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Regional Anaesthesia

Introduction

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.

Anatomy

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:

  • Skin
  • Subcutaneous fat
  • Supraspinous ligament
  • Interspinous ligament
  • Ligamentum flavum
  • Epidural space
  • Dura mata
  • Arachnoid Mata

Tuffier's Line: L4/L5Spinal cord

Physiology

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.

  • Sensory Fibres - Each of the spinal nerves provides sensation to a specific area. This is marked out by a dermotome. Blocking sensory fibres prevents transmission of pain and pressure sensations from that dermatomal region.
  • Motor fibres - in the case of motor fibres the nerve generally relates to the level in which it leaves the cord. The exception is the diaphragm, which is supplied by nerves C3, 4, 5. This allows anaesthetists to give a relatively high block; such as in caesarean section, while still allowing the patient to breathe. Blocking motor fibres causes muscle paralysis.
  • Sympathetic Fibres - Exit at T1-L2. Blocking sympathetic fibres causes a degree of vasodilation, and in a high block (above T4) may lead to bradycardia.
  • Parasympathetic - Exit at the bottom of the spinal column (S2-S4). Blockade of these fibres can lead to urinary retention and the need for urinary catheter insertion.

 

    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.

    Nerve Fibres



    Local Anaesthetics

    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.

    Amides

    Amides

    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.

    Esters

    Esters (now not often used)

    Spinal Injection

    This is the same as in lumbar puncture, except rather than taking a sample of CSF, you will be injecting local anaesthetic.

    Indications

    • Surgical procedures to the lower body such as hernia operations, or caesarean sections.
    • Upper body surgery in combination with general anaesthesia.

     

      Contraindications

      • Relative - systemic sepsis, aortic/mitral stenosis (may result in prefound hypotension), neurological disease.
      • Absolute - intracranial pressure, localised sepsis, clotting issues, refusal.

       

        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 (e.g. Bupivacaine)
        • Opioid analgesia
        • Dextrose solution

         

          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.

          Procedure

          1. Scrub and glove up. Ensure using aseptic non-touch technique through out. 
          2. Patient should be sitting, or lying on their side. Ask them to flex their back (slump shoulders and relax as much as comfortable) as this opens up the intevertebral spaces.
          3. Clean the back with aseptic solution.
          4. Locate a suitable interspinous space using Tuffier's line.
          5. Infiltrate the skin using a small volume of local anaesthetic (lidocaine 1%).
          6. The spinal needle is advanced slowly in the midline, aiming slightly cranially. Resistance is felt upon meeting the ligamentum flavum, followed by loss of resistance in the epidural space. Another loss of resistance may be felt after piercing the dura.
          7. Correct placement is confirmed by the appearance of CSF. CSF should be a clear, colourless fluid.
          8. Attach a syringe firmly to the needle, resistance to the injection will be high and it is easily to spill some of the anaesthetic. Aspirate gently to ensure you are still intrathecal and then slowly inject.
          9. Remove needle and apply dressing to site.
          10. Monitor blood pressure.

          Epidurals vs Spinals

          Epidural anaesthsia - differences to spinal

          • The epidural space is identified by being easy to inject saline into.
          • Due to the use of a larger bore needle, epidurals facilitate the ability to thread a fine plastic tube (catheter) into the space.  The catheter can remain in situ for several days, allowing for continued pain relief (not possible in spinal anaesthesia).
          • As the dura is not breached, there should be no prospect of headache. However as a larger needle is used, if the dura is breached accidentally, severe headache will occur.
          • No dextrose is used as there is no fluid in the epidural space and the solution will naturally move with gravity. Opiates are however often added.
          • Onset is delayed compared to spinal analgesia due to having to diffuse through the fibrous sheath of the spinal nerves and the dural membrane. This is accounts for why a larger dose is required.

           

            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.

              Testing the Block

              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:

              • Cold ethyl chloride spray
              • Cotton wool

               

                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:

                Bromage scale

                There are several versions of this scale.

                Complications of Regional Anaesthesia

                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.

                • Post-Dural puncture headache - this is one of the most common complications. Headache can occur with any breach in the dura, and is classically postural in nature. It is usually relieved by repositioning to the supine position.
                • Hypotension - due to sympathetic blockade. Can be counteracted by fluids and vasoactive agents.
                • Itching - can occur after an opiate has been adminsitered.
                • Urinary retention - due to blockade of parasympathetic fibres.
                • High Block - the patient may complain of tingling/numbness in the hands or difficulty breathing. Numbness in the hands are due to blockade of C6,7,8. Difficulty breathing can be subjective, as it is due to blockade of the intercostal nerves.  Only if blockade extends to C3/4 will actual respiratory depression ensue. High block also causes incressed sympathetic blockade (vasodilation, bradycardia, hypotension). The 'Total Spinal' is where the block reaches the cerebral CSF and causes unconciousness.
                • Nerve damage - due to accidental damage caused by the needle.
                • Spinal cord damage - may be due to dmage from needle, or secondary to a haematoma or abcess which has caused pressure to the cord following the spinal/epidural.
                • Incomplete Block - sometimes the block doesn't goes as high as hoped, or only blocks one side. Can often get the patient to lie on that side to resolve the problem.
                • Failure - failure of the block may require conversion to general anaethetic. Patients should be consented and prepared for this possibility. 
                • Drug toxicity - caused by intravascular injection of local anaesthetic. Symptoms include numbness of the tongue and perioral area, and restlessness, followed by seizures, respiratory failure and coma. Cardiovascular signs often follow these CNS signs.

                Peripheral nerve blocks and their uses



                Specific Peripheral Blocks

                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:

                1. Place a tourniquet around the upper arm.
                2. Empty the arm of blood either by raising above the heart for ~1minute or through use of an Esmarch bandage (surgical tournequet used to provide haemostasis).
                3. Inflate the cuff to 100mmHg above systolic BP.
                4. Inject 30-40ml of 0.5% prilocaine into a vein in the back of the hand.
                5. Give ~20-30min for anaesthetic to take effect, then the fracture can be manipulated.
                6. The cuff should be deflated, and the anaesthetic should wear off.

                 

                *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:

                • Nerve damage
                • Overdose of LA
                • IV adminstration of LA
                • Damage to adjacent structures
                • Haemorrhage
                • Infection
                • Incorrect nerve blocked (e.g. the phrenic nerve may be anaesthetised along with brachial nerve blockade)
                • Skin injury
                • Anaphylaxis

                Comparison of Anaesthetics



                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.

                References

                1. Regional anaesthesia: Collier. J, Longmore. M, Turmezei. T, Mafi. A. (2009) Oxford Handbook of Clinical Specialities. 8th edition. New York. Oxford University Press.
                2. Local anaesthetics & nerve fibres: http://www.ifna-int.org/ifna/e107_files/downloads/lectures/H1LocalAne.pdf
                3. Spinal Anaesthesia: http://www.frca.co.uk/article.aspx?articleid=100126
                4. Spinal anaesthesia: a practical guide http://www.nda.ox.ac.uk/wfsa/html/u12/u1208_01.htm
                5. Epidural Cannulation: http://www.anaesthesiauk.com/article.aspx?articleid=100444
                6. Bromage scale: http://www.frca.co.uk/SearchRender.aspx?DocId=3333&Index=D%3a%5cdtSearch%5cUserData%5cAUK&HitCount=15&hits=3+b+ae+ca+cb+f0+11d+146+147+14d+14e+182+183+18a+1c4+
                7. GA vs RA: Gular. P. 'Regional anaesthesia versus general anaesthesia, morbidity and mortality' Best Practice & Research Clinical Anaesthesiology, 20;2;249–263, 2006.
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