Sepsis is the body’s overwhelming and life-threatening response to an infection, where widespread inflammation can damage its own tissues and organs. This condition is a medical emergency that requires rapid diagnosis and treatment. Among the vital signs monitored, blood pressure is a particularly important indicator for assessing the severity of sepsis.
The Connection Between Sepsis and Blood Pressure
The significant drop in blood pressure seen in sepsis is a direct result of the body’s massive immune and inflammatory response to an infection. The immune system releases a flood of chemicals, such as cytokines, into the bloodstream. In sepsis, their overproduction leads to systemic, or body-wide, effects that compromise the circulatory system, affecting blood pressure in two primary ways.
A primary event is widespread vasodilation, where blood vessels throughout the body relax and widen. This expansion of the circulatory system’s capacity means the existing volume of blood is no longer sufficient to maintain normal pressure, much like how water pressure decreases if it flows through a wider hose. This process is driven by inflammatory mediators that cause the smooth muscles in vessel walls to relax.
Simultaneously, the inflammatory response increases the permeability of capillaries, the body’s smallest blood vessels. The junctions between the cells lining these vessels become leaky, allowing plasma to escape from the bloodstream into surrounding tissues. This fluid shift reduces the overall volume of blood circulating within the vessels, a condition known as hypovolemia.
Defining Hypotension in Sepsis
Low blood pressure, or hypotension, is a defining characteristic of worsening sepsis. Specific numerical values are used to diagnose this state. The most commonly referenced measurement is systolic blood pressure, the top number in a blood pressure reading. A systolic blood pressure below 90 mmHg or a drop of more than 40 mmHg from a person’s baseline reading is a common indicator of sepsis-induced hypotension.
While systolic pressure is a quick reference, a more comprehensive measure used in hospital settings is the Mean Arterial Pressure (MAP). MAP represents the average pressure in the arteries over one full heartbeat and is a more accurate indicator of how well blood is flowing through vital organs. It is calculated from both the systolic and diastolic (bottom number) pressures.
In the context of sepsis, a MAP that falls below 65 mmHg is a significant threshold. This value is used in clinical guidelines to identify when blood flow may be insufficient to supply oxygen to the kidneys, brain, and other organs. Maintaining a MAP at or above this level is a primary goal of initial sepsis treatment to prevent organ damage.
Progression to Septic Shock
Sepsis can advance to a more dangerous state known as septic shock. This condition is a subset of sepsis where the underlying circulatory and cellular problems become so severe that the body’s functions are impaired. The transition to septic shock is marked by persistent low blood pressure that does not respond to initial treatment.
The factor that distinguishes septic shock is hypotension that remains low even after a patient has received substantial intravenous (IV) fluids. While IV fluids are the first line of defense to increase blood volume, in septic shock, the vasodilation and leaky capillaries are so extreme that fluids alone cannot restore adequate blood pressure. At this stage, the body’s circulatory system is failing.
This persistent state of low blood pressure, specifically a MAP remaining below 65 mmHg despite fluid resuscitation, indicates the need for medications to support the cardiovascular system. The diagnosis of septic shock also includes evidence of tissue hypoperfusion, such as an elevated blood lactate level. This progression signifies a much higher risk of mortality and the need for more aggressive medical care.
Medical Interventions for Sepsis-Induced Hypotension
The primary goal of treating sepsis-induced hypotension is to rapidly restore blood volume and pressure to ensure vital organs receive adequate oxygen. The first step is aggressive fluid resuscitation. This involves administering large volumes of intravenous crystalloid solutions, such as normal saline, to quickly expand the amount of fluid within the circulatory system.
This rapid infusion of fluids aims to compensate for both the vasodilation and the fluid that has leaked out of the capillaries. By refilling the circulatory system, the heart has more volume to pump, which should increase blood pressure and improve organ perfusion. The patient’s response is monitored to see if blood pressure rises to a safer level, typically a MAP of 65 mmHg or higher.
If blood pressure remains low despite adequate fluid resuscitation, the patient is considered to be in septic shock, and the next step is the administration of vasopressor medications. These drugs work by constricting blood vessels, which increases vascular resistance and elevates blood pressure. The most commonly used first-line vasopressor is norepinephrine.
Norepinephrine is administered as a continuous intravenous infusion and is titrated, meaning the dose is carefully adjusted, to achieve the target MAP of at least 65 mmHg. By tightening the blood vessels, vasopressors counteract the systemic vasodilation caused by sepsis, helping to redirect blood flow to the core organs. This intervention stabilizes the patient’s circulation while doctors work to treat the underlying infection with antibiotics.