Several factors influence the acute systemic toxicity of local anesthetics:
1. Physico-chemical properties (2),
2. rate and route of administration of the drug (3, 4, 5),
3. blood gas and acid base balance (6, 7, 8, 9, 10, 11),
4. pre-existing medical conditions (12, 13, 14),
5. age (15, 16, 17, 18, 19), and
6. pregnancy (20, 21, 22).
Local anesthetics work by blocking impulse conduction along nerves. They have similar action in other impulse conducting organs such as the brain and heart. The primary target organ of local anesthetic toxicity is the central nervous system (CNS) (2). Early signs of toxic reactions can often be detected by symptoms associated with the CNS, such as changes in speech patterns and muscle twitching (23) (Table 36.4). More severe CNS toxicity may be displayed as seizures or coma (Table 36.6 and Table E.2).
The cardiovascular system (CVS) is generally more resistant to local anesthetic toxicity than is the CNS (Table 57.3, Figure 57.1)(2). However, local anesthetics can have profound effects upon the CVS. Relative potency for CVS toxicity again seems to follow the relative anesthetic potency. More potent agents, such as bupivacaine have been shown to be more cardiotoxic than less potent agents such as lidocaine. There have been laboratory and clinical reports of sudden acute CVS toxicity associated with clinically acceptable dosages, which included ventricular arrhythmias, fibrillation and death. Difficult and prolonged resuscitation has also been reported following intoxication (2) (Chapter 13).
The anesthetic technique should be designed to minimise the risk of accidental over-dosage. A protocol, equipment and pharmacologic agents necessary for the treatment of local anesthetic induced toxicity should be immediately available to the physician (Table 36.5). Early aggressive treatment is essential and prolonged resuscitation may be necessary (Chapter 13).
References:
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Anesthesia, Editor: Miller RD,
Churchill Livingstone, New York.
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