After an epidural dose is accidentally administered into the subarachnoid space, which physiologic effect is not expected with a large-volume subarachnoid injection?

After an epidural dose is accidentally administered into the subarachnoid space, which physiologic effect is not expected with a large-volume subarachnoid injection?

• A. Hypotension
• B. Bradycardia
• C. Respiratory depression
• D. Constricted pupils

Answer: (D)

A large-volume intrathecal (subarachnoid) injection produces a dense neuraxial block that interrupts sympathetic outflow. This leads to vasodilation with a drop in systemic vascular resistance and blood pressure, and it can also cause bradycardia due to reduced sympathetic input to the heart. If the block is high, respiratory depression may occur because of impaired intercostal function or diaphragmatic involvement. Pupil size, however, is governed by cranial nerve–mediated parasympathetic and sympathetic pathways that are not predictably altered by a spinal block. A large intrathecal injection does not reliably cause a consistent constricted (miosed) pupil, so this is not an expected effect. The other listed effects—hypotension, bradycardia, and respiratory depression—are classic consequences of a dense spinal/neuraxial block.

A large-volume intrathecal (subarachnoid) injection produces a dense neuraxial block that interrupts sympathetic outflow. This leads to vasodilation with a drop in systemic vascular resistance and blood pressure, and it can also cause bradycardia due to reduced sympathetic input to the heart. If the block is high, respiratory depression may occur because of impaired intercostal function or diaphragmatic involvement.

Pupil size, however, is governed by cranial nerve–mediated parasympathetic and sympathetic pathways that are not predictably altered by a spinal block. A large intrathecal injection does not reliably cause a consistent constricted (miosed) pupil, so this is not an expected effect. The other listed effects—hypotension, bradycardia, and respiratory depression—are classic consequences of a dense spinal/neuraxial block.