What is cardiac preload?

Cardiac preload refers to the degree of myocardial stretch just prior to contraction, typically measured at the end of diastole (end-diastolic volume). It represents the amount of blood volume present in the ventricles just before they start contracting. Preload is an important determinant of cardiac output, and is influenced by several factors, including:

1. Venous Return: Increased venous return, such as occurs during exercise or fluid resuscitation, leads to increased ventricular filling and thus greater preload.

2. Cardiac Compliance: The compliance of the ventricles, or their ability to stretch and accommodate increased volume, affects preload. Increased ventricular compliance, as seen in certain heart conditions, can result in higher preload.

3. Heart Rate: Changes in heart rate can impact preload. Slower heart rates allow for more time for ventricular filling and increased preload, while faster heart rates can result in decreased preload.

4. Blood Volume: Changes in total blood volume, such as those occurring during hemorrhage or fluid overload, can affect preload. Decreased blood volume reduces preload, while increased blood volume leads to higher preload.

5. Atrioventricular Valve Function: Proper functioning of the atrioventricular (AV) valves is crucial for maintaining preload. Dysfunctional AV valves, such as mitral or tricuspid regurgitation, can result in abnormal flow dynamics and reduced preload.

Preload plays a vital role in the Frank-Starling mechanism of the heart, which describes the relationship between ventricular preload and cardiac output. According to this mechanism, increased preload leads to increased cardiac output, as the stretched myocardial fibers can contract with more force. Understanding the concept of preload is essential in assessing cardiac function and managing various cardiovascular conditions.