What characterizes isometric relaxation in the heart cycle?

Isometric relaxation in the heart cycle refers to a phase during which the ventricles relax without changing their volume, meaning that the heart muscle is relaxing while the blood is not being ejected or filled in the ventricles. During this phase, the atria undergo changes related to venous filling, which is characterized by the presence of a V-wave in the atrial pressure curve.

The V-wave occurs due to the accumulation of blood in the atria, as the veins continue to drain blood into them while the ventricles are still in the isometric relaxation state. The accumulation of blood causes transient elevation in atrial pressure, reflected by the V-wave. This phenomenon indicates that the atria are receiving blood from the veins but are not actively contracting at that moment. It sets the stage for the next phase, where the atria will contract to push blood into the ventricles.

Other characteristics like a sharp contraction of the atria or significant pressure equalization between atria and ventricles do not define isometric relaxation correctly. Instead, during this phase, the focus is on the atrial filling and the subsequent effects on atrial pressure, making the presence of the V-wave a key feature that accurately describes isometric relaxation in the cardiac cycle