Final answer:
When receiving epidural anesthesia during labor, the most important adverse effect to monitor is maternal hypotension due to its potential to cause significant harm to both mother and fetus.
Step-by-step explanation:
The most important adverse effect the practical nurse (PN) should assess for when a client is receiving epidural anesthesia during labor is maternal hypotension. This is because the anesthesia can lead to a sympathetic nervous system blockade which may cause blood vessels to dilate, leading to a drop in blood pressure. This hypotension can endanger both mother and fetus, as it may lead to decreased uteroplacental perfusion and subsequent fetal hypoxia.