Blood pressure measures the force circulating blood exerts against the artery walls. It is a highly regulated function that maintains the necessary pressure to deliver oxygen and nutrients throughout the body. While lower pressure is often associated with better cardiovascular health, an abnormally low reading indicates a condition known as hypotension. This occurs when the force is insufficient to maintain adequate blood flow to the brain and other vital organs, risking symptoms and serious complications.
Understanding the Numerical Thresholds
Blood pressure is expressed using two numbers measured in millimeters of mercury (mm Hg). The higher number is the systolic pressure, reflecting the force in the arteries when the heart contracts. The lower number is the diastolic pressure, representing the force between heartbeats when the heart is at rest.
Hypotension is generally defined as a reading of less than 90/60 mm Hg. This means the systolic pressure is below 90, and the diastolic pressure is below 60. This numerical definition is a general guideline, not a rigid rule for every individual.
Many healthy, fit people naturally have readings below this level without negative effects. A diagnosis of problematic low blood pressure depends less on the absolute number and more on whether the reading causes noticeable signs or symptoms. A sudden drop of 20 mm Hg in systolic pressure, such as from 110 to 90 mm Hg, can be concerning because it may cause immediate symptoms like dizziness.
Recognizing the Signs and Symptoms
When blood pressure falls too low, symptoms are linked to insufficient blood flow reaching the brain. This lack of adequate circulation often causes lightheadedness or unsteadiness, which are the first noticeable signs. A severe reduction in blood supply can cause fainting, known medically as syncope, which is a temporary loss of consciousness.
Other common symptoms include generalized fatigue or weakness. Individuals may also experience blurred vision, nausea, or difficulty concentrating. These signs arise because the body’s cells are not receiving necessary oxygen and nutrients.
Headaches, neck pain, or an irregular heartbeat (palpitations) can also accompany a hypotensive episode. If the low pressure is sustained, these symptoms will persist until the pressure stabilizes. The onset of these signs suggests that the body’s perfusion is being compromised.
Factors That Lead to Low Pressure
A variety of underlying factors can cause acute or chronic low blood pressure. One common cause is dehydration, which reduces the total volume of circulating blood. Fluid loss can result from insufficient water intake, severe vomiting, diarrhea, or intense exercise without adequate rehydration.
Certain medications lower blood pressure as a side effect, including diuretics and other drugs used to treat hypertension. Medications for conditions like Parkinson’s disease, depression, or erectile dysfunction can also contribute to hypotension. These drugs interfere with the body’s natural mechanisms for regulating pressure.
Heart problems may lead to hypotension because the heart cannot pump enough blood to meet the body’s needs. Conditions such as a slow heart rate (bradycardia), heart attack, or heart valve disease reduce the heart’s output, directly lowering the pressure. Endocrine disorders, like an underactive thyroid or adrenal insufficiency (Addison’s disease), disrupt hormone production, which also decreases blood pressure.
Severe, life-threatening conditions cause a rapid drop in pressure. These include septic shock, where widespread infection causes blood vessels to dilate uncontrollably, and anaphylaxis, a severe allergic reaction. Significant blood loss from injury or internal bleeding reduces total volume, leading to a sudden, acute drop. Additionally, a lack of nutrients like vitamin B12 or folate can cause anemia, contributing to lower blood pressure.
Situational Types of Hypotension
Low blood pressure is not always constant but can be triggered by specific body movements or events. These temporary forms are known as situational or positional hypotension.
Orthostatic hypotension, also called postural hypotension, involves a sudden pressure drop when a person stands up quickly from sitting or lying down. This occurs because gravity causes blood to pool in the legs, and the nervous system cannot constrict blood vessels quickly enough to maintain flow to the brain. It is defined by a drop of at least 20 mm Hg systolic or 10 mm Hg diastolic pressure within three minutes of standing. Symptoms like lightheadedness usually resolve shortly after the person sits or lies down.
Postprandial hypotension occurs when pressure decreases significantly one to two hours after eating a meal. This is often seen in older adults or those with nervous system conditions, such as Parkinson’s disease. The digestive system requires a large volume of blood to process food, and the body fails to compensate by constricting blood vessels elsewhere. Eating smaller, low-carbohydrate meals may help mitigate this effect.
Neurally mediated hypotension (NMH) is a pressure drop that happens after standing for a long period. This type is more common in younger adults and children and involves a communication issue between the heart and the brain. Prolonged standing triggers an inappropriate reflex that causes both a drop in blood pressure and a slowing of the heart rate (bradycardia), leading to lightheadedness or fainting.
When Low Pressure Becomes an Emergency
When blood pressure drops severely and remains low, it can progress to shock, a life-threatening condition. Shock is characterized by the circulatory system failing to deliver enough oxygen and nutrients to the vital organs, causing them to begin shutting down. This requires immediate medical attention, as prompt intervention is necessary for survival.
Emergency signs of shock include a weak and rapid pulse, along with rapid, shallow breathing. The skin may become pale, cool, and clammy as the body diverts blood away from the extremities to protect the core organs. Confusion, unresponsiveness, or changes in mental status indicate the brain is not receiving sufficient blood flow.
Acute medical treatment focuses on immediately restoring adequate blood flow and pressure. This often involves administering intravenous (IV) fluids like saline or blood products to increase circulating volume. Medications called vasopressors may also be used to constrict blood vessels and raise the pressure to a target level, typically a mean arterial pressure greater than 65 mm Hg.