What can happen if a local anesthetic is injected into a blood vessel?

Injecting a local anesthetic into a blood vessel can lead to systemic toxicity, which occurs rapidly because the anesthetic agent enters the bloodstream directly. This can result in elevated plasma concentrations of the drug, leading to potential adverse effects on the central nervous system (CNS) and cardiovascular system. Symptoms of systemic toxicity may include dizziness, tinnitus, seizures, and even cardiac arrhythmias or arrest, depending on the amount of anesthetic that enters the circulation.

In contrast, the other options do not correctly describe the consequences of intra-vascular injection. Gradual numbness and delayed onset of action suggest a more controlled and expected response to local anesthetics that occur when injected into the appropriate tissues, rather than directly into the bloodstream. Increased effectiveness of the anesthetic also does not apply in this scenario, as systemic toxicity can hinder the intended local effect and lead to serious complications.