First: The Concept of Cardiac Preload and Postload

First: The Concept of Cardiac Preload and Postload

Cardiac preload refers to the volume at the end of left ventricular diastole; the larger this volume, the stronger the heart’s contractile force, and the greater the stroke volume. The volume at the end of left ventricular diastole is directly related to left atrial pressure, which in turn is linked to pulmonary artery pressure. Therefore, clinical practice often uses pulmonary artery pressure to estimate the volume at the end of ventricular diastole. In healthy individuals, the end-diastolic volume of the left ventricle is approximately 120–130 ml, with an end-diastolic ventricular pressure of 12 mmHg and pulmonary artery pressure of 5 mmHg. Postload refers to the resistance that impedes the ejection of blood from the left ventricle; therefore, peripheral vascular resistance, aortic pressure, and aortic resistance can all serve as indicators for measuring left ventricular postload. Under physiological conditions, cardiac preload remains constant; as the heart beats, left ventricular postload gradually increases, while stroke volume becomes smaller. Normally, the preload and postload of the heart are in dynamic equilibrium.