Final answer:
Negative inotropic agents like certain beta blockers and calcium channel blockers could be potentially harmful to a patient with heart failure and a lowered ejection fraction, as they can exacerbate heart failure by decreasing the strength of heart muscle contraction.
Step-by-step explanation:
The medication potentially harmful to a 70-year-old man with heart failure and an ejection fraction (EF) of 30% is likely to be a negative inotropic agent. Such agents include certain beta blockers and calcium channel blockers, which decrease the strength of heart muscle contraction and can exacerbate heart failure. Since the EKG shows a normal sinus rhythm, drugs specifically aimed at correcting arrhythmias may not be necessary, and the focus should be on optimal management of heart failure. It would also be advisable to review medications that may affect cardiac muscle metabolism and ensure they align with the current cardiac condition. Medical therapy for patients with heart conditions, especially with such a decreased EF, must be carefully tailored to avoid exacerbation of heart failure symptoms.