Preload refers to the resistance or load encountered before myocardial contraction, that is, at the end of diastole, the volume load or pressure borne by the ventricle is preload. In clinic, it is difficult to measure the volume, so the left ventricular end diastolic pressure (LVEDP) is usually used as the index of left ventricular preload. In the absence of mitral valve disease and pulmonary vascular disease, LVEDP is consistent with left atrial pressure, pulmonary vein pressure and pulmonary wedge pressure (capillary pressure PCWP). Right ventricular preload is usually expressed by right ventricular end diastolic pressure or right atrial pressure. Preload is related to venous reflux. In a certain range, with the increase of venous reflux, the preload increases. So, what are the factors that affect venous return? (1) Valve diseases, such as mitral and tricuspid insufficiency, can increase the volume load, while mitral and tricuspid stenosis can reduce the volume load. (2) Internal and external shunt diseases, such as atrial septum, ventricular septal defect and patent ductus arteriosus, can increase the capacity load. (3) Changes in systemic blood volume, such as massive infusion in a short time, hyperthyroidism, chronic anemia, etc. , will increase the volume load. Sweating, diarrhea, blood loss, etc. lead to the reduction of effective circulating blood volume, which can reduce the preload.
Cardiac afterload
Afterload refers to the resistance or load encountered after myocardial contraction, also known as pressure load. Aortic pressure and pulmonary artery pressure are afterloads of left and right ventricles. For the left ventricle, when there is no aortic stenosis or aortic stenosis, the subsequent load mainly depends on: (1) aortic compliance, that is, the ability of aortic contents to expand with the increase of pressure. If the blood vessel wall thickens, the compliance will decrease. (2) Peripheral vascular resistance: It depends on the cross-sectional area of arteriole vascular bed and vascular tension, and is influenced by vascular and humoral factors. (3) Blood viscosity: The higher the blood viscosity, the higher the peripheral vascular resistance. (4) Circulating blood volume. Among them, peripheral vascular resistance is the most important, which is often used as an index of left ventricular afterload in clinic.