A guide to regional anaesthesia

What is regional anaesthesia?

Regional anaesthesia is a type of anaesthesia that involves blocking the nerves to an area, or region of the body by placing local anaesthetic in a specific anatomical position. 

This is different from the other forms of anaesthesia such as general anaesthesia, where a patient is unconscious.  It also differs from local infiltrative and topical anaesthesia. Local infiltrative anaesthesia involves the infiltration of the incised subcutaneous tissue with a local anaesthetic, blocking many nerve endings in the area. This requires a larger dose of anaesthetic. Topical anaesthesia involves a non-infiltrative local application of anaesthetic to mucous membranes.

Regional anaesthesia is a more direct and nuanced approach to rendering an area of the body insensate or anaesthetised compared to these other forms of anaesthesia.

How it works

Local anaesthetics work by reversibly blocking the voltage-gated sodium channels on nerve membranes which block the of transmission of afferent action potentials to the central nervous system. This effectively renders the effected pain fibres non-functional. This results in no central perception of a painful stimulus in any area that the nerve fibre innervates. 

When is regional anaesthesia used

Regional anaesthesia can be used in isolation as the sole form of anaesthesia, or combined with the other forms of anaesthesia. In isolation, regional anaesthesia acts as the anaesthetic block. This requires a cooperative patient, as the patient will be conscious during the procedure. This may be difficult in veterinary practice, and the patient will require restraint in most cases, and sometimes sedation.

When combined with general anaesthesia, regional anaesthesia is used intraoperatively as part of a balanced anaesthetic. An added benefit of the same regional anaesthetic is after the patient wakes from the general anaesthetic, the ongoing effects of the regional anaesthetic block can form the basis of the postoperative analgesic regime.

Why is regional anaesthesia used

Regional anaesthesia is often used for postoperative pain management and intraoperatively in combination with general anaesthetic, due to its notable benefits.  

The benefits of  regional anaesthesia in post operative pain management include improved analgesia and a decreased need for other pain medication such as opioids, which have an increased side effect profile.

Furthermore, there are also benefits to combining regional local anaesthetic block with general anaesthesia as a lower dosage of general anaesthesia is required. This subsequently improves cardiovascular function, provides pre-emptive and post-operative analgesia, and ultimately is more comfortable for the patient.  

Common types of regional anaesthesia 

Spinal

Spinal anaesthesia is a type of neuraxial anaesthesia that involves the application of local anaesthetic agents in the neuraxis. This results in the blockade of spinal nerves. The local anaesthetic is injected into the CSF in the sub arachnoid space between the arachnoid and pia mater. The anaesthetic agent will then diffuse throught the CSF to the nerves. As a result of application location, less anaesthetic agent is required than in an epidural. The spinal block is however less controlled as the agent is mixed in the whole fluid space of CSF.

 Epidural

Spinal anaesthesia is also a type of neuraxial anaesthesia. Local anaesthetic is injected into the epidural space, outside the dura. 

Similar nerves to the spinal anaesthetic are blocked. However, injecting the agent into the epidural space limits the spread of the agent, as the agent must diffuse across the arachnoid mater. This results in a higher anaesthetic dosage requirement. However, it also results in a more controlled blockage as particular spinal roots can be blocked.

Nerve blocks 

Nerve blocks involve the application of local anesthetic close to a specific nerve. The area that is desensitised by a nerve block is determined by the application location of the anaesthetic agent. Larger areas will be blocked with more central application.

As opposed to neuraxial anaesthesia, nerve blocks result in anaesthesia of the area supplied by the particular nerve rather than a dermatomal distribution. Also, given the blockade occures at a nerve level, visceral anaesthia is not possible.

These blocks are used for dental surgery as there are 4 dental blocks that can be performed, which can desensitise most areas of the mouth.

Complications of regional anaesthesia

Process of block

During the application of the anaesthetic, the needle may damage surrounding structures.

This can lead to complications such as bleeding, pneumothorax, and damage to nerves (particularly during intra neural injection).

Effect of block

Adverse physiological effects of nerve blockade can occur such as hypotension during neuraxial block, and damage due to inadvertent effect of anaesthesia such as pressure injury 

Systemic toxicity

Distal organ toxicity can occur if the voltage gated fast sodium channels on membranes of other nerve tissue and cardiac cells are also affected by the anaesthetic. Signs can be neurological or cardiovascular, such as sedation, progressing to twitching, coma and respiratory arrest or arrhythmias, bradycardia, vasodilation and cardiac arrest.

General complications of anaesthesia

• Incomplete anaesthesia resulting in “awareness”
• Complications of sedation particularly unplanned deep sedation due to poorly effective regional anaesthesia

References

Edmondson MA. 2016. Local, Regional, and Spinal Anesthesia in Ruminants. Veterinary Clinics of North America: Food Animal Practice, 32(3), 535-552. 

Grubb T, Lobprise H. 2020. Local and regional anaesthesia in dogs and cats: Descriptions of specific local and regional techniques (Part 2). Vet Med Sci. 6: 218– 234.

Keating S. Small animal local and regional anaesthesia. Vetmed illinois. 

Shroeder C. 2020. Regional Anesthesia and Pain Management in Veterinary Medicine. ASRA pain medicine.

Taylor A & McLeod G. 2020. Basic pharmacology of local anaesthetics. BJA Education, 20(2), 34–41.