Final answer:
The best index of preload is Left ventricular end-diastolic volume (EDV) as it directly measures the volume of blood in the ventricles just before contraction. Other indicators such as central venous pressure can reflect preload but are influenced by other factors, making EDV the most accurate.
Step-by-step explanation:
The best index of preload among the options given is (D) Left ventricular end-diastolic volume (EDV). Preload is an important cardiac parameter that represents the volume of blood in the ventricles at the end of diastole, just before the heart contracts. Preload is directly related to the end-diastolic volume (EDV), which is the amount of blood present in the ventricles right at the end of the filling phase or diastole.
Hence, as EDV increases, so does preload. Measuring the EDV gives the most accurate representation of preload because it provides a direct measurement of the volume filling the ventricles before they contract. While other factors like central venous pressure and pulmonary capillary wedge pressure do give an indication of the preload, they can be influenced by other physiologic variables, therefore, EDV is considered the best index.