Burn Pathophysiology

Burn Pathophysiology: Understanding How Heat Damages the Body Burn injuries cause both immediate local tissue damage and profound systemic effects that can threaten life for weeks after the initial injury. Understanding burn pathophysiology is essential because the physiological mechanisms determine clinical management and patient outcomes. Cellular Damage from Heat When skin is exposed to temperatures above 44 °C (111 °F), proteins within cells begin to denature—that is, they lose their three-dimensional structure and can no longer function. This protein damage is irreversible and occurs progressively as temperature increases. At higher temperatures (above 65 °C), the protein denaturation happens rapidly, causing cells to die almost immediately. This heat-induced cellular breakdown leads to tissue death in the burned area, and the body's inability to repair this damage is why burns require such intensive treatment. The tissue death creates a zone of coagulation where cells have been completely destroyed. Around this area lies a zone of inflammation, where cells are stressed but not yet dead. Understanding this distinction is important because the zone of inflammation is the area clinicians try to salvage through careful management—if blood flow is restored quickly enough and tissue damage is minimized, some of these damaged cells may survive. Local Tissue Architecture and Eschar Formation Burn depth determines which tissue layers are damaged. First-degree burns affect only the epidermis (outer layer), causing redness and pain but no permanent damage. Second-degree burns extend into the dermis and produce blistering and severe pain. Third-degree burns destroy the full thickness of skin, extending into the subcutaneous tissue, and paradoxically cause less pain because nerve endings are destroyed. As burned tissue dies, it forms an eschar—a rigid, leathery covering of dead tissue. While the eschar itself is non-living tissue, it creates a critical problem: it becomes a tight, inelastic barrier that cannot expand as underlying tissues swell with inflammation. This can restrict blood flow to the surrounding living tissue and even restrict chest wall expansion if located on the torso, impairing breathing. Eschar removal (called escharotomy) is sometimes necessary to relieve this restriction. Loss of Skin Barrier Functions Healthy skin provides four critical functions that burned skin immediately loses: Barrier against infection: Skin normally prevents bacteria from entering the body. Burned areas have no barrier, making infection one of the leading causes of death in burn patients. Sensory function: Burned skin cannot sense temperature, pressure, or pain, which normally help protect us from further injury. Thermoregulation: Healthy skin controls body temperature through sweating and blood vessel constriction. Burned skin cannot do this, leaving patients vulnerable to hypothermia (especially dangerous in the first hours after injury when emergency care often involves exposure). Fluid retention: This is perhaps the most immediately life-threatening loss. Intact skin is nearly waterproof. Burned skin allows massive amounts of fluid to evaporate directly from the wound surface, sometimes at rates of several liters per day. Fluid Shifts and Cellular Ionic Imbalance Heat damages cell membranes throughout the burned area and into the surrounding tissues. When cell membranes are disrupted, they can no longer maintain their normal ionic balance. Potassium, which is normally kept inside cells, leaks out into the bloodstream. Meanwhile, sodium and water, which are normally kept outside cells, flood inward, causing cells to swell (edema). This problem extends well beyond the burned tissue itself. The intense inflammatory response in the burned area causes capillaries—the tiny blood vessels that normally contain fluid within the bloodstream—to become leaky. Fluid that should remain in the circulation seeps out into the tissue spaces, following the sodium and water that have shifted out of cells. The result is hypovolemia: a dangerous reduction in circulating blood volume. Although the body still contains the same total amount of fluid, much of it is now trapped in tissue spaces (called third spacing) rather than in the bloodstream. Within hours of a severe burn, this fluid loss can become life-threatening. The Systemic Inflammatory Response Burns larger than approximately 30% of total body surface area (TBSA) trigger a massive systemic inflammatory response—inflammation affecting the entire body, not just the burned area. This happens because the severe tissue damage releases inflammatory chemicals (cytokines) into the bloodstream. This systemic inflammation has several serious consequences: Increased capillary permeability: Inflammatory mediators make blood vessels even leakier throughout the body, not just in the burned area. This worsens hypovolemia. Tissue edema: Fluid accumulates in organs and tissues. If severe enough in the lungs, this can cause breathing problems. In other organs, reduced blood flow can lead to organ failure. Reduced organ perfusion: With fluid shifted out of the bloodstream and cardiac output reduced by hypovolemia, organs receive inadequate blood supply and oxygen. Kidney and gastrointestinal complications: Reduced blood flow to the kidneys can cause acute kidney failure. Reduced blood flow to the gastrointestinal tract can allow bacteria to cross the damaged intestinal barrier, and can also cause stress ulceration (Curling's ulcer). This systemic inflammation is why large burns are medical emergencies even when the burned area itself might seem manageable. Metabolic Changes and the Hypermetabolic State After the initial shock phase, burn patients enter a profound hypermetabolic state—their metabolism becomes dramatically elevated. This is driven by extremely high levels of circulating catecholamines (epinephrine and norepinephrine) and cortisol, hormones released in response to the severe stress of injury. During this hypermetabolic state: Cardiac output and heart rate increase dramatically, placing enormous stress on the heart Overall metabolic rate can increase 100-150% above normal, requiring massive amounts of calories Immune function is paradoxically impaired despite the intense inflammatory response, increasing infection risk This state can persist for months to years after severe burns, affecting long-term recovery The elevated catecholamines and cortisol also have a "catabolic" effect, meaning the body breaks down muscle tissue for fuel. This is why burn patients lose significant muscle mass during recovery, even if they receive adequate nutrition. Burn Shock: The Acute Circulatory Crisis When fluid loss from burns is severe and not rapidly replaced, the body enters burn shock—a state of inadequate circulation characterized by: Decreased cardiac output: The heart cannot pump effectively when blood volume is critically reduced Tachycardia: The heart compensates by beating faster, often at 100+ beats per minute Reduced organ perfusion: Vital organs receive insufficient blood flow and oxygen Potential progression to irreversible shock and death if untreated Burn shock typically develops within the first 24-48 hours after injury. The immediate treatment is aggressive fluid resuscitation—rapidly replacing lost fluids intravenously. The goal is to restore circulating blood volume enough to maintain organ perfusion and prevent irreversible organ failure. This is why burn specialists use specific fluid resuscitation protocols (such as the Parkland formula) to carefully calculate fluid replacement needs based on burn size. The challenge is that giving too little fluid allows shock and organ failure to develop, but giving too much fluid can cause compartment syndrome (dangerously high pressure in tissue spaces) or pulmonary edema. Careful monitoring and adjustment of fluid rates is therefore critical.