Systemic vascular resistance (SVR) represents the opposition to blood flow throughout the body’s circulatory system, excluding the lungs. Maintaining an appropriate SVR is significant for regulating blood pressure and ensuring that organs receive sufficient blood supply. When SVR drops to abnormally low levels, it can substantially affect the body’s ability to circulate blood effectively.
Understanding Systemic Vascular Resistance
Systemic vascular resistance is determined by several factors, including the length and diameter of blood vessels, and the thickness, or viscosity, of the blood itself. Blood vessels, particularly the arterioles, can constrict (vasoconstriction) to narrow their diameter, which increases SVR. Conversely, they can relax (vasodilation) to widen, decreasing SVR. This dynamic adjustment of vessel diameter is a primary way the body regulates SVR and blood pressure.
Blood pressure is a product of cardiac output, the amount of blood pumped by the heart per minute, and SVR. If SVR decreases, blood pressure will fall unless there is a compensatory increase in cardiac output. This balance is regulated by various mechanisms, including neurohumoral influences and local tissue factors that affect vessel tone.
Common Causes of Low SVR
Several conditions and factors can lead to a reduction in systemic vascular resistance. Sepsis, a severe response to infection, is a common cause, where an overactive immune response releases inflammatory substances that cause widespread vasodilation and fluid leakage from capillaries. This systemic vasodilation significantly lowers SVR, contributing to a drop in blood pressure.
Anaphylaxis, a severe allergic reaction, also triggers a rapid and widespread release of histamine and other mediators, leading to substantial vasodilation and reduced SVR. Neurogenic shock, resulting from nervous system damage like a spinal cord injury, disrupts the sympathetic nervous system’s ability to constrict blood vessels, causing them to relax and SVR to decrease. Certain medications that promote vasodilation can also lower SVR. Liver failure can also present with low SVR due to excessive production of vasodilators and altered sensitivity to vasoconstrictors.
Recognizing the Impact of Low SVR
When systemic vascular resistance falls significantly, the primary physiological consequence is a decrease in mean arterial pressure, which can lead to inadequate blood flow to vital organs, a condition known as hypoperfusion. This means organs like the brain, heart, and kidneys may not receive enough oxygen and nutrients to function properly. The body attempts to compensate for this drop in blood pressure, often by increasing heart rate to maintain cardiac output.
Individuals experiencing low SVR may exhibit several observable signs and symptoms. These can include dizziness or lightheadedness due to reduced blood flow to the brain, and a rapid heart rate as the heart works harder to circulate blood. Other signs might include confusion, cool and clammy skin, or decreased urine output, all indicative of poor organ perfusion. A systolic blood pressure below 90 mm Hg is a common indicator of concern. The presence of these symptoms suggests a medical situation requiring prompt attention.
Approaches to Managing Low SVR
Managing low systemic vascular resistance involves strategies aimed at restoring adequate blood pressure and ensuring sufficient blood flow to organs. One immediate approach includes fluid administration to increase circulating blood volume and improve cardiac output, which helps raise blood pressure. This is relevant in situations with fluid leakage from vessels, as seen in some types of shock.
Medications known as vasopressors are frequently used to address low SVR. These drugs work by causing blood vessels to constrict, thereby increasing SVR and elevating blood pressure. Norepinephrine is often a first-line vasopressor in conditions like septic shock, while other agents like epinephrine or vasopressin may also be used. Beyond immediate interventions, identifying and treating the underlying cause of low SVR is important for long-term recovery. Medical management of low SVR requires professional assessment and care.