Blood pressure is measured using two numbers. The top number, systolic pressure, measures the force when the heart contracts and pushes blood out. The bottom number, diastolic pressure, reflects the pressure in the arteries when the heart is at rest between beats. This resting phase is important because the heart muscle receives its oxygenated blood supply during this time.
A diastolic reading consistently below 60 millimeters of mercury (mm Hg) is often considered low. If the systolic reading remains normal, this is known as isolated diastolic hypotension. Whether a low reading is problematic depends on the overall health context and the presence of symptoms.
Physical Signs of Low Diastolic Pressure
The symptoms associated with low diastolic pressure arise directly from insufficient blood flow, or perfusion, to the body’s tissues, particularly the brain, during the heart’s relaxation phase. A common and immediate physical sign is dizziness or lightheadedness, often noticed when a person changes position too quickly, such as standing up from sitting or lying down. This sensation, sometimes called orthostatic hypotension, occurs because the low diastolic pressure is not high enough to quickly deliver blood to the brain.
The lack of adequate blood flow often leads to a feeling of fatigue or generalized weakness. Low diastolic pressure can reduce the oxygen supply to the heart muscle, potentially leading to chest pain, or angina. Some people may experience blurred or fading vision, which indicates that the eyes and visual centers of the brain are temporarily lacking sufficient blood and oxygen.
When the blood pressure drop is more significant, individuals may feel nauseous or experience fainting (syncope). Fainting is a protective mechanism that restores blood flow to the brain by bringing the head to the same level as the heart. Confusion or an inability to concentrate can also signal diminished cerebral blood flow, particularly in older individuals.
Underlying Factors that Lower Diastolic Pressure
A number of physiological or medical conditions can lead to a disproportionately low diastolic pressure, often by affecting the resistance within the blood vessels or the overall blood volume. Arteries naturally lose elasticity with age, a process that can cause the systolic pressure to rise while the diastolic pressure falls, leading to isolated diastolic hypotension in older adults. Certain heart valve problems, such as aortic regurgitation, can allow blood to flow backward into the heart after it has been pumped out, causing a rapid drop in the pressure between heartbeats.
Dehydration is a common cause, as a reduction in total blood volume lessens the pressure exerted on the artery walls during the resting phase of the heart. This can result from insufficient fluid intake, excessive sweating, or the use of diuretics. Medications are another significant factor, particularly vasodilators like alpha-blockers, which work by widening blood vessels and thus decrease the systemic vascular resistance that maintains diastolic pressure.
Systemic issues like severe infections, known as sepsis, can trigger a widespread inflammatory response that leads to excessive blood vessel dilation, drastically lowering diastolic pressure. Endocrine disorders, such as adrenal insufficiency, can also interfere with hormones that regulate blood pressure, contributing to a lower reading. Pregnancy commonly causes a temporary drop in diastolic pressure, especially during the first and second trimesters, due to hormonal changes and increased blood vessel capacity.
Knowing When to Contact a Doctor
If a low diastolic reading is discovered without symptoms, routine monitoring with a primary care provider is usually sufficient. However, if low diastolic pressure is accompanied by persistent or worsening symptoms, medical evaluation is necessary to identify the root cause. Contact a doctor if symptoms like frequent dizziness, recurring fainting episodes, or consistent fatigue begin to interfere with daily life.
Immediate medical attention is required if symptoms are severe or suggest a state of shock. Signs of an emergency include:
- Sudden confusion
- Rapid and shallow breathing
- A weak or irregular pulse
- Cold, pale, and clammy skin
Individuals taking blood pressure medication or who have existing heart conditions should contact their physician promptly if they notice a significant drop in their diastolic number or develop new symptoms. Keeping a detailed record of home blood pressure readings and associated symptoms provides valuable information.