaine	1-2%	3-5 mL	Fast	1-3 h
Bupivacaine	0.25-0.5%	3-5 mL	Slower	6-12 h
Bupivacaine + epinephrine 1:200,000	0.25-0.5%	3-5 mL	Moderate	8-12 h
Ropivacaine	0.5%	3-5 mL	Moderate	6-10 h

Important: The intercostal space has the highest systemic absorption rate of any peripheral nerve block site. Calculate maximum total dose carefully - toxic serum levels can be reached rapidly, especially when blocking multiple levels.

Post-Procedure Monitoring

Observe for 30 minutes post-block
Routine chest radiograph is not indicated unless clinical signs appear
Bedside lung ultrasound (assess for absent sliding sign = pneumothorax) is preferred
Warning signs: cough, dyspnea, oxygen desaturation, chest pain → immediate evaluation
Tintinalli's Emergency Medicine, p. 287
Barash Clinical Anesthesia, 9th ed., p. 2993

Complications Summary

Complication	Mechanism	Rate
Pneumothorax	Pleural puncture (especially posteriorly where only thin fascia separates nerve from pleura)	<0.1% (Moore); ~1.4% per level (Shanti)
Local anesthetic systemic toxicity (LAST)	High vascular uptake in intercostal space; cumulative dose across multiple levels	Dose-dependent
Intravascular injection	Needle in intercostal artery/vein (VAN relationship - artery just above nerve)	Prevented by aspiration
Hemothorax	Intercostal vessel laceration	Rare
Incomplete block	Block site too anterior (distal to lateral cutaneous branch), or inadequate dermatomal spread	Technique-dependent

Roberts and Hedges', p. 675-676
Barash Clinical Anesthesia, 9th ed., p. 2993

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