What determines preload and afterload?
Preload is the initial stretching of the cardiac myocytes (muscle cells) prior to contraction. It is related to ventricular filling. Afterload is the force or load against which the heart has to contract to eject the blood.
What 4 factors affect cardiac output?
Cardiac output is the amount of blood the heart pumps in 1 minute, and it is dependent on the heart rate, contractility, preload, and afterload. Understanding of the applicability and practical relevance of each of these four components is important when interpreting cardiac output values.
What improves preload?
Preload is increased by the following: Increased central venous pressure (CVP), e.g., from decreased venous compliance due to sympathetic activation; increased blood volume; respiratory augmentation; increased skeletal pump activity.
What determines afterload?
Afterload is the pressure that the heart must work against to eject blood during systole (ventricular contraction). Afterload is proportional to the average arterial pressure. As aortic and pulmonary pressures increase, the afterload increases on the left and right ventricles respectively.
How does preload affect afterload?
Afterload per se does not alter preload; however, preload changes secondarily to changes in afterload. Increasing afterload not only reduces stroke volume, but it also increases left ventricular end-diastolic pressure (LVEDP) (i.e., increases preload).
How do you reduce afterload?
Sympatholytics act to reduce afterload by inhibiting the binding of norepinephrine to post-junctional alpha receptors preventing them from causing smooth muscle contraction. The effect is more significant on the arterial system, but they also have some venodilating properties.
How does preload affect cardiac output?
Preload is related to cardiac performance through the Frank-Starling law of the heart; a decrease in preload diminishes the force of ventricular contraction and therefore decreases stroke volume. As a result, preload reduction generally results in a decrease in cardiac output.
What factor directly influences preload?
Factors affecting preload Preload is affected by venous blood pressure and the rate of venous return. These are affected by venous tone and volume of circulating blood. Preload is related to the ventricular end-diastolic volume; a higher end-diastolic volume implies a higher preload.
How do you increase afterload?
Afterload is increased when aortic pressure and systemic vascular resistance are increased, by aortic valve stenosis, and by ventricular dilation. When afterload increases, there is an increase in end-systolic volume and a decrease in stroke volume.