Systemic vascular resistance (SVR) represents the opposition blood encounters as it flows through the body’s vast network of blood vessels. When blood vessels widen or relax excessively, this resistance decreases, making it easier for blood to flow. SVR is a primary factor in determining overall blood pressure and plays a role in circulatory system health.
How Blood Pressure is Regulated
Blood pressure, the force of blood against artery walls, is primarily determined by two main factors: cardiac output and systemic vascular resistance. Cardiac output refers to the volume of blood the heart pumps per minute, while SVR reflects the degree of constriction or relaxation in the blood vessels. These two components work in tandem, with changes in one often affecting the other to maintain stable blood pressure.
A balance between cardiac output and SVR is necessary for maintaining healthy blood pressure. The body uses mechanisms to regulate blood vessel tone, influencing SVR. The autonomic nervous system, particularly its sympathetic branch, constantly adjusts the diameter of arterioles to control blood flow and resistance. Hormones, such as angiotensin II and norepinephrine, also constrict vessels, while substances like nitric oxide promote relaxation.
Causes of Decreased Systemic Vascular Resistance
A decrease in systemic vascular resistance can stem from various medical conditions.
Sepsis
Severe infections, particularly sepsis, are a common cause. The body’s inflammatory response triggers widespread vasodilation. This inflammation releases chemical mediators like nitric oxide and prostaglandins, which relax smooth muscle in blood vessel walls. Blood vessels expand, reducing resistance to blood flow.
Anaphylaxis
Anaphylaxis, a severe allergic reaction, also causes a rapid, widespread decrease in SVR. The immune system releases chemicals such as histamine and leukotrienes. These substances cause extensive vasodilation and increased vascular permeability, leading to a sudden drop in blood pressure.
Medications
Certain medications can intentionally or unintentionally lower SVR. Vasodilator drugs, prescribed to manage hypertension, directly relax blood vessel walls. Some anesthetic agents used during surgery can also induce widespread vasodilation.
Liver Disease
Advanced liver disease, such as cirrhosis, can lead to chronic vasodilation and decreased SVR. The liver’s impaired function results in the accumulation of vasodilating substances in the bloodstream, including nitric oxide and carbon monoxide. This leads to widening of blood vessels, particularly in the splanchnic circulation.
Adrenal Insufficiency
Adrenal insufficiency, where adrenal glands produce insufficient cortisol and aldosterone, can impair the body’s ability to maintain vascular tone. Cortisol deficiency can lead to reduced SVR. Aldosterone deficiency can contribute to hypovolemia and low blood pressure.
Neurogenic Shock
Neurogenic shock, often from spinal cord injuries, disrupts the sympathetic nervous system’s control over blood vessel constriction. This prevents signals that normally keep blood vessels partially constricted. Without this sympathetic tone, blood vessels below the injury dilate uncontrollably, leading to a widespread decrease in SVR. This loss of sympathetic input results in pooling of blood in the periphery, reducing venous return to the heart.
Effects on the Body
The immediate consequence of decreased systemic vascular resistance is hypotension, or low blood pressure. When blood vessels are wide, pressure within them drops. This reduction in pressure can be sudden and severe.
Low blood pressure due to decreased SVR can lead to insufficient blood flow, or reduced organ perfusion, to organs such as the brain, kidneys, and heart. When these organs do not receive adequate oxygen and nutrients, their function can be impaired. Prolonged or severe hypoperfusion can result in organ damage or failure. For instance, reduced blood flow to the brain can cause altered mental status or loss of consciousness.
Individuals with decreased SVR exhibit symptoms including dizziness, lightheadedness, and fainting. Confusion or disorientation may also occur as brain perfusion diminishes. The body often attempts to compensate by increasing heart rate (tachycardia) to pump more blood, but this may not be sufficient to restore adequate blood pressure. The skin might appear cool, clammy, or pale as blood is shunted away from the periphery to maintain flow to core organs.
Addressing Decreased Systemic Vascular Resistance
Managing decreased systemic vascular resistance involves identifying and treating the underlying cause of widespread vasodilation. For instance, in cases of sepsis, prompt administration of antibiotics is crucial to eliminate the infection.
Approaches to support blood pressure and organ perfusion include fluid administration. Intravenous fluids increase circulating blood volume, which helps raise blood pressure and improve blood flow to tissues. This strategy aims to compensate for the expanded vascular space caused by vasodilation.
In severe instances where fluid administration alone is insufficient, medications called vasopressors may be used. These drugs, such as norepinephrine or dopamine, work by causing blood vessels to constrict, thereby directly increasing SVR and raising blood pressure. They are typically reserved for situations of profound hypotension where immediate vascular tone support is necessary. The goal is to restore adequate blood flow to all organs.