Systemic Absorption of Injected Local Anesthetics

1. Depends on vascularity of site of injection: intravenous > tracheal > intercostal > paracervical > epidural > brachial plexus > sciatic > subcutaneous.

2. The presence of vasoconstrictors causes vasoconstriction at the site of administration. The decreased absorption decreases the peak local anesthetic concentration in the blood and facilitates neuronal uptake, enhances the quality of analgesia and prolongs the duration of the block, and limits toxic side effects. The effect is greater on short-acting agents (extends duration of lidocaine by at least 50%) than long-acting agents (little or no effect on bupivacaine).

3. Lipid-soluble agents are more slowly absorbed.

Distribution of Local Anesthetics

1. Highly perfused organs are responsible for the initial uptake (α phase), which is followed by a slower redistribution (β phase) to moderately perfused tissues (muscle and gut).

2. Increased lipid solubility is associated with greater plasma protein binding and greater tissue uptake from an aqueous compartment.

3. Muscle provides the greatest reservoir for distribution of local anesthetic agents in the bloodstream because of large mass.

Systemic Effects

Neurologic Effects

Intravenous (IV) lidocaine decreases cerebral blood flow and attenuates the rise in intracranial pressure that accompanies intubation in patients with decreased intracranial compliance. The central nervous system (CNS) is vulnerable to local anesthetic toxicity and is the site of premonitory signs of rising blood concentrations in awake patients such as circumoral numbness, tongue paresthesia, and excitatory signs. Muscle twitching heralds the onset of tonic-clonic seizures. Higher concentrations lead to respiratory depression and coma. Highly lipid-soluble local anesthetics produce seizures at lower blood concentrations than less potent agents. Benzodiazepines and hyperventilation raise the threshold of local anesthetic-induced seizures. Both respiratory and metabolic acidosis reduce the seizure threshold.

Pulmonary Effects

Lidocaine depresses the hypoxic drive. Apnea can result from phrenic or intercostal nerve paralysis or depression of the medullary respiratory center after direct exposure to local anesthetic agents. Apnea after a “high” spinal or epidural is nearly always the result of hypotension rather than phrenic block. Local anesthetics relax bronchial smooth muscle and may be effective in blocking the reflex bronchoconstriction sometimes associated with intubation.

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