Vandana Lahiri

The five main groups of anaesthetic
techniques are:
1. Topical Anaesthesia
2. Infiltration Anaesthesia
3. Conduction Anaesthesia
4. Central Neuraxial Block
5. General Anaesthesia

The first two techniques can be grouped under local anaesthesia whereas the next two are generally grouped under regional anaesthesia.

1. Topical (surface) Anaesthesia
It is used mainly for nasal operations and for endoscopies. Attention to total drug dosage and careful selection of those patients who will tolerate the experience is important.

2. Infiltration Anaesthesia
Infiltration anaesthesia, with or without sedation, is most commonly used for minor superficial operations or operations in the middle ear.

3. Conduction Anaesthesia
It is the technique whereby a local anaesthetic agent is introduced via a needle to the immediate proximity of a specific nerve, series of nerves or a nerve trunk in order to produce analgesia over the sensory distribution i.e. Nerve Blocks.
In conduction blocks, the duration of surgical analgesia depends on the accuracy of initial injection, the properties of the drug in use and the percentage concentration injected. Therefore, if the surgical procedure takes longer than anticipated and the block wears off, general anaesthesia with its concomitant risks has to be superimposed. The other major drawback is the time required for the block to be executed and for analgesia to develop. It is because of the induction time and inflexibility of a ‘one shot’ technique that these blocks are not used frequently.

4. Central Neuraxial Block
This includes spinal and epidural anaesthesia. Spinal anaesthesia is induced by injecting small amounts of local anaesthetic into the cerebro-spinal fluid (CSF). The injection is usually made in the lumbar spine below the level at which the spinal cord ends (L2). Spinal anaesthesia is easy to perform and has the potential to provide excellent operating conditions for surgery below the umbilicus.

The epidural anaesthesia is given by injecting the local anaesthetic solution in the epidural space. It is a potential space that lies between the dura and the periosteum lining the inside of the vertebral canal. It extends from the foramen magnum to the sacral hiatus. The anterior and posterior nerve roots in their dural covering pass across this potential space to unite in the intervertebral foramen to form segmental nerves.

Local Anaesthetic Drugs
Most commonly used are:

Lignocaine : 1, 2, 4 or 10%
Maximum dose - 4 mg/kg plain or 7 mg /kg with adrenaline

Bupivacaine : 0.25 or 0.5%
Maximum dose - 2 mg/kg plain or 3 mg/kg with adrenaline

Toxic side effects of local anaesthetic drugs occur when excessive blood levels occur. This is usually due to:

• l Accidental rapid intravenous injection.
• Rapid absorption, such as from a very vascular site i.e. mucous membranes. Intercostal nerve blocks will give a higher blood level than subcutaneous infiltration, whereas plexus blocks are associated with the slowest rates of absorption and therefore give the lowest blood levels.
• Absolute overdose if the dose used is excessive.

Reducing the risk of toxicity

• Decide on the concentration of the local anaesthetic that is required for the block to be performed. Calculate the total volume of drug that is allowed.
• Use the least toxic drug available.
• Use lower doses in frail patients or at extremes of ages.
• Always inject the drug slowly (slower than 10 ml /minute) and aspirate regularly looking for blood to indicate an accidental intravenous injection.
  l Injection of a test dose of 2-3 ml of local anaesthetic containing adrenaline will often (but not always) cause a significant tachycardia if accidental intravenous injection occurs.
• Most nerve blocks are more dependent on volume of drug injected than the total dose. Therefore if more volume is needed it is better to dilute the local anaesthetic with 0.9% saline than to add more local anaesthetic and increase the dose unnecessarily.
• Add adrenaline (epinephrine) to reduce the speed of absorption. The addition of adrenaline will reduce the maximum blood concentration by about 50%. Usually adrenaline is added in a concentration of 1:200,000, with a maximum dose of 200 micrograms. The addition of adrenaline will make no difference to the toxicity of the local anaesthetic if it is injected intravenously.
• Make sure that the anaesthetist or a trained nurse monitors the patient closely during the administration of the local anaesthetic and the surgery.

Signs and Symptoms of Local Anaesthetic Toxicity
The systemic toxic effects due to local anaesthetic overdose primarily involve the central nervous and cardiovascular systems. In general the Central Nervous System (CNS) is more sensitive to local anaesthetics than the Cardiovascular System (CVS). Therefore CNS manifestations tend to occur earlier. Brain excitatory effects occur before the depressant effects.

CNS signs and symptoms
Early or mild toxicity : light-headedness, dizziness, tinnitus, circumoral numbness, abnormal taste, confusion and drowsiness. Patients often will not volunteer information about these symptoms unless asked. Throughout the injection talk to the patient asking them how they feel. Any suggestion of confusion should alert you to the possibility of toxicity and you should stop any further injection.

Severe toxicity: tonic-clonic convulsion leading to progressive loss of consciousness, coma, respiratory depression, and respiratory arrest.

Depending on the drug and the speed of the rise in blood level the patient may go from awake to convulsing within a very short time.

CVS signs and symptoms
Early or mild toxicity: tachycardia and rise in blood pressure. This will usually only occur if there is adrenaline in the local anaesthetic. If no adrenaline is added then bradycardia with hypotension will occur.

Severe toxicity: Usually about 4 - 7 times the convulsant dose needs to be injected before cardiovascular collapse occurs. Collapse is due to the depressant effect of the local anaesthetic acting directly on the myocardium. Bupivacaine is considered to be more cardiotoxic than lignocaine. Severe and intractable arrhythmias can occur with accidental iv injection.

The acute toxicity of local anaesthetics is due to the speed of rise of blood concentration. Therefore a rapid injection of a small volume may cause toxicity. 

Essential Precautions

• l Always secure intravenous access before injection of any dose that may cause toxic effects.
• l Always have adequate resuscitation equipment and drugs available before starting to inject.

Treatment of Toxicity
If a patient you are attending shows any signs or symptoms of toxicity during injection of local anaesthetic stop the injection and assess the patient.

Treatment is based on the A B C D of Basic Life Support

Call for help while treating the patient
A. Ensure an adequate airway, give oxygen in high concentration if available.
B. Ensure that the patient is breathing adequately. Ventilate the patient with a self inflating bag if there is inadequate spontaneous respiration. Intubation may be required if the patient is unconscious and unable to maintain an airway.
C. Treat circulatory failure with intravenous fluids and vasopressors such as ephedrine if hypotension occurs. Adrenaline may be used cautiously intravenously if ephedrine is either not available or not effective in correcting the hypotension. Treat arrhythmias. Start chest compressions if cardiac arrest occurs.
D. Drugs to treat convulsions such as Diazepam./ Thiopentone intravenously may also be used in theatre. Observe the patient closely after any reaction.

Treatment of local anaesthetic toxicity is likely to have a good outcome if toxicity is recognised and basic resuscitation is started early. Monitor patients closely when using local anaesthetics. If a reaction occurs:

• Prevent hypoxia which will cause brain damage and make convulsion or arrhythmias more difficult to control.
• Ensure that hypotension and arrhythmias are treated early.
• Ensure that convulsions are adequately treated.
• Most reactions are short-lived if the above advice is followed. 

Local complications of local anaesthetic blocks
The most important is damage to the nerve. Permanent nerve damage is very rare. It may be caused by accidentally injecting local anaesthetic within the nerve itself (intraneural) or by traumatising the nerve with the needle point. Two signs of intraneural injection are severe pain on attempted injection and marked resistance to injection. Either of these warning signs should prompt the operator to stop injecting and reposition the needle. Intraneural injection may also be less likely if a short-bevel needle is used. Paraesthesia is the “electric shock-like” feeling felt as the nerve is touched by the needle. It should be a warning sign that nerve damage may occur if the needle is inserted further.

It is also possible to cause a haematoma by puncturing an artery with the needle - pressure applied to the site for 5 minutes will minimize the haematoma.