When could cardiac output decrease?

Low blood pressure. Cardiac output is related to the heart rate, and volume of blood the heart can pump with each heart beat. A low cardiac output occurs when the pump function of the heart is compromised by an event such as a heart attack or when the blood volume is reduced by and event such as dehydration, or bleeding.
Output. Cardiac output is made up of stroke volume and heart rate. Decreasing heart rate decreases cardiac output. Also decreasing stroke volume (dehydration or anemia) can cause decreased output. There are also other hemodynamic states that may cause a decreased cardiac output.