A heartbeat is a two-part pumping action that takes about a second. As blood collects in the upper chambers (the right and left atria), the heart’s natural pacemaker (the sinoatrial or SA node) sends out an electrical signal that causes the atria to contract. This contraction pushes blood through the tricuspid and mitral valves into the resting lower chambers (the right and left ventricles). This part of the two-part pumping phase (the longer of the two) is called diastole.

The second part of the pumping phase begins when the ventricles are full of blood. The electrical signals cause the ventricles to contract and push blood through the pulmonary and aortic valves. This is called systole.

If the diastole part of the two-part pumping phase is abnormal, this is called diastolic dysfunction.

In some people with heart failure, the ventricles do not relax properly during diastole. If the ventricles are not relaxed, the pressure inside of them will increase as blood from the next heartbeat tries to enter. This can lead to extra pressure and fluid in the blood vessels of your lungs (called pulmonary congestion) or in the blood vessels leading back to your heart (called systemic congestion).

Diastolic dysfunction often occurs in people with certain types of cardiomyopathy.