Toxicity from Local Anaesthetic Drugs

Issue 10 (1999) Article 8: Page 1 of 1

Toxicity from Local Anaesthetic Drugs

Dr Henry Bukbirwa,
Senior Lecturer/Head Anaesthetic Department,
Makerere University Medical School, Uganda
Dr David A Conn,
Consultant Anaesthetist,
Royal Devon & Exeter Hospital, Exeter, UK

Introduction		CVS Signs & Symptoms
Reducing the Risk of Toxicity		Essential Precautions
Signs and Symptoms of Local Anaesthetic Toxicity		Treatment of Toxicity
CNS Signs & Symptoms		Case History

Introduction

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

Accidental rapid intravenous injection.
Rapid absorption, such as from a very vascular site ie 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 according to the table below.
Use the least toxic drug available.
Use lower doses in frail patients or at the extremes of ages.
Always inject the drug slowly (slower than 10ml /minute) and aspirate regularly looking for blood to indicate an accidental intravenous injection.
Injection of a test dose of 2-3ml 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 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. This is made up by taking an ampoule of adrenaline with a concentration of 1:1,000 = 1mg/ml =0.1mg%. From this you take 0.1ml (zero point one millilitres) and add it to each 20ml of the local anaesthetic. The addition of adrenaline will make no difference to the toxicity of the local anaesthetic if it is injected intravenously.
Make sure that the patient is monitored closely by the anaesthetist or a trained nurse during the administration of the local anaesthetic and the following surgery.

Drug	Maximum dose for infiltration	* Maximum dose for plexus anaesthesia
Lignocaine	4mg/kg	5mg/kg
Lignocaine with adrenaline	7mg/kg	7mg/kg
Bupivacaine	2mg/kg	2mg/kg
Bupivacaine with adrenaline	3mg/kg	3mg/kg
Prilocaine	6mg/kg	7mg/kg
Prilocaine with adrenaline / octapressin	8mg/kg	8mg/kg
* when performing intercostal blocks, reduce the dose for infiltration by 25%

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 & 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 & 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

secure intravenous access before injection of any dose that may cause toxic effects.

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 (10mg boluses) if hypotension occurs. Adrenaline may be used cautiously intravenously in boluses of 0.5 - 1ml of 1:10,000 (1mg in 10ml) 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 stop fitting such as Diazepam 0.2-0.4mg/kg intravenously slowly over 5 minutes repeated after 10 minutes if required, or 2.5mg - 10 mg rectally. Thiopentone 1-4 mg/kg 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 fitting or arrhythmias more difficult to control.
Ensure that hypotension and arrhythmias are treated early.
Ensure that fits are adequately treated.
Most reactions are short-lived if the above advice is followed.

Case History

A 20 year old mother who had just delivered a baby started to fit and then developed a cardiac arrest whilst a midwife was injecting lignocaine prior to suturing her episiotomy. Prompt resuscitation with airway, intubation and ventilation, chest compressions, intravenous fluid and adrenaline saved her life. When the ampoule of lignocaine was checked it was found that the midwife had used 10mls of 10% lignocaine for infiltration (1000mg), more than 5 times the maximum permitted dose for infiltration.