Depends on dosing. Dobutamine is an agent that helps the weak heart pump better by stimulating beta receptors in the heart. At lower doses it doesnt affect the blood vessels but higher doses it can also lower the tone of the ateries and makes it easier for the heart to pump. This tone is aka systemic vascular resistence. By lowering the svr the cariac out put will increase. Nipride (nitroprusside) has no beta stim effects only svr.
Vasodilitation. Nitroprusside very potently dilates blood vessels and reduces the workload of the heart. Dobutamine stimulates cardiac contraction but also constricts blood vessels in a dose dependent fashion which can counteract the nitroprusside effect. The combination has been used to treat cardiogenic shock.