Complex sentence: In those receiving heparin through an intravenous infusion, a complex of symptoms ("systemic reaction") may occur when the infusion is commenced: fever, chills, high blood pressure, a fast heart rate, shortness of breath and chest pain.
Split into simpler sentences: In those receiving heparin through an intravenous infusion, a complex of symptoms ("systemic reaction") may occur when the infusion is started. These include fever, chills, high blood pressure, a fast heart rate, shortness of breath, and chest pain.