Anaphylaxis is a severe, rapidly progressing allergic reaction that can be life-threatening. It occurs quickly after allergen exposure. This widespread immune response can cause a sudden and significant drop in blood pressure, a major factor in its severity.
How Anaphylaxis Lowers Blood Pressure
When the body encounters an allergen during anaphylaxis, immune cells such as mast cells and basophils rapidly release potent chemical mediators, including histamine and platelet-activating factor (PAF). Histamine, for instance, is known to cause a fall in blood pressure.
These mediators trigger two primary effects that lead to a significant drop in blood pressure. First, they cause widespread vasodilation, the widening of blood vessels. This reduces peripheral vascular resistance, causing blood pressure to fall. Second, these chemicals increase vascular permeability, meaning the blood vessel walls become leaky. Fluid escapes from the bloodstream into surrounding tissues, reducing the total circulating blood volume. The combination of widespread vasodilation and fluid loss can overwhelm the circulatory system, leading to dangerously low blood pressure, a condition known as anaphylactic shock.
Recognizing Signs of Low Blood Pressure in Anaphylaxis
A sudden drop in blood pressure, known as hypotension, can manifest through several observable symptoms. These indicate the circulatory system is struggling to deliver enough blood to the body’s organs. Individuals may experience dizziness or lightheadedness, and in severe cases, they might faint or collapse due to reduced blood flow to the brain.
The skin can also show changes, often becoming pale (pallor) as blood is diverted away from the surface to more vital organs. The skin might also feel cold and clammy to the touch. A weak and rapid pulse is another common sign, as the heart attempts to compensate for the decreased blood pressure by beating faster to maintain blood circulation.
Immediate Action and Medical Care
Immediate action is important when someone is experiencing anaphylaxis, especially if signs of low blood pressure are present. The most important step is the immediate administration of epinephrine using an auto-injector. Epinephrine works rapidly by acting on the body’s systems. This causes blood vessels to constrict, which helps to raise blood pressure and improve blood flow to vital organs. Epinephrine also relaxes the smooth muscles in the airways, helping to alleviate breathing difficulties.
After administering epinephrine, emergency services (such as 911 in the United States or equivalent local numbers) must be called immediately. While waiting for medical help, the person should lie flat on their back with their legs elevated, if possible, to improve blood flow to the brain and heart. However, if lying flat compromises their breathing, they should be positioned to ease respiration. Monitor the person for any changes in symptoms or consciousness until paramedics arrive. Even if symptoms improve after the epinephrine injection, seeking prompt medical evaluation in an emergency room is necessary to ensure symptoms do not return, as a second reaction, known as biphasic anaphylaxis, can occur hours later.