A low bottom number on your blood pressure reading, called diastolic pressure, usually reflects changes in how flexible your arteries are, though it can also be driven by medications, dehydration, or heart conditions. The bottom number measures the pressure in your arteries between heartbeats, when your heart is resting and refilling with blood. A diastolic reading below 60 mmHg is generally considered low, and readings that drop well below that range deserve attention.
What the Bottom Number Actually Measures
Your blood pressure reading has two numbers. The top number (systolic) captures the force when your heart contracts and pushes blood out. The bottom number (diastolic) captures the residual pressure in your arteries while your heart relaxes between beats. Normal blood pressure is defined as below 120/80 mmHg, and overall low blood pressure is typically classified as anything under 90/60 mmHg.
The diastolic number depends heavily on how well your arteries bounce back after each heartbeat. Healthy arteries are flexible and elastic, so they expand when blood surges through and then spring back to maintain steady pressure during the resting phase. When arteries lose that springiness, they can’t recoil as effectively, and the pressure between beats drops.
Aging Is the Most Common Cause
If you’re over 50, the most likely explanation is simply age-related stiffening of your arteries. Starting around age 55, arteries naturally lose elasticity. Stiffer arteries expand with the force of each heartbeat but have a harder time springing back between beats, which causes the diastolic number to fall while the systolic number stays the same or even rises. This is why many older adults see a widening gap between their two blood pressure numbers.
Atherosclerosis, the buildup of fatty plaque inside artery walls, accelerates this process. Plaque makes blood vessels rigid, producing the same drop in diastolic pressure that aging causes on its own. Risk factors like smoking, high cholesterol, and diabetes all speed up plaque buildup and can contribute to a lower bottom number earlier in life.
Another possible contributor is a condition called endothelial dysfunction, where the small arteries on the heart’s surface constrict instead of relaxing and opening. This disrupts normal blood flow patterns and can pull diastolic pressure downward.
Medications That Lower the Bottom Number
Blood pressure medications are a frequent culprit. Drugs designed to bring high blood pressure under control sometimes overshoot, pushing diastolic pressure lower than intended. The classes most likely to do this include diuretics (water pills), alpha blockers, and beta blockers. If your bottom number started dropping after a medication change or dose increase, that connection is worth raising with your prescriber.
Other medications can also lower blood pressure as a side effect. Drugs for Parkinson’s disease, certain antidepressants, and sildenafil (especially when combined with heart medications) are all known to push readings down. The effect is sometimes more noticeable on the diastolic number because these drugs relax blood vessel walls, reducing the baseline tension that maintains pressure between heartbeats.
Dehydration and Other Everyday Factors
When your body is low on fluids, your total blood volume drops. Less blood in the system means less pressure pushing against artery walls, and the diastolic number is often the first to reflect that. Dehydration can happen from obvious causes like heat, exercise, or illness, but also from simply not drinking enough throughout the day. General fluid intake recommendations are about 125 ounces per day for men and 91 ounces for women, though hot weather and physical activity increase those needs.
The relationship between dehydration and blood pressure is more complicated than it first appears. When you’re dehydrated, sodium levels in your blood rise, triggering the release of a hormone that constricts blood vessels to compensate. So in some cases, mild dehydration can temporarily raise blood pressure. But when fluid loss is significant or sustained, the drop in blood volume wins out and pressure falls. If your low readings come and go, fluctuating hydration could be a factor.
Heart Conditions That Reduce Diastolic Pressure
Certain heart problems directly lower diastolic pressure because they reduce the amount of blood your heart circulates with each cycle. A very slow heart rate (bradycardia), heart valve problems, and heart failure can all cause the bottom number to fall. In these cases, the heart either isn’t beating often enough or isn’t pumping forcefully enough to maintain adequate pressure between beats.
Leaky heart valves are a particularly clear example. If the aortic valve doesn’t close tightly, blood flows backward into the heart during the resting phase, draining pressure from the arteries and pulling the diastolic reading down. This type of low diastolic pressure tends to be persistent rather than coming and going.
Why a Low Bottom Number Matters for Heart Health
A low diastolic reading isn’t just an abstract number. Your heart muscle gets most of its own blood supply during the diastolic phase, between beats. When diastolic pressure drops too low, blood flow to the heart muscle itself can be compromised. This is especially concerning when the top number remains elevated, creating a wide gap between the two readings (called pulse pressure).
A large study published in the Journal of the American College of Cardiology found that people with a diastolic pressure below 60 mmHg were roughly twice as likely to show signs of microscopic heart muscle damage compared to those with a diastolic reading of 80 to 89 mmHg. Those with readings between 60 and 69 mmHg had about 1.5 times the risk. The association was strongest in people who also had a systolic reading above 120 mmHg, meaning the combination of a high top number and a low bottom number carried the most risk. Low diastolic pressure was also independently linked to coronary heart disease events and higher mortality, though not to stroke.
Current clinical guidelines reflect this concern. While there’s no hard cutoff for a “too low” diastolic number, experts suggest it may be prudent to keep diastolic pressure from falling below 70 mmHg during treatment, and particularly not below 60 mmHg.
Symptoms to Pay Attention To
Some people with low diastolic pressure feel perfectly fine and only discover it during a routine check. Others notice symptoms that reflect reduced blood flow. Dizziness or lightheadedness, especially when standing up, is one of the more common signs. Fatigue that doesn’t improve with rest, blurred vision, difficulty concentrating, and feeling faint can all point to blood pressure that’s dropping too low. Orthostatic hypotension, a sudden pressure drop when you stand, affects an estimated 7% to 10% of adults with high blood pressure and is more common in older adults.
The symptoms that matter most are the ones that affect your daily functioning or put you at risk for falls. Occasional mild lightheadedness after standing too quickly is common and usually harmless. Persistent dizziness, episodes of near-fainting, or unexplained fatigue that limits your activity are more meaningful signals that something needs evaluation. If your bottom number is consistently below 60 and you’re experiencing any of these symptoms, that combination warrants a conversation with your doctor, particularly if you’re taking blood pressure medication that could be adjusted.