Systemic adverse reactions involving the central nervous system and the cardiovascular system usually result from high plasma levels due to excessive dosage, rapid absorption, or inadvertent intravascular injection. In addition, use of inappropriate doses or techniques may result in extensive spinal blockade leading to hypotension and respiratory arrest.
A small number of reactions may result from hypersensitivity, idiosyncrasy, or diminished tolerance to normal dosage.
Excitatory CNS effects (nervousness, dizziness, blurred vision, tremors) commonly represent the initial signs of local anesthetic systemic toxicity. However, these reactions may be very brief or absent in some patients in which case the first manifestation of toxicity may be drowsiness or convulsions merging into unconsciousness and respiratory arrest.
Cardiovascular system reactions include depression of the myocardium, hypotension (or sometimes hypertension), bradycardia, and even cardiac arrest.
Allergic reactions are characterized by cutaneous lesions of delayed onset, or urticaria, edema, and other manifestations of allergy. The detection of sensitivity by skin testing is of limited value. As with other local anesthetics, hypersensitivity, idiosyncrasy and anaphylactoid reactions have occurred rarely. The reaction may be abrupt and severe and is not usually dose related.
The following adverse reactions may occur with spinal anesthesia: Central Nervous System: postspinal headache, meningismus, arachnoiditis, palsies, or spinal nerve paralysis. Cardiovascular: hypotension due to vasomotor paralysis and pooling of the blood in the venous bed. Respiratory: respiratory impairment or paralysis due to the level of anesthesia extending to the upper thoracic and cervical segments. Gastrointestinal: nausea and vomiting.
Treatment of Reactions. Toxic effects of local anesthetics require symptomatic treatment: there is no specific cure. The physician should be prepared to maintain an airway and to support ventilation with oxygen and assisted or controlled respiration as required. Supportive treatment of the cardiovascular system includes intravenous fluids and, when appropriate, vasopressors (preferably those that stimulate the myocardium, such as ephedrine). Convulsions may be controlled with oxygen and by the intravenous administration of diazepam or ultrashort-acting barbiturates or a short-acting muscle relaxant (succinylcholine).
Intravenous anticonvulsant agents and muscle relaxants should only be administered by those familiar with their use and only when ventilation and oxygenation are assured. In spinal and epidural anesthesia, sympathetic blockade also occurs as a pharmacological reaction, resulting in peripheral vasodilation and often hypotension. The extent of the hypotension will usually depend on the number of dermatomes blocked. The blood pressure should therefore be monitored in the early phases of anesthesia. If hypotension occurs, it is readily controlled by vasoconstrictors administered either by the intramuscular or the intravenous route, the dosage of which would depend on the severity of the hypotension and the response to treatment.