The axillary sheath and single-injection axillary block

Area of Science:

• Anatomy
• Regional Anesthesia
• Surgical Technique

Background:

• Incomplete axillary brachial plexus blocks are a common issue with single-injection techniques.
• Failure to block terminal nerves of the brachial plexus is attributed to the circumferential type of block.
• Understanding axillary sheath anatomy is crucial for improving block efficacy.

Purpose of the Study:

• To investigate the internal anatomy of the axillary sheath and its septa.
• To determine the effect of varying injected fluid volumes on dye spread within the axillary sheath.
• To elucidate the mechanism of failure in brachial plexus blocks.

Main Methods:

• Microscopic examination of longitudinally and transversely sectioned axillary sheaths to analyze internal structure and septa.
• Injection of 10, 20, and 40 cc methylene-blue into axillary sheath compartments in cadavers.
• Transverse and longitudinal dissection and examination of injected sheaths and septa at 10x magnification to assess dye spread and septal integrity.

Main Results:

• Septa originating from the deep surface of the axillary sheath compartmentalize individual nerves.
• These septa act as physical barriers to fluid spread under normal physiological conditions.
• Increasing injected fluid volume created defects in the septa, leading to leakage into adjacent compartments.

Conclusions:

• The internal septa of the axillary sheath are significant barriers to complete brachial plexus blockade.
• Higher injected volumes can overcome these barriers by creating defects, facilitating wider spread.
• This finding has implications for optimizing injection volumes in brachial plexus anesthesia techniques.