Low diastolic blood pressure: what a low bottom number means

What the bottom number actually measures

Systolic pressure is the pressure in the arteries while the heart contracts and pushes blood out. Diastolic is the pressure during the heart’s resting phase between beats. Harvard Health describes a healthy diastolic pressure as critical for the coronary vessels to supply the heart muscle with enough oxygen.

That last line is the one to hold onto, because it’s what explains why the bottom number matters at all. Dr Jason Guichard, a cardiologist at the University of Alabama at Birmingham, sets out the mechanism in an interview with UAB News: most of your arteries feed your organs during systole, but the coronary arteries sit around the aortic valve and get blood only when that valve closes, which happens in diastole. A low diastolic pressure means a low coronary artery pressure, which means the heart muscle is doing its work on less blood and oxygen than it would like.

Why it falls while the top number climbs

Here’s the pattern that brings most people to this question. The top number has been creeping up for years, and the bottom number has quietly been heading the other way.

Harvard Health explains it through the arteries themselves. Systolic pressure tends to increase with age as blood vessels weaken and narrow, while diastolic pressure often declines after age 50, because arteries become less elastic. An artery that’s gone stiff expands with the systolic push but doesn’t spring back as easily between heartbeats, so the pressure in the gap between beats drops. Harvard also names endothelial dysfunction, where the coronary arteries on the heart’s surface constrict instead of opening, as another possible cause.

Guichard describes the same change from the clinic side: as you get older your vessels become a little more stiff, and that tends to push the systolic number up and the diastolic number down.

If the top number is the one out of range while the bottom one stays normal, there’s a name for that too: isolated systolic hypertension. Harvard describes it as a common type of high blood pressure in older adults, happening because less elastic arteries have trouble accommodating surges of blood.

When it is worth a conversation

Most of the time the honest answer is that a low bottom number gets kept an eye on, and not much else. The BHF says most people with low blood pressure have no noticeable symptoms at all.

Symptoms are what change the picture. The NHS says to get your blood pressure checked if you keep getting lightheadedness or dizziness, feeling sick, blurred vision, generally feeling weak, confusion or fainting, and to see a GP if symptoms such as dizziness and fainting keep coming back. The BHF’s version of the list adds feeling tired. If the symptoms arrive when you stand up or change position suddenly, the NHS says that may be postural hypotension, a type of low blood pressure with its own name and its own pathway.

The research is where that “below 60” figure comes from. A 2011 paper in the journal Hypertension by Guichard and Ali Ahmed, described in the same UAB News interview, coined the term “isolated diastolic hypotension” for a diastolic below 60mmHg sitting under a systolic that is still normal, which they put at above 100mmHg. Older adults who fitted that description, they found, were at increased risk of developing new-onset heart failure.

A 2023 cohort study in the Journal of Clinical Hypertension followed 25,858 US adults whose blood pressure was already inside the recommended range, which the study defined as under 130mmHg systolic and under 80mmHg diastolic. Within that group, a diastolic below 60mmHg was associated with a higher risk of death from any cause than a diastolic between 70 and 80mmHg, with a hazard ratio of 1.30 (95% confidence interval 1.12 to 1.51). It helps to know who that figure doesn’t describe. If your top number is 150, you wouldn’t have been in that cohort at all. It describes people whose systolic was already fine.

There’s a second half to that study, and it’s the half that matters if you take blood pressure medicines. When the researchers split the group by medication, a diastolic below 60 in people not taking blood pressure medicines carried the raised risk, at a hazard ratio of 1.46 (95% confidence interval 1.21 to 1.75). In people whose diastolic sat below 60 while on blood pressure medicines, it did not, at a hazard ratio of 0.99 (95% confidence interval 0.73 to 1.36). Their reading of it was that the risk was already there, and bringing the diastolic down a little further with medication didn’t add to it.

Guichard’s own emphasis is the same one. His point about watching the bottom number is that nobody should be driving their numbers down so far they can’t play with their grandchildren or walk round the block. He calls anything between 60 and 90 good in an older adult. Below 60 is where people start to feel uncomfortable, and that’s where he thinks a closer look belongs.

What to log before you see your GP

Guichard’s answer, for people whose medicines can’t be changed, is closer monitoring: more frequent appointments, and an eye on any cardiovascular or heart failure symptoms. That’s a job a home monitor happens to be good at, and it’s the part you can arrive with already done.

• Both numbers, every time. A diastolic on its own tells a GP far less than the pair does.
• Same time of day, same technique. The NHS notes blood pressure varies depending on the time of day, so two readings only compare if they were taken the same way. We’ve set the method out in how to measure blood pressure correctly at home.
• Any symptom, next to the reading it belongs to. Dizzy standing up, tired by mid-afternoon, a near-faint in the kitchen.
• Every medicine you take, including anything you buy over the counter.

Then leave it alone for a few weeks. One low morning is noise. Six to eight weeks of readings taken the same way is what turns a handful of numbers into a trend, and a trend is the thing a GP can work with. If a check has already suggested your blood pressure might be high, the NHS says you may be asked to use a monitor at home over the next few days, to come back for another check, or to wear a portable monitor that takes readings across 24 hours. What happens during a 24-hour blood pressure monitor on the NHS covers that appointment.

Separately from all of this, the NHS says to get help from 111 if you often have headaches or blurred vision, or chest pain that comes and goes. It says to call 999 for sudden chest pain or discomfort that does not go away, for pain spreading to your arm, neck, jaw, stomach or back, or for chest pain alongside sweating, feeling sick, lightheadedness or breathlessness. Those aren’t bottom-number questions.

The bottom line

A bottom number drifting down while the top one climbs is usually the arteries getting stiffer, and it’s the most overlooked thing on a home monitor’s display. It isn’t a number to panic about, and it isn’t a number to ignore either. Write down both figures at the same time each day, note any dizziness or tiredness beside them, keep going for six to eight weeks, and take the trend to your GP. That’s the version of this a GP can do something with. Changing a dose is their decision. Having the readings ready is yours.

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