Diastolic refers to the relaxed phase of the cardiac cycle when the chambers of the heart are re-filling with blood. It is the pressure in the arteries when the heart is at rest between beats. The diastolic reading is the bottom number in a blood pressure measurement, and it is important to know both systolic and diastolic numbers. The diastolic pressure plays a critical role in helping coronary vessels supply oxygen to the heart muscle. A too-low diastolic number may signal a higher risk of heart issues, and older adults with low diastolic blood pressure are at increased risk for developing new-onset heart failure. A normal diastolic number is lower than 80 mm Hg.