Diastolic blood pressure is the pressure in your arteries between heartbeats, when your heart is relaxing and refilling with blood. It’s the bottom number in a blood pressure reading. A normal diastolic reading is below 80 mmHg, and anything at or above that threshold is considered elevated.
What Happens During Diastole
Your heart works in two phases. During systole, the heart muscle contracts and pushes blood out into your arteries. During diastole, the muscle relaxes. At the molecular level, calcium is removed from the heart’s muscle fibers and the protein bonds that drive contraction release. This lets the heart chambers expand and fill with blood again before the next beat.
The pressure that remains in your arteries during this resting phase is your diastolic blood pressure. Even though the heart isn’t actively pumping, your blood vessels maintain some baseline pressure because of their elastic recoil and the volume of blood inside them. Think of it like a garden hose that still holds some water pressure even when you briefly release the nozzle.
This resting phase also serves a critical purpose for the heart itself. The heart muscle gets most of its own blood supply during diastole. The coronary arteries, which feed oxygen to the heart, are compressed during contraction and open up between beats. If diastolic pressure drops too low or the resting period between beats becomes too short (as it does at very high heart rates), the inner layers of the heart muscle can become starved for oxygen, especially if there’s any narrowing in the coronary arteries.
What Your Diastolic Number Means
The 2025 guidelines from the American Heart Association and American College of Cardiology break blood pressure into four categories. Both numbers matter, and if your systolic and diastolic readings fall into different categories, the higher category applies.
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129, diastolic still below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 or higher or diastolic 90 or higher
Notice that a diastolic reading of 80 or above places you in the hypertension range even if your top number looks fine. This matters because many people focus only on the systolic number and overlook the bottom one.
When Only the Bottom Number Is High
Some people have a normal systolic reading (below 130) but a diastolic reading of 80 or above. This is called isolated diastolic hypertension, and it’s diagnosed when that pattern shows up at two or more office visits. It’s more common in younger adults and tends to reflect increased resistance in smaller blood vessels.
Isolated diastolic hypertension isn’t harmless. It raises your lifetime risk of heart attack, increases the chance of dying from cardiovascular disease, and makes congestive heart failure more likely. These risks are greatest for women and people under 60. If your doctor flags a high diastolic number even though the top number is normal, it’s worth taking seriously.
How Diastolic Pressure Changes With Age
Systolic and diastolic pressure don’t age the same way. Systolic pressure tends to climb throughout life as blood vessels stiffen and narrow. Diastolic pressure, on the other hand, typically rises until about age 50 and then begins to decline.
The reason comes down to arterial elasticity. Healthy, flexible arteries stretch when the heart pumps and then snap back between beats, maintaining diastolic pressure. As arteries stiffen with age, they still expand under the force of each heartbeat (raising systolic pressure) but lose their ability to spring back effectively. That reduced recoil means less pressure is maintained during the resting phase, so the diastolic number drops. This is why older adults often have a wide gap between their two numbers, a pattern called wide pulse pressure.
When Diastolic Pressure Is Too Low
Low blood pressure is generally defined as a reading below 90/60 mmHg. While some people naturally run low without any problems, a diastolic number that drops significantly can cause noticeable symptoms: dizziness, blurred or fading vision, fatigue, trouble concentrating, nausea, or fainting. A sudden drop of just 20 mmHg in either number can be enough to make you lightheaded.
One common form is orthostatic hypotension, where blood pressure falls when you stand up after sitting or lying down. It’s especially common in older adults and can be triggered by dehydration, prolonged bed rest, pregnancy, or certain medications. Because diastolic pressure drives blood flow to the heart muscle during its resting phase, a persistently low diastolic reading can reduce oxygen delivery to the heart, particularly in people who already have narrowed coronary arteries.
How Diastolic Pressure Is Measured
When a clinician takes your blood pressure with a cuff and stethoscope, they’re listening for specific sounds as air is slowly released from the cuff. The systolic number is recorded when they first hear a tapping sound as blood begins to flow through the compressed artery. The diastolic number is recorded when that sound disappears completely, indicating the artery is no longer being squeezed shut at any point during the heartbeat. Automated cuffs use sensors to detect the same transition, though they measure oscillations in cuff pressure rather than listening for sounds directly.
For the most accurate reading, sit quietly for five minutes beforehand with your feet flat on the floor and your arm supported at heart level. A single reading can be misleading. Blood pressure fluctuates throughout the day, which is why a diagnosis of hypertension requires elevated readings on multiple occasions.
Lowering a High Diastolic Reading
The same lifestyle changes that lower systolic pressure also bring down diastolic readings. Reducing sodium intake is one of the most effective steps. A crossover trial published in JAMA found that switching from a high-sodium to a low-sodium diet lowered median diastolic pressure by about 3 mmHg, and some people showed even greater reductions depending on their individual salt sensitivity.
Regular aerobic exercise, maintaining a healthy weight, limiting alcohol, and eating a diet rich in fruits, vegetables, and whole grains (the pattern known as the DASH diet) all contribute to lower readings on both numbers. These changes work best in combination. If lifestyle adjustments aren’t enough to bring your diastolic pressure below 80, medication may be appropriate, particularly if you have other risk factors for heart disease.