In general four. Cardiac output is influence by the amount of blood the heart recieves, the resistance against which the blood is ejected, the strength of contraction and the heart rate. For ex. CO increases with heart rate, increase in blood volume, increase in cardiac contraction and less resistance to ejection because of lower peripheral resistance. Clinical conditions that affects any 4 will alter cardiac output.
Many. High cardiac out put can be from sepsis, thyroid disease, arterial-venous fistulas, anemia, vitamin defiency etc. Low out put can be from a heart attack and a weak heart muscle, virus induced cardiomyopathy, severe valvular heart disease. Just to name a few.