Why does an interscalene block make patients breathless?
Anatomy — Brachial and Lumbosacral Plexuses and Peripheral Nerve Blocks
The question
A 68-year-old man with moderate chronic obstructive pulmonary disease receives an ultrasound-guided interscalene brachial plexus block with 20 mL of levobupivacaine 0.5% for shoulder arthroplasty. Twenty minutes later he complains of breathlessness. His voice is normal, his pupils are equal, oxygen saturation has fallen from 95% to 89% on air, and there is reduced air entry at the ipsilateral lung base. Which anatomical explanation is most likely?
- Blockade of the recurrent laryngeal nerve
- Spread of local anaesthetic to the phrenic nerve on the anterior surface of the anterior scalene muscle
- Blockade of the cervical sympathetic chain
- Pneumothorax from puncture of the cervical pleura
- Blockade of the long thoracic nerve with paralysis of serratus anterior
The answer
Spread of local anaesthetic to the phrenic nerve on the anterior surface of the anterior scalene muscle
Blockade of the recurrent laryngeal nerve — Recurrent laryngeal nerve spread can occur with blocks in this region, but it causes hoarseness and sometimes stridor. This patient's voice is normal, and a unilateral vocal cord palsy would not reduce basal air entry.
Spread of local anaesthetic to the phrenic nerve on the anterior surface of the anterior scalene muscle — Correct. The phrenic nerve (C3, 4, 5) descends along the anterior surface of anterior scalene, separated from the plexus by only millimetres at this level. Ipsilateral hemidiaphragm paresis occurs in the vast majority of standard interscalene blocks.
Blockade of the cervical sympathetic chain — Sympathetic chain blockade produces Horner syndrome: ptosis, miosis, and anhidrosis. The stem states that the pupils are equal, which excludes this diagnosis. Furthermore, sympathetic blockade does not impair ventilation.
Pneumothorax from puncture of the cervical pleura — This is a reasonable consideration, and the reason a chest radiograph is often obtained. However, the pleural dome sits well below and posterior to the interscalene groove. Pneumothorax is far less likely at this level than with a supraclavicular approach, and diaphragm paresis is a much more common explanation for this presentation.
Blockade of the long thoracic nerve with paralysis of serratus anterior — The long thoracic nerve arises from C5, C6, and C7 and can be blocked at this level, but its blockade causes scapular winging. It has no role in ventilation and does not account for reduced breath sounds.
Explanation
The brachial plexus arises from the ventral rami of C5 to T1. At the interscalene level, the roots and upper trunks lie in the groove between anterior and middle scalene, deep to the prevertebral fascia, approximately at the level of the cricoid cartilage and the C6 transverse process. This is a compact, fascia-bounded space in the posterior triangle, and the clinical behaviour of the block is best understood by considering the adjacent structures.
The phrenic nerve is the nearest important neighbour. It arises from C3, 4, and 5, forms on the lateral border of anterior scalene, and descends across its anterior surface, separated from the plexus by only millimetres at the level of injection and by a fascial layer that does not reliably contain local anaesthetic. Ipsilateral hemidiaphragm paresis is not truly a complication in the usual sense; it is an expected accompaniment. With a standard large-volume interscalene injection, it occurs in close to 100% of patients, and FVC and FEV1 typically drop by approximately one quarter. Low-volume, ultrasound-guided techniques and more selective alternatives (such as suprascapular plus axillary nerve block) reduce the incidence substantially but do not eliminate it.
Most healthy patients will not notice the loss of one hemidiaphragm. Patients with limited respiratory reserve will, which is why significant lung disease is a relative contraindication to interscalene block, and why bilateral interscalene blocks are never performed. If a patient becomes breathless in recovery with reduced air entry on the blocked side, diaphragm paresis is the presumptive diagnosis until proven otherwise. It resolves as the block wears off, though it can outlast the sensory block when a long-acting agent has been used.
The other structures surrounding the groove produce the remainder of the block's side-effect profile, each with a characteristic clinical sign. The cervical sympathetic chain produces Horner syndrome. The recurrent laryngeal nerve produces hoarseness. The vertebral artery, carotid sheath, and internal jugular vein represent the vascular risks, and vertebral artery injection can cause seizures at very small doses. The plexus itself is incompletely covered from above: the inferior trunk (C8, T1) is often spared, which is why interscalene block is excellent for the shoulder and unreliable for the hand, particularly in the ulnar territory.
Tips for the candidate
Regional anatomy questions are best approached by considering what lies adjacent to the target structure rather than by memorising complication lists. For each block, the essential knowledge comprises the target, the two or three named neighbouring structures, and the clinical sign each produces when blocked. For the interscalene groove: phrenic nerve blockade causes breathlessness, sympathetic chain blockade causes Horner syndrome, recurrent laryngeal nerve blockade causes hoarseness, and the plexus itself spares C8/T1.
The normal findings stated in a stem deserve as much attention as the abnormal ones. "Voice is normal" excludes recurrent laryngeal nerve involvement in a single clause. "Pupils are equal" excludes sympathetic chain blockade. These details are included deliberately, and overlooking them leaves the candidate choosing between three plausible options instead of one.
Root values should be committed to memory, as several questions depend on them. Phrenic nerve: C3, 4, 5. Upper trunk: C5, C6. Middle trunk: C7. Lower trunk: C8, T1. Long thoracic nerve: C5, 6, 7. Suprascapular nerve: arises from the upper trunk. These values allow prediction of what a block at any given level will and will not cover.
A block-selection question is commonly encountered alongside the anatomical one. When a stem presents a patient with severe COPD requiring shoulder surgery, the teaching point is typically that interscalene block carries near-certain phrenic paresis and a more selective alternative should be considered. Understanding why the anatomy necessitates that recommendation is more durable than memorising the recommendation itself.