Blood pressure (BP) is the force exerted by circulating blood against the walls of the body’s arteries. When this pressure becomes too high, it is known as hypertension, which is the leading preventable cause of stroke worldwide. A stroke occurs when blood flow to a part of the brain is interrupted, causing brain cells to die. Determining the exact blood pressure reading that causes a stroke is complex because risk is a function of both a sudden, extreme spike and the cumulative damage from years of unmanaged high pressure.
Defining the Acute Threshold for Stroke Risk
A sudden, severe rise in blood pressure is known as a hypertensive crisis, which is medically defined as a systolic pressure of $180$ millimeters of mercury (mmHg) or higher, or a diastolic pressure of $120$ mmHg or higher. This numerical threshold alone represents a serious health situation that requires immediate medical evaluation. A hypertensive crisis is classified into two types: urgency and emergency.
A hypertensive urgency involves these high numbers without evidence of new or worsening organ damage. A hypertensive emergency is defined by the same high blood pressure readings plus signs of acute organ damage, which includes an acute stroke, heart attack, or kidney failure. In this context, the acutely high pressure is a direct cause of the resulting brain injury.
If a person’s blood pressure is measured at or above $180/120$ mmHg and they are experiencing stroke symptoms, they are in a life-threatening hypertensive emergency. Immediate medical intervention is necessary to carefully lower the pressure and limit the extent of brain injury. Paradoxically, in an acute ischemic stroke, a very high blood pressure of up to $220/120$ mmHg may be initially tolerated or even protective, as it helps push blood past a blockage to the deprived brain tissue.
How Chronic Hypertension Sets the Stage
While acute spikes can trigger a stroke, the majority of strokes are rooted in the long-term, structural damage caused by sustained, uncontrolled hypertension. Chronic high pressure puts constant strain on the inner lining of blood vessels, known as the endothelium. This persistent stress leads to endothelial dysfunction, which is the first step in vascular pathology.
The continuous force promotes vascular remodeling, where the muscular wall of the arteries thickens and stiffens, narrowing the vessel’s internal diameter. This stiffening increases resistance to blood flow and makes the arteries less flexible. This chronic injury also accelerates atherosclerosis, causing fatty plaques to build up within the artery walls.
In the brain’s smaller vessels, chronic hypertension causes a specific type of damage called lipohyalinosis, where the vessel walls become hardened and weakened. This weakening can lead to the formation of small outpouchings called microaneurysms. These damaged, fragile vessels are then primed to either rupture or become easily blocked, even if the pressure is only moderately elevated.
Blood Pressure’s Role in Ischemic Versus Hemorrhagic Stroke
The type of stroke that occurs is directly related to the underlying vascular damage caused by different blood pressure patterns. Ischemic strokes, which account for about $87\%$ of all cases, occur when a blood vessel supplying the brain becomes blocked by a clot. This type of stroke is primarily linked to chronic hypertension, as the sustained high pressure drives the development of atherosclerosis.
The plaque buildup narrows the arteries, and a clot can form directly on the roughened surface of the plaque or travel from elsewhere in the body to lodge in the narrowed cerebral vessels. Therefore, a person with a history of chronic hypertension is at a substantially elevated risk for an ischemic stroke, even if their blood pressure at the moment of the event is not in the crisis range.
Hemorrhagic strokes, which involve bleeding into the brain, are more closely associated with extreme, acute spikes in blood pressure. This sudden surge of pressure can overwhelm the integrity of an already weakened cerebral artery, causing it to burst. Uncontrolled high blood pressure is considered the most common cause of hemorrhagic stroke, especially when vessels are already damaged by chronic hypertension.
When to Seek Emergency Medical Attention
Recognizing the symptoms of a stroke is more important than knowing the exact blood pressure number, as brain tissue loss occurs rapidly. A simple way to remember the signs is the acronym F.A.S.T.: Face drooping, Arm weakness, Speech difficulty, and Time to call $911$. Any one of these symptoms warrants an immediate call for emergency medical services.
If you are monitoring your blood pressure at home and it reads $180/120$ mmHg or higher, seek emergency medical attention if you are simultaneously experiencing symptoms like a severe headache, vision changes, chest pain, or shortness of breath. These symptoms suggest the high pressure is actively causing organ damage, constituting a hypertensive emergency. If the reading is high but there are no symptoms, contact a healthcare professional immediately for guidance.