Toxicity of Local Anaesthetics

Toxicity may be a feature of the specific drug chosen, the dose given or the technique used.

Overdoses

Overdoses may be absolute or relative in nature.

There may be an absolute overdose where the dose exceeds the recommended maximum.

There may be a relative overdose where there has been:

• An exceptionally rapid absorption, but the dose is within the recommended limit
• An inadvertent intravascular injection or injection into a very vascular site
• Inadequate attention to co-morbidities such as severe liver disease, malnourishment and also in the elderly. A dose reduction should be considered

There can also be very rapid absorption from mucosal surfaces. These circumstances result in a rate of absorption which cannot be matched by the rate of distribution, breakdown and excretion.

The use of adrenaline

The use of adrenaline to prolong local anaesthetic action and slow absorption is widely used, but care should be taken not to induce adrenaline toxicity.

A maximum of 0.5mg of adrenaline is the maximum that should be injected at any one time i.e. 100mls of 1:200000 solution.

Only pre-prepared solutions should be used.

Toxicity

Toxicity can be categorised as either local or systemic:

Local

Local toxicity includes:

• Infection
• Haematoma
• Local tissue damage e.g. intraneural injection
• Unwanted nerve block
• Necrosis through ischaemia from vasoconstrictor
• Pneumothorax (in blocks around the neck or axilla)

Systemic

Cardiovascular toxicity includes:

• Cardiac depression – reduced BP, tachycardia, reduced cardiac output and acute cardiac dilatation
• Peripheral vasodilatation, except cocaine–vasoconstriction

Respiratory toxicity includes:

• Medullary depression
• Bronchospasm from hypersensitivity – although extremely rare. Bronchospasm may also be induced in those with a psychogenic element to their illness such as in some asthma sufferers
• Relaxation of bronchial musculature

With CNS toxicity:

• The higher cortex tends to be excited whereas the mid-brain is depressed
• Loss of inhibitory neurones leads to cortical excitability, e.g. fits or tremor
• Depression of the mid-brain leads to respiratory collapse.

Non-specific toxicity includes

• Methaemoglobinaemia
• Hypersensitivity reactions:
  • The preservative methylparaben, used in multidose vials may cause such reactions
  • Reactions may be local (rash/dermatitis) or generalised
  • All reactions are unusual; true anaphylaxis is extremely rare and has only been reported in individual case reports. It has been suggested that patients who declare that they are ‘allergic’ should be skin tested, but this should only be considered in those who can describe a clear history of severe reaction. Many cases of ‘allergy’ involving collapse can be attributed to vasovagal faints, and mechanisms not related to hypersensitivity
• Intravenous injections
• Psychological reactions e.g. anxiety leading to vasovagal collapse

Learning bite

Toxicity can occur even within the maximum recommended dose.