The low number on a blood pressure reading is called diastolic pressure, and it measures the force of blood against your artery walls when your heart is resting between beats. In a reading like 120/80, the bottom number (80) is the diastolic value. It reflects the lowest pressure your arteries experience during the cardiac cycle, and a healthy diastolic reading falls below 80 mmHg.
What the Bottom Number Tells You
Your heart works in a two-phase cycle: squeeze and relax. The top number (systolic) captures the pressure during the squeeze, when blood is being pumped out. The bottom number captures the pressure during the pause between beats, when your heart refills with blood. Think of it as the baseline pressure always present in your blood vessels, even when your heart isn’t actively pushing.
This resting pressure matters more than people realize. During the relaxation phase, your coronary arteries, the vessels that feed your heart muscle itself, receive most of their blood supply. A healthy diastolic pressure ensures enough oxygen reaches the heart. When that number drops too low, the heart can be starved of its own fuel even while it continues pumping blood to the rest of your body.
Normal Ranges and What’s Too Low
The standard blood pressure categories, based on guidelines from the American Heart Association, break down diastolic pressure like this:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
Most discussions about blood pressure focus on readings that are too high. But diastolic pressure can also be too low. A reading below 60 mmHg is generally considered the threshold for concern, particularly if the top number remains normal or elevated. This pattern, sometimes called isolated diastolic hypotension, affects a meaningful portion of adults. In one large study of over 5,000 older adults, about 14% had diastolic readings below 60 with systolic pressure at 100 or above.
Overall low blood pressure, where both numbers drop, is typically defined as a reading below 90/60 mmHg. But the bottom number deserves its own attention because it can dip independently of the top number, especially as people age or take blood pressure medications.
Why a Low Bottom Number Is Risky
A persistently low diastolic reading isn’t just a harmless quirk. Because the heart muscle relies on diastolic pressure to receive its own blood supply, a number that stays below 60 can impair that oxygen delivery. This is especially pronounced in people who already have narrowed coronary arteries.
A secondary analysis of the large SPRINT trial, published in the Journal of the American Heart Association, tracked patients whose systolic pressure was well-controlled below 130 mmHg but whose diastolic pressure dropped below 60. Compared to those with a diastolic reading between 60 and 79, the low-diastolic group had roughly 32% higher risk of major cardiovascular events. The increased risk showed up across the board: 37% higher for coronary heart disease, 33% higher for stroke, 30% higher for heart failure, and 31% higher for cardiovascular death. These are not small differences, and they emerged in people whose top number looked perfectly fine.
The practical takeaway: the bottom number isn’t just the less important half of the reading. A diastolic value that consistently sits below 60 warrants a conversation with your doctor, particularly if you’re on blood pressure medication that may be pushing it down too far.
What Causes a Low Diastolic Reading
Diastolic pressure naturally declines with age. As arteries stiffen over the decades, systolic pressure tends to rise while diastolic pressure drops. This is why isolated low diastolic readings are more common in people over 60.
Blood pressure medications can also push the bottom number lower than intended. When treatment is aimed at bringing down a high systolic number, the diastolic number often comes along for the ride. Dehydration, prolonged bed rest, and significant blood loss are other common causes. Heart valve problems, particularly a leaky aortic valve, can lower diastolic pressure because blood flows backward during the resting phase instead of maintaining forward pressure.
Symptoms to Watch For
Many people with mildly low diastolic pressure feel nothing unusual. Symptoms tend to appear when the number drops low enough to reduce blood flow to the brain or other organs. The most common signs include dizziness or lightheadedness (especially when standing up), fatigue that doesn’t improve with rest, blurry vision, and difficulty concentrating. In more severe cases, fainting can occur. These symptoms overlap with general low blood pressure and aren’t unique to diastolic drops, so a reading is the only way to know what’s actually happening with your numbers.
Getting an Accurate Reading
A single blood pressure check can be misleading. Your diastolic number fluctuates throughout the day based on stress, activity, caffeine, and even your body position. If you’re monitoring at home, a few simple steps make the difference between a useful reading and a meaningless one.
Sit in a chair with your back supported for at least five minutes before measuring. Keep your feet flat on the floor with your legs uncrossed. Rest the arm you’re using on a table so it’s level with your heart, and let it relax completely. The cuff should wrap around your upper arm just above the elbow, directly against skin rather than over a sleeve. Using a cuff that’s too small for your arm will produce inaccurate results.
Take two or three readings a minute apart and average them. Track your numbers over days and weeks rather than reacting to any single measurement. Patterns matter far more than individual readings, and a consistently low bottom number tells a different story than one that dips occasionally.