Preload

Indeed, GTN works predominantly by venodilation through activation of the nitric oxide pathway. This causes preload to be reduce. Nitrates, such as GTN, also reduce afterload through vasodilation. However this is not as prominent and reduction of preload. Together, reduction of preload and afterload are beneficial in the treatment of heart failure. 

Over the next few days, Mrs. Reed makes a good recovery and she is finally discharged and advised to follow up with her cardiologist.

Further developments

Mrs. Reed's cardiologist reviews her progress and notes a significant deterioration in her ejection fraction. He notes that in addition to lung crackles, ankle oedema is now also evident. He decides to add another drug to Mrs. Reed's standard heart failure regimen, which already includes lisinopril (ACEI) and bisoprolol (beta-blocker). The cardiologist explains that this additional drug will not only improve her symptoms but also halt the progression of the disease.

Which drug was most likely prescribed?