Cardiac output. Basically an increase in demand for oxygen by the body. This may be physiological (exercise) or pathological (sepsis - infection - fever) or increased metabolism (hyper active thyroid gland - hyperthyroidism) or abnormal conducts (shunting of blood).
Many things. When you increase heart rate, you increase cardiac output. There are medications that increase the heart muscles contractility that can increase cardiac output when it is low. Adrenaline itself can increase cardiac output.