Which cardiac symptom indicates possible toxic level of local anesthetic?

Bradycardia indicates a possible toxic level of local anesthetic because it can occur as a result of compromise in cardiac function caused by the anesthetic agent. Local anesthetics can affect the conduction system of the heart, leading to a slower heart rate. This is particularly significant because bradycardia may reflect an underlying cardiac toxicity that can potentially lead to more serious arrhythmias or even cardiac arrest.

In contrast, other symptoms such as hypertension, tachycardia, and arrhythmias can be manifestations of various other medical conditions or responses to different stimuli, and while they may also be associated with local anesthetic toxicity in certain contexts, bradycardia is more specifically associated with the direct effects of toxic levels of these agents impairing the normal electrical conduction pathways in the heart.