What factor is most likely to contribute to a significant decrease in cardiac output?

A significant decrease in cardiac output can occur due to an increase in afterload. Afterload refers to the resistance that the left ventricle must overcome to eject blood during systole. When afterload increases, the heart has to work harder to pump blood against this greater resistance, which can lead to a reduced stroke volume. As stroke volume is a key component of cardiac output (Cardiac Output = Stroke Volume x Heart Rate), a decreased stroke volume directly impacts cardiac output, leading to a significant overall decrease.

Increased afterload might be due to conditions such as hypertension or aortic stenosis, which create an additional burden on the heart. If the heart cannot generate enough force to overcome this resistance, it may result in lower volumes of blood being ejected with each heartbeat, thereby decreasing cardiac output.

In contrast, factors that are associated with increases in heart rate, preload, or stroke volume typically do not contribute to a significant decrease in cardiac output. Increased heart rate can enhance cardiac output, and an increase in preload generally indicates more filling of the heart, which can often lead to an increased stroke volume through the Frank-Starling mechanism. Therefore, increased afterload is the primary factor leading to a decrease in cardiac output in this context.