Blood pressure measures the force blood exerts against artery walls. It is given as two numbers: the top number is the systolic pressure, and the bottom number is the diastolic pressure. Systolic pressure reflects the maximum force when the heart contracts and pushes blood out. Diastolic pressure represents the lowest pressure remaining in the arteries when the heart muscle is fully relaxed between beats. A low diastolic reading means the residual pressure during this resting phase is lower than expected, suggesting the circulatory system is not maintaining sufficient tension when the heart is not actively pumping.
Understanding the Physiological Importance of Diastolic Pressure
The diastolic number is significant because it is the main driving force for the heart’s own blood supply. The heart muscle, or myocardium, receives oxygen-rich blood almost entirely during the diastolic phase. When the heart contracts, the major coronary arteries that feed the muscle are compressed, limiting blood flow.
Diastolic pressure serves as the primary determinant of coronary perfusion pressure, the gradient that pushes blood into the heart tissue. A healthy diastolic reading is considered below 80 millimeters of mercury (mmHg). A reading below 60 mmHg is defined as low, often called isolated diastolic hypotension if the systolic number remains normal.
If this pressure drops too low, the heart muscle may not receive enough blood and oxygen to sustain its function. The pressure must be high enough to overcome the resistance within the heart’s own vessels to ensure proper tissue nourishment.
Primary Causes of Low Diastolic Readings
Low diastolic pressure often results from decreased resistance in the peripheral blood vessels, known as poor vascular tone. As people age, large arteries naturally stiffen and lose elasticity. This causes the systolic pressure to rise while allowing the diastolic pressure to fall because the arteries cannot properly recoil and maintain pressure during the heart’s resting period.
Certain medical treatments frequently cause reduced diastolic readings, particularly medications used to manage high blood pressure. Vasodilators, alpha-blockers, and diuretics work by relaxing blood vessel walls or reducing fluid volume, which can sometimes lower the diastolic number too aggressively.
Underlying heart conditions can also lead to low diastolic pressure. Aortic regurgitation, where the aortic valve does not close completely, causes blood to leak backward into the heart during diastole. This backflow rapidly reduces the pressure in the arteries, resulting in a low diastolic reading.
Systemic issues affecting overall blood volume or vessel tone are another contributing factor. Severe dehydration reduces the total volume of blood circulating, directly lowering both systolic and diastolic pressures. Other conditions like severe infection (sepsis), which causes widespread dilation of blood vessels, or chronic anemia can also lead to sustained low diastolic readings.
Potential Health Risks Associated with Very Low Diastolic Pressure
A sustained low diastolic pressure, particularly below 60 mmHg, carries health risks due to its impact on coronary perfusion. The most immediate concern is inadequate oxygen supply to the heart muscle, leading to myocardial ischemia. If the pressure gradient driving blood into the heart tissue is too low, the muscle is starved of oxygen and can weaken over time, potentially leading to heart failure.
When diastolic pressure is low while systolic pressure remains normal or high, it creates a wide gap known as a widened pulse pressure. This wide pulse pressure is recognized as a significant independent risk factor for cardiovascular events. It reflects the stiffness of the major arteries and resulting inefficient blood flow dynamics.
Low diastolic pressure can manifest with noticeable symptoms related to poor blood flow to the brain and other organs. Individuals may experience lightheadedness, dizziness, fatigue, or blurred vision. Fainting, medically termed syncope, is a possibility if the pressure is low enough to cause a temporary loss of consciousness.
An isolated diastolic pressure below 60 mmHg is associated with an increased risk of all-cause and cardiovascular death, especially in adults not taking antihypertensive medications. When these low readings occur alongside symptoms or are a persistent finding, they warrant a medical consultation to investigate the underlying cause and determine if medication adjustments or other treatments are necessary.