Regional anaesthesia is the loss of sensation affecting only a part of the body produced by the application of an anaesthetic agent to the nerves supplying that region. Regional anaesthesia can be divided into central and peripheral techniques. Central techniques include spinal and epidural anaesthesia (also known as neuoaxial blocks). Peripheral techniques can be further divided into plexus blocks (e.g. Brachial) and single nerve blocks.
Regional anaesthesia is a safe and effective procedure. When performed by an anaesthetist with adequate knowledge of the relavant anatomy, physiology and pharmacology, it has the potential to provide excellent operating conditions for the surgeon as well as satisfactory pain relief for the patient. For some operations, outcome has even been found to be improved with neuroaxial techinqiues; with fewer deaths and DVTs found in one study in hip fracture surgery.
Epidural and spinal anaesthesia has been performed for well over 100 years. Providing a valuable alternative to general anaethesia (GA) where GA would be otherwise contraindicated, regional anaesthesia has also revolutionised pain relief in areas such as child birth and post-operative analgesia following major surgery.
Below are the basic practical aspects, covering spinal and epidural anaesthesia, as well as some of the possible indications and complications involved.
Before learning to do the procedure, you should revise the relevant anatomy.
The spinal cord usually ends at L1/L2 in adults, however there is some variation. Nerve roots hanging down below this level form the cauda equina, sometimes called a 'horses tail'. The spinal cord and cauda equina lie within the dural sac, bathed within cerebrospinal fluid (CSF). Puncture above this level is associated with an increased risk of damaging the spinal cord.
Outside the dural sac is the epidural space; this is actually a 'potential' space, as the surfaces of the dura mater and ligamentum flavum are pressed closely together, due to surrounding tissue pressure.
An important anatomical landmark is the line joining the top of the iliac crests, this is known as 'Tuffier's line' and it signifies L4/L5 in 35% of patients.
A common exam question is to ask which layers the needle will pierce before reaching CSF, you should try and remember these, as it will also help you in guiding your needle:
Local Anaesthetic can affect a number of different nerve fibres; sensory, motor and autonomic. The autonomic fibres can be further broken down into sympathetic and parsympathetic. Some nerves are easier to block than others, and which nerves are blocked depends on the level at which the block is given.
There are several types nerve fibres. The classification of a nerve fibre impacts on its sensitivity to local anaesthesia. The sensitivity to blockade is shown in the table below.
In general, the thicker the nerve in diameter, the less sensitive it is to anaesthesia.
This has important consequences in regional anaesthesia; it allows a so called 'mobile' block in which motor fibres can be spared, while the patient is still pain free (e.g in epidural). This is possible as motor fibres are thicker and thus less sensitive to anaesthesia. Epidurals are useful in labour and are very popular with patients.
Local anaesthetics work by impairing membrane permeability to sodium; resulting in a membrane stabilizing affect. This causes blockade of impulse propagation, preventing transmission of pain induced signals to higher centres.
NB: Lidocaine hurts less when injected warm, or at a lower concentration.
Certain preparations are available containing adrenaline, which induces local vasoconstriction resulting in dcreased blood loss in minor surgical procedures. Preparations containing adrenaline should be used with caution as they can have systemic side effects, especially in those with CVS disease or with raised BP. Adrenaline is contraindacated in peripheral procedures including digital/penile block and the nose or ears, as induced ischaemia could result in gangrene.
This is the same as in lumbar puncture, except rather than taking a sample of CSF, you will be injecting local anaesthetic.
Before carrying out the procedure, ensure you have undertaken a comprehensive pre-preoperative assessment. Patients should be fully monitored and have IV access, as in a general anaesthetic. Adequate pre-loading should have occured, with the patient being given IV fluids immediately prior to the spinal injection.
What is injected?
Local anaesthetic in dextrose solution is often used as it is denser and tends to sink when the position of the patient is altered. The onset of action for spinal injection is rapid, at around 5 minutes. This lasts for around 1-6 hours. Often opiates are added to enhance the analgesic action.
The needle used in spinal injection is very fine (usually 25G), as if a large hole is created in the dura, this causes CSF to leak and a drop in pressure; causing severe headache.
Combined Spinal and Epidural
As epidural analgesia has the disadvantage of taking longer to take effect, many anaesthetists combine the use of intrathecal opioids with epidural analgesia, allowing the combination of rapid onset of pain relief (intrathecal injection) with maintenance analgesia for extended periods (epidural). This is of particular use in labour.
Reliable measures should be used for assessment. This can be either light touch or temperature. Use of pinprick pain sensation is unecesary as it is both a poor predictor, and it confers an unecessary risk of transmission of viruses if blood is drawn. Recommended techniques include:
Test by first touching a site where sensation should be normal (e.g. the arm). Next progressively work upwards from the legs to find where the patient begins to appreciate sensation again. Ask the patient to close their eyes.
Muscle blockade can also be assessed using the bromage scale:
There are several versions of this scale.
Complications of central neuroaxial block are estimated to occur in about 1/1000 -1/1,000,000. Serious complications are relatively low, and the incidence is higher in spinal than in epidural anaesthesia.
Bier Block (Intravenous regional anaesthesia)
This is a simple method of providing anaesthesia for surgical proceudres to the body's extremities such as the hand/forearm. Here is an example of Bier Block usage in distal radial fracture:
*Do NOT use bupivacaine for this block. If the cuff is accidentally deflated, this is cardiotoxic and potentially fatal*
Possible complications of peripheral nerve blocks:
Advantages and disadvantages are dependent on patient and surgical factors. A full pre-operative assessment should always be undertaken, and wishes of the patient taken into consideration.
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