Please explain why does tachycardia not always have an effect on cardiac output?

Homeostasis: Cardiac output is regulated to meet the needs of the body. Heart rate is only one determinant of cardiac output. If heart rate goes up, cardiac output can be maintained by decreasing the amount of blood pumped by each beat by increasing the resistance against which the heart pumps, decreasing the filling of the heart, or decreasing the the contractility of the heart muscle.