The bottom number of your blood pressure reading, called diastolic pressure, reflects the pressure in your arteries between heartbeats, when your heart is relaxing. A diastolic reading below 60 mmHg is generally considered low. The most common reason it drops is stiffening of the arteries, which happens naturally with age, but medications, heart valve problems, and dehydration can also pull that number down.
What the Bottom Number Actually Measures
Your blood pressure reading has two numbers. The top number (systolic) captures pressure when your heart contracts. The bottom number (diastolic) captures pressure when your heart rests between beats. That resting pressure depends heavily on how elastic your arteries are. Flexible arteries absorb and slowly release blood flow, keeping diastolic pressure in a healthy range. When arteries lose that flexibility, blood rushes through too quickly during the heartbeat and pressure drops too low between beats.
The gap between the two numbers is called pulse pressure. A normal gap is roughly 40 mmHg. When the bottom number drops while the top stays the same or rises, that gap widens, and a wide pulse pressure is itself a marker of cardiovascular strain. Data from the Framingham Heart Study found that every 10 mmHg increase in pulse pressure independently raised the risk of coronary heart disease by 23%.
Arterial Stiffness and Aging
The single most common cause of an isolated low diastolic number is arterial stiffness. As you age, the walls of your large arteries, especially the aorta, gradually lose elasticity. They become less like rubber bands and more like rigid pipes. When the heart pumps blood into a stiff aorta, pressure spikes high during the beat (raising systolic) but drops quickly once the beat ends (lowering diastolic). This is why many people over 60 notice their top number creeping up while their bottom number drifts down.
Stiffening also changes the timing of blood flow waves that bounce back from smaller arteries. In a healthy system, these reflected waves return during the resting phase and help maintain diastolic pressure. In stiff arteries, the reflected wave arrives too early, during the heartbeat itself, adding to systolic pressure while robbing diastolic pressure. This pattern is especially pronounced in people with long-standing high blood pressure, diabetes, or chronic kidney disease.
Heart Valve Problems
A leaky aortic valve, known as aortic regurgitation, is a well-known cause of low diastolic pressure. Normally the aortic valve shuts tightly after each heartbeat, keeping blood in the aorta so pressure stays up between beats. When the valve doesn’t close properly, blood leaks backward into the heart during the resting phase, and aortic pressure drops rapidly. The result is a high top number, a low bottom number, and a noticeably wide pulse pressure. Chronic severe aortic regurgitation can produce some of the widest pulse pressure gaps seen in clinical practice.
Medications That Lower Diastolic Pressure
Several classes of medication can push your bottom number down, sometimes more than intended. The effect is often most noticeable when you stand up, a phenomenon called orthostatic hypotension, where blood pools in your legs and your body can’t compensate fast enough.
- Diuretics (water pills) increase sodium loss and reduce blood volume, which lowers pressure across the board but can disproportionately affect diastolic readings, especially in older adults.
- Alpha-blockers, often prescribed for prostate enlargement or high blood pressure, directly reduce the resistance in blood vessel walls.
- Nitrates, used for chest pain, widen veins and reduce the amount of blood returning to the heart.
- Beta-blockers slow the heart rate and reduce the force of each beat, which can impair the body’s ability to maintain pressure when you change position.
- Tricyclic antidepressants cause low blood pressure in 10 to 50% of patients who take them, largely through their effect on blood vessel tone.
- Antipsychotic medications trigger orthostatic drops in up to 40% of users.
- Benzodiazepines (anti-anxiety drugs) can cause a significant blood pressure drop within seconds of standing.
If you suspect a medication is responsible, the fix often involves adjusting the dose or switching to an alternative. Don’t stop any prescription without talking to whoever prescribed it.
Other Common Causes
Dehydration is one of the simplest explanations. When blood volume drops because you aren’t drinking enough fluid, or you’re losing fluid through illness or heavy sweating, there’s less blood in the system to maintain pressure between heartbeats. Prolonged bed rest can have a similar effect because the cardiovascular system decondititions when you’re not upright regularly.
Thyroid disorders, adrenal insufficiency, and severe infections can also lower diastolic pressure, though these conditions typically affect both numbers and come with other obvious symptoms.
Symptoms of Low Diastolic Pressure
Many people with a mildly low bottom number feel perfectly fine and never need treatment. When diastolic pressure drops enough to reduce blood flow to organs, though, symptoms show up. The most common are dizziness or lightheadedness, especially when standing. Fatigue, weakness, and a sluggish or lethargic feeling are also typical. Some people experience blurred vision, nausea, confusion, or difficulty concentrating. In more pronounced cases, fainting can occur.
These symptoms tend to be worse in hot weather, after meals (when blood diverts to the digestive system), or after standing for long periods.
Why It Matters for Your Heart
Your heart muscle is unique in the body: it receives most of its own blood supply during diastole, the resting phase between beats. That means the bottom number on your blood pressure reading directly determines how well your heart feeds itself. When diastolic pressure drops below 60 mmHg, coronary blood flow can fall short, particularly if you already have any narrowing in your coronary arteries.
A study tracking this relationship found that people with diastolic pressure below 60 mmHg had a 69% higher risk of coronary heart disease events and a 48% higher risk of dying from any cause compared to those with diastolic readings in the 80 to 89 range. Crucially, this risk was concentrated in people who already had calcium buildup in their coronary arteries, a sign of existing atherosclerosis. People with clean coronary arteries and the same low diastolic readings did not face elevated risk. This suggests that a low bottom number is most dangerous when the heart’s blood supply is already compromised.
Effects on Brain Health
Chronically low diastolic pressure also appears to affect the brain over time. A long-running study of adults over age 75, the Bronx Aging Study, found that each 10 mmHg drop in diastolic pressure was associated with a 20% increase in dementia risk. People with persistently low diastolic pressure (at or below 70 mmHg) over a two-year period had more than double the risk of developing dementia compared to those with normal readings. The connection was strongest for Alzheimer’s disease specifically, not vascular dementia, suggesting that chronic low perfusion may contribute to the neurodegenerative process itself rather than just causing small strokes.
What You Can Do About It
Low diastolic pressure without symptoms rarely requires intervention. When symptoms are present, treatment depends entirely on the cause. If a medication is responsible, adjusting the prescription is the most direct fix. If arterial stiffness is the driver, the focus shifts to managing cardiovascular risk factors more broadly.
For day-to-day management, several practical strategies help. Drinking more water increases blood volume and supports pressure between heartbeats. Using a bit more salt in your diet, if your doctor agrees, can raise blood pressure modestly. Moving slowly when going from lying down to sitting to standing gives your body time to adjust. Compression stockings press on your lower legs and push blood back toward your heart, which can be especially helpful if you stand for long stretches. Cutting back on alcohol helps too, since even moderate drinking is dehydrating and lowers blood pressure.
Keeping track of your readings at home, noting both numbers and the time of day, gives you and your healthcare provider a much clearer picture than a single reading in a clinic. Patterns matter more than any single measurement.