Hydralazine by itself is only an arterial vasodilator and does not reduce left ventricular filling pressures to the same extent as nitrates and ACE inhibitors do. In fact, when used alone it can stimulate sympathetic tone reflexively. The combination of hydralazine and nitrates has been shown to decrease mortality as well as improve the left ventricular ejection fraction and exercise capacity in patients with heart failure. However the combination of hydralazine and nitrates has been found to be less effective than ACE inhibitors.
The major of use of this combination is in those patients who are intolerant of ACE inhibitors.
2009 ACC/AHA Focused Update and 2005 Guidelines for the Diagnosis and Management of Chronic Heart Failure in the Adult (DO NOT EDIT) 
Combination of Hydralazine and Nitrates in Patients Presenting With Heart Failure (DO NOT EDIT) 
↑ 2.02.1 Hunt SA, Abraham WT, Chin MH, Feldman AM, Francis GS, Ganiats TG, Jessup M, Konstam MA, Mancini DM, Michl K, Oates JA, Rahko PS, Silver MA, Stevenson LW, Yancy CW, Antman EM, Smith SC Jr, Adams CD, Anderson JL, Faxon DP, Fuster V, Halperin JL, Hiratzka LF, Jacobs AK, Nishimura R, Ornato JP, Page RL, Riegel B; American College of Cardiology; American Heart Association Task Force on Practice Guidelines; American College of Chest Physicians; International Society for Heart and Lung Transplantation; Heart Rhythm Society. ACC/AHA 2005 Guideline Update for the Diagnosis and Management of Chronic Heart Failure in the Adult: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Update the 2001 Guidelines for the Evaluation and Management of Heart Failure): developed in collaboration with the American College of Chest Physicians and the International Society for Heart and Lung Transplantation: endorsed by the Heart Rhythm Society. Circulation. 2005 Sep 20; 112(12): e154-235. Epub 2005 Sep 13. PMID 16160202
↑Loeb HS, Johnson G, Henrick A, Smith R, Wilson J, Cremo R, Cohn JN (1993). "Effect of enalapril, hydralazine plus isosorbide dinitrate, and prazosin on hospitalization in patients with chronic congestive heart failure. The V-HeFT VA Cooperative Studies Group". Circulation. 87 (6 Suppl): VI78–87. PMID8500244. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)CS1 maint: Multiple names: authors list (link)