Final answer:
Chest tightness is the adverse effect of greatest concern to the nurse.
Step-by-step explanation:
The adverse effect of greatest concern to the nurse in a client receiving sumatriptan (Imitrex) for migraine headaches is chest tightness. Chest tightness could be a sign of a severe reaction to sumatriptan, such as a heart attack. The nurse should closely monitor the client for any other signs of cardiac distress, such as shortness of breath, chest pain, or changes in heart rate or rhythm.