Final answer:
To maintain cardiac output, an increase in heart rate is needed if stroke volume decreases, and vice versa.
Step-by-step explanation:
If stroke volume (SV) decreases, the heart must compensate by increasing heart rate (HR) to maintain cardiac output (CO). Cardiac output is the product of HR multiplied by SV and is a crucial measure of heart function. When stroke volume drops, a higher HR is needed to pump the same amount of blood per minute. Conversely, if HR decreases, the stroke volume must increase to maintain CO. This can be achieved through mechanisms that increase venous return to the heart, enhance the force of cardiac muscle contraction, or through other factors that affect preload and afterload.