Which of the axillary, interscalene, or supraclavicular brachial plexus blocks is least likely to cause phrenic nerve blockade and diaphragmatic paresis on the side of the block?

Axillary Block is Least Likely to Cause Phrenic Nerve Blockade

The axillary block is least likely to cause phrenic nerve blockade and hemidiaphragmatic paralysis among the three options, with essentially a 0% risk, while interscalene block carries the highest risk (approaching 100%), and supraclavicular block falls in between with variable risk depending on technique.

Risk Hierarchy for Phrenic Nerve Involvement

Based on guideline recommendations, the blocks rank as follows for respiratory safety 1:

• Axillary block: Essentially no risk of phrenic nerve involvement
• Supraclavicular block: Intermediate risk (can range from 9-70% depending on injection technique)
• Interscalene block: Highest risk (92-100% incidence of hemidiaphragmatic paralysis)

Evidence-Based Rationale

Guideline Recommendations

The 2020 Anaesthesia guidelines explicitly state that "axillary or infraclavicular brachial plexus block may be chosen over supraclavicular brachial plexus block, and superior trunk block or other alternatives are preferred over interscalene block" when attempting to minimize respiratory compromise 1. This recommendation prioritizes blocks that are least likely to interfere with respiratory function.

Research Supporting the Hierarchy

Interscalene blocks consistently demonstrate the highest risk:

• 95% incidence of hemidiaphragmatic paralysis in comparative studies 2
• 92.45% incidence in recent randomized trials 3
• This occurs because the phrenic nerve originates from C3-C5 nerve roots in close proximity to the injection site

Supraclavicular blocks show variable but reduced risk:

• When performed with posterolateral injection technique: 9% incidence of hemidiaphragmatic paralysis 2
• Traditional supraclavicular approach: 70% incidence 4
• The risk depends heavily on injection location relative to the brachial plexus

Axillary blocks spare the phrenic nerve entirely because the injection site is distal to where the phrenic nerve branches from the cervical plexus.

Clinical Decision Algorithm

For patients with normal respiratory function:

• Any of the three blocks can be used based on surgical requirements
• Interscalene provides most complete shoulder coverage

For patients with compromised respiratory reserve (COPD, obesity, heart failure, baseline hypoxemia):

1. First choice: Axillary block (if surgical site permits distal arm/elbow surgery)
2. Second choice: Supraclavicular block with posterolateral injection technique (9% HDP risk) 2
3. Avoid: Interscalene block unless absolutely necessary

Important Caveats

• Axillary blocks do not adequately cover the shoulder joint for shoulder surgery, as they miss the suprascapular and axillary nerve distributions to the glenohumeral joint 5
• For shoulder surgery specifically, axillary block alone is insufficient and would require combination with suprascapular nerve block 5
• The question asks specifically about phrenic nerve risk among these three blocks, where axillary is clearly safest, but surgical coverage must also be considered in practice

The anatomic distance from the phrenic nerve determines risk: the more distal the block (axillary < supraclavicular < interscalene), the lower the risk of hemidiaphragmatic paralysis.