Which factor does not directly influence Cardiac Index?

The correct choice reflects that blood pressure does not directly influence cardiac index. Cardiac index is a measure of the cardiac output of a patient normalized for their body surface area, indicating how well the heart is functioning in relation to a person's size.

Heart rate directly impacts cardiac output, which is a fundamental component of cardiac index since cardiac index is calculated using cardiac output divided by body surface area. Body surface area itself is critical as it ensures the measurement is adjusted for individual differences in size, which can affect how much blood is circulating effectively.

While blood pressure is a vital statistic and can be related to overall cardiac health and function, it does not play a direct role in calculating cardiac index like the other factors do. Blood pressure is an outcome of cardiac output and vascular resistance but is not a defining input in the formula for cardiac index. Thus, blood pressure is not a direct influencer in this context.