What condition is most commonly associated with unconsciousness during a postretrobulbar block?

Unconsciousness during a postretrobulbar block is most commonly associated with intravascular injection. This procedure, which is performed to anesthetize the eye and surrounding structures, carries a risk of inadvertently injecting local anesthetic into a blood vessel. Such an event can lead to significant systemic effects, including sedation or loss of consciousness, as the anesthetic enters systemic circulation and affects the central nervous system. The rapid onset of these symptoms can result from high concentrations of the anesthetic reaching the brain, causing changes in consciousness.

In contrast, while respiratory failure, cardiac arrest, and cerebral hypoxia are serious conditions that could also impact consciousness, they are not the most direct or immediate consequences of a postretrobulbar block. They may occur in rare situations or as secondary complications but do not typically follow an intravascular injection as promptly.