What happens to atrial preload when a woman at 35 weeks of pregnancy is laid in a supine position?

When a woman at 35 weeks of pregnancy is laid in a supine position, the weight of the gravid uterus can compress the inferior vena cava, which is responsible for venous return from the lower extremities back to the heart. This compression can lead to a reduction in venous return, which ultimately decreases atrial preload—the amount of blood filling the atria before contraction.

In the supine position, the increased pressure on the inferior vena cava interferes with blood flow, causing a reduction in the volume of blood filling the right atrium. This can have a notable effect on cardiac output and overall hemodynamics. The decrease in preload can lead to symptoms such as hypotension, dizziness, or syncope, commonly referred to as supine hypotensive syndrome.

While there may be increased uterine circulation or aortic compression involved, these do not directly connect to the changes in atrial preload in a supine position. The critical factor is the mechanical effect of the uterus on the inferior vena cava, resulting in the marked reduction of atrial preload.