Blood pressure: why a reading of 120/40 mmHg can also be a concern

What exactly is blood pressure?

Blood pressure is the force that blood exerts on the walls of the arteries during circulation.

It is expressed by two numbers, usually in millimeters of mercury (mmHg).

For example: 120/80 mmHg.

The first number corresponds to systolic pressure and the second to diastolic pressure.

Both reflect different moments of the cardiac cycle and provide complementary information about the functioning of the heart and arteries.

What is systolic pressure, or the 'upper' number?

Systolic pressure is the maximum blood pressure reached during the contraction of the ventricles, particularly when the left ventricle pumps blood into the aorta.

During this phase, known as systole, the heart drives blood into the systemic circulation.

Its value depends on multiple factors, including the volume of blood ejected, the ejection velocity, and the elastic properties of the large arteries.

When we talk about a blood pressure reading of '16', we are generally referring to a systolic pressure of 160 mmHg.

A persistently elevated systolic pressure increases the workload of the heart and can, over time, promote cardiovascular, cerebral, and renal damage.

What is diastolic pressure, or the 'minimum' reading?

Diastolic pressure is the minimum arterial pressure recorded during the relaxation phase of the heart, before the next heartbeat.

During diastole, the left ventricle relaxes and fills with blood in preparation for the next contraction.

But this does not mean that arterial circulation comes to a stop.

The large arteries, especially the aorta, store part of the energy from systolic ejection through elastic distension. During diastole, their elastic recoil helps maintain blood flow.

That is why diastolic pressure reflects, among other factors, peripheral vascular resistance, heart rate, and the elastic behaviour of the arterial system.

And there is a fundamental physiological aspect: a large part of the perfusion of the left cardiac muscle occurs during diastole.

Why does the heart receive a large part of its blood supply during diastole?

The heart is a muscular pump that needs to continuously receive oxygen and nutrients.

This supply arrives through the coronary arteries.

During systole, when the left ventricle contracts, the pressure within the cardiac muscle partially compresses the small vessels that run through it.

This compression is especially significant in the inner layers of the myocardium, closest to the ventricular cavity.

During diastole, the muscle relaxes, the compression of the intramyocardial vessels decreases, and perfusion improves.

That is why, unlike what many people imagine, the relaxation of the heart is essential for its own blood supply.

This phenomenon is particularly important in the left ventricle.

What happens when diastolic pressure is too low?

A very low diastolic pressure can reduce the pressure available to drive blood through the coronary circulation.

But the phenomenon is more complex than simply looking at the number on the blood pressure monitor.

In simplified terms, one of the main determinants of the coronary perfusion gradient of the left ventricle is the difference between aortic diastolic pressure and the pressure present within the left ventricle at the end of diastole.

We can express this in approximate terms as follows:

Coronary perfusion gradient ≈ aortic diastolic pressure − left ventricular end-diastolic pressure.

This is not a formula that can diagnose perfusion insufficiency on its own, but it explains an important physiological mechanism.

Let us imagine an aortic diastolic pressure of 40 mmHg.

If there is also an elevated pressure within the left ventricle, the gradient available to perfuse certain regions of the myocardium may decrease.

This situation becomes particularly relevant in patients with coronary artery disease, left ventricular hypertrophy, or certain cardiac conditions.

Diastolic pressure is not simply the second number on the blood pressure monitor: it plays a role in the conditions that allow the heart to receive blood during its relaxation phase.

Is a blood pressure of 120/40 mmHg necessarily dangerous?

Not necessarily, but it is a reading that warrants attention, especially if confirmed.

A blood pressure of 120/40 mmHg means that the systolic pressure is 120 mmHg and the diastolic pressure is 40 mmHg.

We cannot state that every person with that reading is experiencing cardiac ischemia.

We must assess age, symptoms, cardiovascular history, medication, heart rate, and repeated measurements.

In a young, healthy person with no symptoms, a relatively low diastolic pressure may carry a different interpretation than it would in a 78-year-old with coronary artery disease and antihypertensive treatment.

However, a diastolic pressure persistently close to 40 mmHg should not automatically be considered normal simply because the systolic reading is 120.

It is important to confirm the reading and investigate its clinical context.

What about a blood pressure of 120/50 mmHg?

A blood pressure of 120/50 mmHg also presents a considerable difference between the systolic and diastolic figures.

Its interpretation depends on the clinical situation.

It can be observed without significant symptoms, but it can also appear in people with arterial stiffness, certain valvular disorders, or under the effect of medications.

The key is not to establish a rigid rule whereby any pressure of 50 mmHg is dangerous.

The key is to understand that diastolic pressure deserves its own assessment, especially when it remains very low, is accompanied by symptoms, or occurs in people with cardiovascular disease.

What is pulse pressure?

Pulse pressure, also called differential pressure, is the difference between systolic and diastolic pressure.

It is calculated in a simple way:

Pulse pressure = systolic pressure − diastolic pressure.

For example, in a person with a blood pressure of 120/80 mmHg: 120 − 80 = 40 mmHg.

In another person with a blood pressure of 120/40 mmHg: 120 − 40 = 80 mmHg.

Both have the same systolic pressure, but present a very different pulse pressure.

This difference can provide information about the mechanical properties of the arteries and certain cardiovascular conditions.

What does it mean to have a very wide pulse pressure?

A wide pulse pressure can appear when there is a large difference between systolic and diastolic pressure.

In older people, one of the frequent causes is the loss of elasticity of the large arteries.

When the aorta is less distensible, systolic pressure may rise while diastolic pressure remains low or decreases.

This can produce readings such as 150/60 mmHg, with a pulse pressure of 90 mmHg.

But a wide pulse pressure does not always mean the same thing.

It can also be observed in certain valvular diseases, hyperdynamic circulation states, and other clinical situations.

It must therefore be interpreted together with age, physical examination, and the rest of the cardiovascular data.

What is the relationship between aging and diastolic pressure?

Over the years, the large arteries can progressively lose some of their elasticity.

The aorta contains elastin and other structural components that allow it to expand during systole and return to its shape during diastole.

Vascular aging may be accompanied by changes in elastin, collagen, and the structure of the arterial wall.

When the aorta becomes stiffer, its ability to buffer the pressure variations produced by each heartbeat is reduced.

The result can be a higher systolic pressure and a relatively low diastolic pressure.

This combination is especially common in older individuals with isolated systolic hypertension.

However, aging does not explain all cases of low diastolic pressure and should not be used to rule out other conditions.

What is isolated systolic hypertension?

Isolated systolic hypertension refers to the situation in which systolic pressure is elevated while diastolic pressure does not reach the hypertension threshold.

For example: 160/65 mmHg.

Systolic pressure is clearly elevated. Diastolic pressure is not.

This does not mean that the patient has a less significant form of hypertension.

Isolated systolic hypertension is clinically relevant and common in older age.

Furthermore, it poses a therapeutic challenge: when we lower systolic pressure with medications, we may also lower diastolic pressure.

This is why treatment must be individualized, particularly in people with coronary artery disease, frailty, or symptoms of hypotension.

What is aortic regurgitation, and why can it cause a low diastolic pressure?

Aortic regurgitation is a disease of the aortic valve in which the valve does not close properly.

Under normal conditions, after the left ventricle ejects blood into the aorta, the valve closes to prevent blood from flowing back into the heart.

When aortic regurgitation is present, a portion of the blood can return from the aorta into the left ventricle during diastole.

This can contribute to a reduction in aortic diastolic pressure.

In significant cases, a very wide pulse pressure may develop.

Clinical examination can detect certain findings, such as a diastolic murmur, but the key study for confirming and quantifying the abnormality is usually a Doppler echocardiogram.

A blood pressure of 120/40 mmHg does not by itself diagnose aortic regurgitation. However, if it is accompanied by a murmur, dyspnea, palpitations, or other findings, this possibility must be considered.

What other situations can cause a low diastolic pressure?

In addition to arterial stiffness and aortic regurgitation, there are other possible causes.

Peripheral vasodilation can reduce vascular resistance and contribute to a drop in diastolic pressure.

This can occur in certain physiological, pharmacological, or pathological circumstances.

Some situations of hyperdynamic circulation, such as hyperthyroidism or significant anemia, can be associated with a wide pulse pressure.

Dehydration and loss of circulating volume can cause low blood pressure, although they frequently affect both systolic and diastolic pressure.

Medications should also be considered, especially when there have been recent dose changes or combinations of antihypertensive agents.

Therefore, a low diastolic pressure is a finding we need to explain, not a diagnosis in itself.

Can medications for hypertension lower the minimum pressure too much?

Yes, in certain individuals.

Antihypertensive drugs reduce blood pressure through different mechanisms: vasodilation, modification of circulating volume, reduction of renin-angiotensin system activity, or changes in heart rate and cardiac function, among others.

When treating elevated systolic pressure, diastolic pressure may also decrease.

This does not mean that antihypertensive treatment is harmful.

On the contrary: adequately controlling hypertension significantly reduces cardiovascular risk.

But we must assess the individual response.

If a patient repeatedly presents with very low diastolic pressure readings, dizziness, falls, or symptoms consistent with low blood pressure, it may be necessary to review the treatment.

An antihypertensive medication should never be stopped or modified on one's own based on a single measurement.

What is the J-curve of blood pressure?

The so-called J-curve describes an association observed in some studies between blood pressure and cardiovascular risk.

When we plot risk against different blood pressure levels, we may find that risk increases with very high pressures, decreases in intermediate ranges, and rises again among some individuals with very low pressures.

This relationship has been studied particularly in patients with coronary artery disease.

The proposed physiological explanation is that an excessively low diastolic pressure could compromise coronary perfusion in certain patients.

However, there is a fundamental scientific distinction: association does not necessarily imply causality.

People with very low diastolic values may also be older, have arterial stiffness, frailty, or cardiovascular diseases that account for part of the observed risk.

The 2024 European guidelines specifically highlight that the observational J-curve does not in itself demonstrate that intensive blood pressure reduction is harmful.

Therefore, we should not use the J-curve as an argument for leaving significant hypertension untreated.

We should use it to understand the need for individualised clinical evaluation.

What do the European guidelines say about diastolic pressure?

The 2024 European Society of Cardiology (ESC) guidelines propose, for most adults receiving antihypertensive treatment, a blood pressure target of around 120–129/70–79 mmHg, provided it is well tolerated.

These guidelines place particular emphasis on the strength of the evidence regarding systolic pressure reduction.

They also acknowledge that the evidence on specific diastolic targets is less robust.

Furthermore, they warn that there are no robust data demonstrating the cardiovascular safety of achieving a diastolic pressure below 50 mmHg through treatment, and that there is a potential for harm.

This does not mean that 49 mmHg is a universal threshold of immediate danger.

It means that we must act with caution, assess symptoms and clinical context, and avoid automatically interpreting very low readings as a therapeutic success.

Is it true that the ideal blood pressure is always 120/80 mmHg?

Not exactly.

The figure 120/80 mmHg is a very well-known reference, but it does not represent a universal physiological obligation.

Needs and goals can vary depending on:

• Age and frailty.

• The presence of coronary artery disease or heart failure.

• Symptoms of hypotension.

• Cardiovascular and renal history.

• Tolerance to treatment.

• The overall clinical context.

A healthy person may present readings below 120/80 without any underlying disease.

And in certain frail patients, it may be necessary to individualize targets in order to avoid adverse effects.

Medicine is not about forcing all patients to reach exactly the same number.

Can a low diastolic pressure cause dizziness or fainting?

Yes, although symptoms depend on overall blood pressure, cardiac output, and the organs' capacity for autoregulation.

When circulatory pressure is insufficient to maintain adequate perfusion, the following may occur:

• Dizziness.

• Weakness.

• Blurred vision.

• Feeling of faintness.

• Syncope.

• Intolerance upon standing.

However, these symptoms are not specific to a low diastolic pressure.

They can also occur due to arrhythmias, anemia, dehydration, metabolic disorders, and other conditions.

That is why we must evaluate the patient as a whole, not just the number.

What is orthostatic hypotension?

Orthostatic hypotension is a drop in blood pressure that occurs when moving from a lying or seated position to standing.

It is classically defined as a decrease of at least 20 mmHg in systolic pressure or 10 mmHg in diastolic pressure within the first three minutes of standing up.

It can cause dizziness, instability, and falls, especially in older individuals.

Its diagnosis requires comparing measurements in different positions following an appropriate protocol.

It is important not to confuse a persistently low diastolic pressure with orthostatic hypotension: they can coexist, but they are distinct phenomena.

Does a low diastolic pressure also affect the brain and kidneys?

Cerebral and renal perfusion depends on multiple factors, including mean arterial pressure, cardiac output, and autoregulation mechanisms.

It does not depend exclusively on diastolic pressure.

The brain and kidneys have mechanisms that allow them to keep their blood flow relatively stable within certain pressure ranges.

But when circulatory pressure drops too much, especially suddenly, these mechanisms may prove insufficient.

That is why significant hypotension can be associated with neurological disturbances, syncope, or acute kidney injury.

It is important to distinguish this situation from an isolated diastolic pressure of 50 mmHg in a stable, symptom-free individual.

These are not clinically equivalent situations.

What is mean arterial pressure?

Mean arterial pressure, or MAP, is an estimate of the average pressure that drives blood through the circulation during the cardiac cycle.

Under normal conditions and with a normal heart rate, we can approximate it using:

MAP ≈ diastolic pressure + (systolic pressure − diastolic pressure) / 3.

For example, for 120/80 mmHg: MAP ≈ 80 + 40/3 = 93 mmHg, approximately.

For 120/40 mmHg: MAP ≈ 40 + 80/3 = 67 mmHg, approximately.

This shows that two people with the same systolic pressure can have different mean arterial pressures.

However, this formula is an approximation and loses accuracy under certain conditions, particularly when there are significant changes in heart rate or in the shape of the arterial waveform.

Furthermore, a given MAP value alone is not sufficient to conclude that coronary, cerebral, or renal perfusion is adequate.

What symptoms associated with low blood pressure require urgent attention?

An abnormal blood pressure accompanied by significant symptoms should be assessed promptly.

The following are particularly concerning:

• Chest pain.

• Difficulty breathing.

• Loss of consciousness.

• Acute confusion.

• Sudden neurological weakness.

• Signs of poor perfusion, such as cold skin, profuse sweating, or a general deterioration in condition.

In these situations, we should not limit ourselves to repeatedly taking measurements at home.

An urgent medical assessment is necessary.

A diastolic pressure of 40 mmHg without symptoms does not carry the same significance as that same reading accompanied by chest pain or syncope.

Can a blood pressure monitor give an inaccurate reading of diastolic pressure?

Yes.

Automatic blood pressure monitors estimate readings using algorithms that interpret the oscillations produced in the cuff.

A measurement may be inaccurate if:

• The cuff is the wrong size.

• The arm is positioned incorrectly.

• The person talks or moves.

• No rest was taken before measuring.

• An arrhythmia exists that makes estimation difficult.

• The device is not validated.

That is why an unexpected reading, such as 120/40 mmHg, must be confirmed with correct technique.

If it persists, it may be useful to compare it with a clinical measurement and, depending on the case, carry out ambulatory monitoring.

How should we correctly measure blood pressure at home?

Technique matters enormously.

It is recommended to use a validated automatic blood pressure monitor, preferably an upper-arm device, with a cuff suited to its circumference.

Before measuring:

1. Avoid exercise, tobacco and caffeine for approximately 30 minutes.

2. Empty the bladder if necessary.

3. Remain seated and calm for five minutes.

4. Keep your back supported and your feet flat on the floor, without crossing your legs.

5. Position the arm resting at heart level.

6. Do not talk during the measurement.

7. Take two separate readings approximately one minute apart and record the results.

To monitor progress, having several correctly taken measurements is more useful than interpreting a single figure in isolation.

What is ambulatory blood pressure monitoring (ABPM)?

Ambulatory blood pressure monitoring (ABPM) allows blood pressure to be recorded over an extended period, usually 24 hours.

The device takes automatic measurements during everyday activities and overnight rest.

It can be useful for:

• Confirming hypertension.

• Investigating discrepancies between home and clinic readings.

• Detecting hypotension related to specific times of day.

• Assessing the response to medications.

• Studying the nocturnal blood pressure pattern.

ABPM does not replace clinical evaluation, but it can provide very valuable information when readings are variable or difficult to interpret.

What tests may be necessary when diastolic pressure is persistently low?

It depends on the clinical findings.

First, we need to confirm that diastolic pressure is genuinely low and determine whether symptoms are present.

We can then review medical history, treatments, heart rate, hydration status, and cardiovascular examination.

Depending on the context, the following may be indicated:

Electrocardiogram, to study the heart rhythm and certain signs of heart disease.

Echocardiogram, especially when there is suspicion of valvular disease, ventricular hypertrophy, or structural abnormalities.

ABPM (Ambulatory Blood Pressure Monitoring), to study blood pressure throughout the day and night.

Blood work, when we suspect anemia, renal, thyroid, electrolyte, or other abnormalities.

Not all patients need all of these tests.

The clinical examination should guide their selection.

What happens if my reading is 160/50 mmHg?

This combination deserves special attention.

There is a clearly elevated systolic pressure and a relatively low diastolic pressure.

The pulse pressure is: 160 − 50 = 110 mmHg.

This wide difference may be related to arterial stiffness or other cardiovascular conditions.

Furthermore, the elevated systolic pressure still represents a risk factor that requires appropriate treatment.

It would not be correct to say: 'Since the lower number is low, we cannot treat the upper number.'

Nor would it be correct to reduce the pressure without considering symptoms, medical history, and tolerance.

The goal is to control hypertension safely, tailoring the strategy to each individual.

What happens if I have 100/50 mmHg?

The interpretation is different.

The systolic pressure is also relatively low.

In a young person, without symptoms and with usually similar readings, it may not represent a disease.

But if it appears suddenly or is accompanied by dizziness, weakness, or syncope, we need to investigate.

This shows that diastolic pressure should never be interpreted in isolation from systolic pressure and the patient's clinical status.

What if I have 120/40 mmHg but feel perfectly fine?

The first thing is to confirm that the measurement is correct.

If it is an isolated reading and the patient is asymptomatic, it does not automatically mean that there is an emergency.

But if it recurs, especially in an older person or someone with a cardiovascular history, it is reasonable to arrange a medical assessment.

It may be necessary to evaluate pulse pressure, valvular function, treatments, and other factors.

The absence of symptoms is reassuring, but it does not completely replace a medical assessment when a persistent abnormality is present.

Why is it important not to treat only the numbers?

Because blood pressure does not exist separately from the patient.

A reading of 120/50 mmHg can have very different meanings in:

• A young, athletic person.

• A pregnant woman.

• An 80-year-old patient with arterial stiffness.

• A person with aortic insufficiency.

• A coronary patient taking several antihypertensive medications.

• A person with dehydration or acute illness.

Interpretation depends on physiology, the underlying condition, and how things evolve over time.

The same number does not necessarily imply the same risk.

What should we remember about diastolic pressure?

For years, everyday conversations about hypertension have focused mainly on the upper pressure reading.

And understandably so: systolic hypertension plays an enormous role in cardiovascular prevention.

But diastolic pressure also contains relevant information.

It helps us understand the dynamics of arterial circulation, plays a part in the conditions of coronary perfusion, and, together with systolic pressure, allows us to calculate pulse pressure.

A very low diastolic pressure may simply be an individual characteristic with no significant consequences, but it can also appear in association with arterial stiffness, valvular disease, pharmacological effects, or certain hemodynamic disturbances.

That is why, when a blood pressure monitor reads 120/40 mmHg, we should not simply say: 'The upper number is perfect.'

We should ask ourselves: Why is the lower number so low? Is this a real measurement? Does it repeat? Does the person have symptoms? What is this person's medical history?

Because in cardiovascular medicine, the goal is not to achieve the lowest number possible.

It consists of maintaining a blood pressure level that reduces cardiovascular risk and is appropriate and well tolerated for each patient.

Blood pressure is not interpreted by looking at the systolic number alone. It is interpreted by understanding both figures and, above all, the person in front of us.