Anesthesiology APPLIED Exam › Regional anesthesia and acute pain

Interscalene block and its complications

T1Regional anesthesia and acute pain·767 words · ~4 min read·v0.1DRAFT

Why examiners ask this

The interscalene block is the best analgesia for shoulder surgery and also the block with the most predictable side effect, hemidiaphragmatic paresis. Examiners use it to test judgment about patient selection (COPD, obesity, contralateral phrenic palsy), application of knowledge about anatomy and alternatives, and adaptability when the patient becomes dyspneic, hoarse, or hypotensive in the beach-chair position. It is also the natural setting for questions about nerve injury and consent.

The 60-second answer

For arthroscopic or open shoulder surgery, I offer an interscalene block because it covers the shoulder joint and reduces opioid use. I target the C5–C6 roots or upper trunk between the anterior and middle scalene under ultrasound, with a low volume, about 10–15 mL of ropivacaine 0.5%, to reduce spread to the phrenic nerve. I consent for phrenic nerve block, Horner syndrome and hoarseness, and for the rare risks of nerve injury, LAST, and intrathecal or vertebral artery injection. In a patient who cannot tolerate a 25% fall in vital capacity, I choose a diaphragm-sparing alternative: a superior trunk or supraclavicular block, or a suprascapular nerve block with an axillary nerve block. I monitor for respiratory compromise and LAST after the block.

Decision points and the "why"

Phrenic nerve block is near-universal — it lies on the anterior scalene near C5, and conventional volumes block it nearly every time, cutting FVC by about 25%. Low volume, injection lateral to the plexus, and more distal blocks reduce but do not eliminate it.
Patient selection — severe COPD, morbid obesity, contralateral phrenic or recurrent laryngeal palsy, and limited respiratory reserve push me to an alternative. A patient who has had a contralateral pneumonectomy is an absolute no.
Alternatives — suprascapular plus axillary nerve blocks give less complete analgesia but spare the diaphragm; the superior trunk and supraclavicular blocks reduce but do not eliminate phrenic involvement.
Awake or asleep — I block awake or lightly sedated so the patient reports paresthesia or pain on injection; injection pressure monitoring adds a second safeguard against intraneural injection.
Beach-chair hypotension and bradycardia — the Bezold-Jarisch reflex in a preload-depleted, hypercontractile heart, often after epinephrine-containing infiltration. I treat with fluids, ephedrine or atropine, and keep cerebral perfusion pressure in mind because the brain is above the cuff.
Rebound pain — the single-shot block wears off at 12–18 hours; I start oral multimodal analgesia before it does, or use a catheter or dexamethasone to extend duration.

The complication they will inject

In the recovery room, a 64-year-old with obesity and mild COPD becomes dyspneic with SpO2 89% on 2 L nasal cannula after a 20 mL interscalene. I sit her up, increase oxygen, and examine her: reduced breath sounds and paradoxical abdominal motion on the blocked side suggest phrenic paresis, but I exclude pneumothorax with lung ultrasound, looking for lung sliding. I check the diaphragm with ultrasound and a blood gas for hypercarbia. Most patients settle with oxygen, upright positioning and reassurance; if she tires or the PaCO2 rises, I use non-invasive ventilation and, rarely, intubation until the block wears off. I document and discuss it with her.

Pitfalls that cost points

Promising a block with no phrenic involvement.
Offering an interscalene to a patient with severe COPD without discussing alternatives.
Forgetting pneumothorax and LAST in the differential for post-block dyspnea.
Treating beach-chair hypotension without considering cerebral perfusion.
Omitting the rebound-pain plan.

Rapid-why

Q: Why Horner syndrome? A: Spread to the cervical sympathetic chain causes ptosis, miosis and anhidrosis; it is benign and resolves with the block.

Q: Why hoarseness? A: Spread to the recurrent laryngeal nerve; dangerous only if the contralateral cord is already paralyzed.

Q: Why can a small dose cause seizures in this block? A: Inadvertent injection into the vertebral artery delivers drug directly to the brain; as little as 1–2 mL can seize.

Q: Which part of the shoulder does an interscalene block miss? A: It often spares the C8–T1 roots, so the ulnar distribution is missed, and the posterior portal skin, supplied by upper thoracic dorsal rami, often needs local infiltration.

Numbers to know

Item	Value
Phrenic block with conventional volume	Near 100%
FVC reduction	~25%
Low-volume interscalene	5–15 mL ropivacaine 0.5%
Single-shot duration	12–18 h; longer with dexamethasone
Roots targeted	C5–C6 (upper trunk)

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