Medical School · Year 2 · Pathology · includes a quiz and discussion video

Lecture 04: Hemodynamic Disorders

Unit 2.11: Pathology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the pathophysiology of edema and its types
2. Explain hyperemia and congestion
3. Describe thrombosis and its consequences
4. Explain embolism and its types
5. Describe infarction patterns and morphology
6. Explain the pathophysiology of shock

Lecture Outline

I. Edema

Edema refers to the abnormal accumulation of fluid within the interstitial tissue spaces or body cavities, resulting from disturbances in the mechanisms that normally maintain fluid balance between the intravascular and extravascular compartments. The two major categories of edema fluid are transudate, which is protein-poor and results from alterations in hemodynamic forces, and exudate, which is protein-rich and results from increased vascular permeability in inflammatory conditions. Lymphedema occurs when lymphatic drainage is obstructed, causing accumulation of protein-rich fluid in affected tissues. Anasarca describes severe, generalized edema affecting the entire body, typically seen in advanced heart failure, nephrotic syndrome, or severe liver disease.

The Starling forces govern fluid movement across capillary walls and determine the balance between filtration and reabsorption. Capillary hydrostatic pressure, generated by cardiac output and blood pressure, pushes fluid out of vessels into the interstitium. Plasma oncotic pressure, determined primarily by albumin concentration, draws fluid back into the vascular compartment. Interstitial hydrostatic pressure and interstitial oncotic pressure represent the corresponding forces in the tissue compartment. Under normal conditions, slight net filtration occurs and excess fluid is removed by lymphatic drainage, maintaining tissue fluid homeostasis.

The causes of edema can be classified according to which Starling force is disturbed. Increased hydrostatic pressure occurs in heart failure, where venous congestion raises capillary pressure, and in deep venous thrombosis or pregnancy where venous outflow is obstructed. Decreased plasma oncotic pressure results from hypoalbuminemia due to nephrotic syndrome with urinary protein loss, cirrhosis with decreased hepatic albumin synthesis, or malnutrition with inadequate protein intake. Lymphatic obstruction from tumors, surgical lymph node dissection, or parasitic infections such as filariasis causes accumulation of protein-rich lymphedema. Sodium and water retention in renal failure expands plasma volume and increases hydrostatic pressure.

The clinical manifestations of edema depend on the underlying cause and the tissues affected. Pulmonary edema from left heart failure causes dyspnea and may be life-threatening if severe, with fluid accumulating in alveolar spaces and impairing gas exchange. Peripheral edema in dependent areas such as the ankles and sacrum occurs in heart failure and is typically pitting, leaving an indentation when compressed. Ascites, fluid accumulation in the peritoneal cavity, occurs in cirrhosis and malignancy, causing abdominal distension and discomfort. Cerebral edema from trauma, stroke, or tumors is particularly dangerous due to the rigid cranial vault, with potential for herniation and death as brain tissue is displaced.

<image>Panel A: Diagram illustrating the four Starling forces - capillary hydrostatic pressure, plasma oncotic pressure, interstitial hydrostatic pressure, and interstitial oncotic pressure - with arrows showing direction of fluid movement. Panel B: Flowchart showing the major causes of edema categorized by mechanism including increased hydrostatic pressure (heart failure, DVT), decreased oncotic pressure (nephrotic syndrome, cirrhosis), lymphatic obstruction (tumor, filariasis), and sodium retention (renal failure). Panel C: Clinical photographs showing pitting edema of the lower extremity with finger indentation, ascites with abdominal distension, and pulmonary edema chest X-ray. Panel D: Comparison of transudate versus exudate characteristics including protein content, specific gravity, and clinical contexts.</image>

II. Hyperemia and Congestion

Hyperemia refers to an active increase in blood flow to a tissue due to arteriolar dilation, resulting in increased blood volume within the microcirculation. This is an active process driven by smooth muscle relaxation in arteriolar walls, typically occurring in response to physiologic demands for increased tissue perfusion. Exercise-induced hyperemia delivers additional oxygen and nutrients to working muscles. Inflammatory hyperemia represents the vascular response to injury, with vasodilation mediated by histamine, prostaglandins, and nitric oxide. The affected tissue appears red or erythematous because the vessels contain well-oxygenated blood.

Congestion represents a passive process in which blood accumulates in tissues due to impaired venous outflow, in contrast to the active arteriolar dilation of hyperemia. The stagnant blood becomes deoxygenated, giving congested tissues a bluish-red or cyanotic appearance rather than the bright red of hyperemia. Congestion may be acute or chronic, with chronic congestion leading to tissue hypoxia, parenchymal atrophy, and eventual fibrosis. The most clinically significant forms of congestion affect the lungs and liver as consequences of left-sided and right-sided heart failure respectively.

Chronic passive congestion of the lungs occurs as a consequence of left ventricular failure or mitral valve disease, with elevated left atrial pressure transmitted retrograde to the pulmonary veins and capillaries. The congested pulmonary capillaries leak red blood cells into the alveolar spaces, where hemoglobin is converted to hemosiderin by alveolar macrophages. These hemosiderin-laden macrophages, termed heart failure cells, are characteristic of chronic pulmonary congestion and can be identified in sputum or bronchoalveolar lavage specimens. With prolonged congestion, fibrosis of the alveolar septa develops, producing the pathologic condition known as brown induration of the lungs.

Chronic passive congestion of the liver results from right heart failure or constrictive pericarditis, with elevated central venous pressure transmitted to the hepatic veins and sinusoids. The characteristic gross appearance is the nutmeg liver, in which the congested centrilobular regions appear dark red against the paler peripheral zones that remain relatively spared. Histologically, the centrilobular sinusoids are dilated and filled with blood, and with severe or prolonged congestion, centrilobular hepatocyte necrosis occurs. Long-standing cardiac congestion may eventually produce cardiac cirrhosis, though this is uncommon with modern management of heart failure. The centrilobular vulnerability reflects the position of these hepatocytes at the end of the hepatic arterial and portal venous supply, making them most susceptible to hypoxia.

<image>Panel A: Comparison of hyperemia versus congestion showing arteriolar dilation with bright red oxygenated blood in hyperemia versus venous outflow obstruction with blue-red deoxygenated blood in congestion. Panel B: Chronic pulmonary congestion showing dilated capillaries, intra-alveolar hemorrhage, hemosiderin-laden macrophages (heart failure cells), and eventual fibrosis producing brown induration. Panel C: Gross photograph of nutmeg liver showing alternating dark congested centrilobular zones and pale peripheral zones, with corresponding histologic section showing centrilobular congestion. Panel D: Hepatic lobule diagram showing why centrilobular hepatocytes are most vulnerable to hypoxia based on their position at the end of the sinusoidal blood supply.</image>

III. Hemorrhage

Hemorrhage refers to the extravasation of blood from vessels into tissues, body cavities, or the external environment, occurring when vessel wall integrity is compromised. The clinical terminology for hemorrhages of different sizes includes petechiae for minute hemorrhages 1-2 millimeters in diameter typically caused by platelet disorders, low platelet counts, or defective platelet function. Purpura describes slightly larger hemorrhages of 3 millimeters or more, while ecchymoses are larger subcutaneous hematomas greater than 1-2 centimeters, commonly known as bruises. A hematoma is a localized collection of blood, usually clotted, that forms a palpable mass within tissue.

The causes of hemorrhage span a broad range of conditions affecting vessel wall integrity and hemostatic mechanisms. Trauma represents the most common cause, with mechanical injury disrupting vessel walls. Atherosclerosis weakens arterial walls and may lead to aneurysm formation and rupture. Inflammatory conditions including vasculitis can erode vessel walls. Neoplasms may directly invade vessels or erode into them through tissue destruction. Coagulation factor deficiencies, as in hemophilia, cause prolonged bleeding after injury. Platelet disorders, including thrombocytopenia and qualitative platelet defects, manifest with mucocutaneous bleeding.

The clinical significance of hemorrhage depends on several factors including the volume of blood lost, the rate of blood loss, and the site of bleeding. Acute loss of more than 20% of blood volume can cause hypovolemic shock, while slower blood loss may be compensated by fluid shifts and eventual increased erythropoiesis. The location of hemorrhage is critically important; relatively small hemorrhages in the brain or pericardium can be rapidly fatal due to mass effect or cardiac tamponade, while much larger hemorrhages into the retroperitoneum or thigh may be survivable. Chronic, occult blood loss, particularly from the gastrointestinal tract, causes iron deficiency anemia that may be the presenting finding of an underlying malignancy.

The resolution of ecchymoses follows a characteristic color progression that reflects the sequential degradation of hemoglobin. Fresh hemorrhage appears red to blue due to the presence of deoxyhemoglobin. Over the first few days, the hemorrhage takes on a blue-green color as hemoglobin is converted to biliverdin. By four to seven days, the yellow color of bilirubin predominates. Finally, by one to two weeks, the brown color of hemosiderin appears as iron is deposited. This color sequence allows forensic estimation of hemorrhage age and is clinically useful in assessing the timing of bruises, though the exact timing varies with hemorrhage size and location.

<image>Panel A: Classification of hemorrhages by size showing petechiae (1-2 mm), purpura (3+ mm), ecchymoses (>1-2 cm), and hematoma (localized collection) with characteristic clinical photographs. Panel B: Causes of hemorrhage organized by mechanism including vessel wall abnormalities (trauma, atherosclerosis, vasculitis), coagulation defects (hemophilia), and platelet disorders (thrombocytopenia). Panel C: Diagram showing critical hemorrhage locations where small blood collections can be rapidly fatal including intracranial hemorrhage with brain herniation and pericardial hemorrhage with cardiac tamponade. Panel D: Timeline of ecchymosis color evolution from red-blue (fresh) through blue-green (biliverdin) to yellow (bilirubin) to brown (hemosiderin) with molecular basis of each color.</image>

IV. Thrombosis

Thrombosis refers to the pathologic formation of a blood clot (thrombus) within an intact blood vessel, representing inappropriate activation of hemostatic mechanisms. The conditions predisposing to thrombosis were described by Virchow in the 19th century and remain valid today as Virchow's triad: endothelial injury, abnormal blood flow, and hypercoagulability. While any one of these factors may be sufficient to initiate thrombosis, they commonly occur in combination. The relative importance of each factor differs between arterial and venous thrombosis, with endothelial injury predominating in arterial disease and stasis being most important in venous thrombosis.

Endothelial injury is the most important factor in arterial thrombosis, exposing thrombogenic subendothelial matrix and tissue factor to the bloodstream. Atherosclerotic plaque rupture represents the classic example, triggering the thrombosis that causes myocardial infarction and stroke. Endothelial activation without overt injury can also promote thrombosis by inducing procoagulant changes including increased tissue factor expression and decreased thrombomodulin expression. Causes of endothelial injury and dysfunction include hypertension, turbulent flow, hyperlipidemia, products from cigarette smoke, and inflammatory cytokines.

Abnormal blood flow contributes to thrombosis through two mechanisms: stasis and turbulence. Stasis allows platelets to contact the endothelium, prevents dilution of activated clotting factors, and impairs the influx of clotting inhibitors. Venous thrombosis characteristically occurs in settings of stasis including immobilization after surgery, prolonged bed rest, and venous obstruction from pregnancy or tumors. Turbulent flow causes endothelial injury and creates local pockets of stasis. Atrial fibrillation creates stasis in the fibrillating atrium, promoting thrombus formation in the left atrial appendage. Aneurysms contain regions of stasis that predispose to mural thrombus formation.

Hypercoagulability encompasses conditions that increase the propensity for blood to clot, classified as primary (genetic) or secondary (acquired). Primary hypercoagulability includes inherited deficiencies of anticoagulant proteins such as antithrombin, protein C, and protein S, as well as gain-of-function mutations such as Factor V Leiden and prothrombin G20210A. Factor V Leiden, which renders Factor V resistant to inactivation by activated protein C, is the most common inherited thrombophilia, present in approximately 5% of Caucasians. Secondary hypercoagulability occurs in numerous acquired conditions including malignancy, pregnancy, oral contraceptive use, prolonged immobilization, and the antiphospholipid antibody syndrome.

<image>Panel A: Virchow's triad diagram showing the three factors predisposing to thrombosis - endothelial injury, abnormal blood flow (stasis and turbulence), and hypercoagulability - with examples of each and their relative importance in arterial versus venous thrombosis. Panel B: Endothelial injury from atherosclerotic plaque rupture showing exposure of subendothelial collagen and tissue factor initiating platelet adhesion and coagulation cascade activation. Panel C: Stasis in venous thrombosis showing immobile legs with blood pooling in deep veins and dilution failure of activated clotting factors. Panel D: Hypercoagulability causes showing genetic factors (Factor V Leiden pathway diagram) and acquired factors (malignancy, pregnancy, oral contraceptives) with their mechanisms.</image>

V. Thrombus Morphology and Fate

The morphology of thrombi provides clues to their origin and distinguishes them from postmortem clots. Lines of Zahn are alternating pale layers of platelets and fibrin with darker layers of red blood cells, visible grossly or microscopically in thrombi formed in flowing blood. This laminated appearance indicates that the thrombus formed ante-mortem and is therefore pathologically significant. Postmortem clots lack this laminated structure because blood is stagnant and all components settle together. Arterial thrombi tend to be pale or gray-white because they are rich in platelets, while venous thrombi tend to be red because they contain more entrapped erythrocytes and form in slower-flowing blood.

Arterial and venous thrombi differ in their composition, location, and clinical significance. Arterial thrombi typically form at sites of endothelial injury, most commonly over atherosclerotic plaques, and grow in the direction of blood flow. They are platelet-rich and relatively resistant to fibrinolysis, explaining why antiplatelet therapy is emphasized in arterial thrombotic disease. Venous thrombi, also called red or stasis thrombi, form in areas of slow blood flow and propagate in the direction of blood flow, potentially extending considerable distances. They are fibrin-rich and more susceptible to fibrinolysis. The deep veins of the lower extremity are the most common site of venous thrombosis.

The fate of a thrombus depends on the balance between thrombotic growth and the host response. Propagation occurs when the thrombus grows by continued platelet aggregation and fibrin deposition, potentially causing complete vascular occlusion. Embolization occurs when part or all of the thrombus detaches and travels to distant sites, with venous thrombi embolizing to the pulmonary circulation and arterial thrombi embolizing systemically. Dissolution results from fibrinolytic activity and is most effective when thrombi are small and fresh. Organization occurs when the thrombus becomes invaded by granulation tissue, with eventual incorporation into the vessel wall. Recanalization may occur when new vascular channels form through the organized thrombus, partially restoring blood flow.

The clinical manifestations of thrombosis depend on the vessel involved and the degree of occlusion. Coronary artery thrombosis causes myocardial infarction, the leading cause of death in developed countries. Cerebral arterial thrombosis causes ischemic stroke. Deep venous thrombosis of the lower extremity may be asymptomatic or cause leg swelling and pain, but the major danger is pulmonary embolism. Mural thrombi in the heart, forming on damaged endocardium after myocardial infarction or in dilated cardiomyopathy, can fragment and embolize to the systemic circulation, causing stroke or peripheral arterial occlusion. Left atrial thrombi in atrial fibrillation are a major source of cardioembolic stroke.

<image>Panel A: Gross and microscopic appearance of Lines of Zahn showing alternating pale (platelet-fibrin) and dark (red blood cell) layers in a thrombus formed in flowing blood, compared to uniform appearance of postmortem clot. Panel B: Comparison of arterial thrombus (pale, platelet-rich, at site of atherosclerosis) versus venous thrombus (red, RBC-rich, propagating with blood flow) with their typical locations. Panel C: Diagram showing the four potential fates of a thrombus: propagation, embolization, dissolution, and organization with recanalization. Panel D: Clinical consequences of thrombosis at different sites including coronary (MI), cerebral (stroke), deep venous (DVT/PE), and intracardiac (systemic embolization).</image>

VI. Embolism

An embolus is an intravascular mass that travels through the blood to lodge at a site distant from its origin, causing partial or complete vascular occlusion. Thromboembolism, the embolization of fragments of thrombi, accounts for the vast majority of embolic events, approximately 99%. Other types of emboli include fat emboli from bone fractures, air or gas emboli from trauma or decompression, amniotic fluid emboli during labor, and tumor emboli that spread cancer through the vascular system. Emboli travel through vessels of progressively smaller caliber until they lodge in a vessel too small to permit further passage.

Pulmonary embolism (PE) originates from venous thrombi, with approximately 95% arising from deep vein thrombi in the legs. The embolus travels through the right heart to the pulmonary arterial system, where it lodges in vessels sized to trap it. The clinical consequences range from asymptomatic to sudden death depending on the size and number of emboli and the patient's cardiopulmonary reserve. Massive pulmonary embolism, in which a saddle embolus straddles the bifurcation of the main pulmonary artery, causes acute right heart failure and cardiovascular collapse with high mortality. Smaller emboli may cause pulmonary infarction, particularly when they obstruct medium-sized vessels in patients with pre-existing cardiac or pulmonary disease.

Systemic (arterial) embolism refers to emboli traveling through the systemic arterial circulation to lodge in downstream organs. Approximately 80% originate from intracardiac thrombi, most commonly mural thrombi on the left ventricular wall following myocardial infarction or in dilated cardiomyopathy, and left atrial thrombi associated with atrial fibrillation. Aortic atherosclerotic plaques are another source of cholesterol and thrombotic emboli. Paradoxical embolism occurs when a venous thrombus crosses from the right to left heart through a patent foramen ovale or other septal defect, enabling systemic embolization. The most common destinations are the lower extremities and the brain, with stroke being a dreaded consequence.

Fat embolism syndrome develops one to three days after long bone fractures, when marrow fat enters ruptured venous sinusoids and travels to the lungs and systemic circulation. The classic triad includes pulmonary insufficiency with hypoxemia, neurologic symptoms from cerebral fat embolization, and petechial rash on the skin, often in the axillae and conjunctivae. The pathogenesis involves both mechanical obstruction by fat globules and the toxic effects of free fatty acids released by lipase action. Air embolism occurs when air enters the venous circulation, typically during surgery, central line placement, or trauma; more than 100 mL of air is generally required to cause symptoms. Amniotic fluid embolism is a rare but catastrophic complication of labor characterized by sudden dyspnea, cyanosis, shock, and often disseminated intravascular coagulation.

<image>Panel A: Pathway of pulmonary embolism from deep leg vein thrombus through the right heart to the pulmonary arteries, showing saddle embolus at the main pulmonary artery bifurcation. Panel B: Sources of systemic arterial emboli including left ventricular mural thrombus post-MI, left atrial thrombus in atrial fibrillation, and aortic atheroma, with common destination sites. Panel C: Fat embolism syndrome showing timeline after long bone fracture, clinical triad of pulmonary, neurologic, and dermatologic manifestations, and histology showing fat globules in pulmonary vessels. Panel D: Amniotic fluid embolism showing histologic appearance of squamous cells and mucin in pulmonary vessels with the clinical syndrome of sudden cardiopulmonary collapse during labor.</image>

VII. Fat and Air Embolism

Fat embolism syndrome represents a systemic manifestation of fat globule embolization, occurring primarily after fractures of long bones where fatty marrow is disrupted and fat enters damaged venous sinusoids. The syndrome typically develops one to three days after the inciting injury, allowing time for the biochemical effects of fat to develop in addition to the mechanical obstruction. The clinical presentation includes pulmonary manifestations with dyspnea, tachypnea, and hypoxemia that may progress to acute respiratory distress syndrome. Neurologic manifestations range from restlessness and irritability to delirium, stupor, and coma, reflecting fat embolization to the cerebral circulation.

The characteristic petechial rash of fat embolism syndrome appears in non-dependent regions including the conjunctivae, oral mucosa, skin of the upper trunk, and axillary folds. The petechiae result from fat microemboli damaging small vessels in these areas, with platelets consumed in the microthrombi that form. The diagnosis is primarily clinical, as there is no specific confirmatory test, though fat globules may be found in urine, sputum, or bronchoalveolar lavage specimens. The pathophysiology involves both mechanical obstruction by fat globules and a biochemical component in which lipoprotein lipase in the lungs releases free fatty acids that injure the alveolar epithelium and pulmonary vasculature.

Air embolism occurs when sufficient volumes of gas enter the venous or arterial circulation to cause mechanical obstruction of blood flow. Venous air embolism may occur during central venous catheter placement or removal, certain surgical procedures, and chest wall trauma. The air travels to the right heart and pulmonary circulation, where large volumes can cause outflow obstruction and cardiovascular collapse. Arterial air embolism is less common but more dangerous, occurring when air gains access to the systemic arterial circulation and lodges in cerebral or coronary vessels. The lethal volume for venous air embolism is generally considered to be greater than 100 mL introduced rapidly.

Decompression sickness, colloquially known as "the bends," results from nitrogen gas coming out of solution and forming bubbles in tissues and blood when ambient pressure decreases rapidly. Divers who ascend too quickly, or aviators who experience rapid decompression, develop gas bubbles primarily in the joints causing severe pain, but also in the lungs (the "chokes"), and potentially in the spinal cord causing paralysis. Treatment requires recompression in a hyperbaric chamber to redissolve the nitrogen bubbles, followed by slow decompression. Caisson disease represents chronic decompression injury with multiple foci of ischemic necrosis of bone, particularly affecting the femoral heads and humeral heads, developing in divers and others with repeated decompression exposures.

<image>Panel A: Fat embolism syndrome timeline showing long bone fracture at day 0, latent period, then clinical syndrome at days 1-3 with pulmonary, neurologic, and cutaneous manifestations. Panel B: Pathophysiology of fat embolism showing mechanical obstruction by fat globules plus biochemical injury from free fatty acid release by lipoprotein lipase. Panel C: Air embolism showing sources (central line, surgery, trauma), pathway through right heart to pulmonary circulation, and the volume threshold for clinical effects. Panel D: Decompression sickness showing nitrogen bubble formation in tissues during rapid ascent, affected sites (joints, lungs, spinal cord), and treatment with hyperbaric recompression.</image>

VIII. Infarction

Infarction refers to ischemic necrosis of tissue resulting from occlusion of the vascular supply, most commonly by thrombosis or embolism. The term derives from the Latin word for "stuffing," reflecting the original observation of blood stuffed into hemorrhagic infarcts. Infarcts are classified as white (anemic) or red (hemorrhagic) based on their gross appearance, with this distinction reflecting the vascular anatomy and other factors affecting whether blood enters the infarcted tissue. Almost all infarcts are caused by arterial occlusion, with venous occlusion being an uncommon cause in organs with extensive collateral venous drainage.

Several factors determine the severity of tissue damage following vascular occlusion and influence whether infarction occurs. The nature of the vascular supply is critical; tissues supplied by a single vessel without collaterals (end-arteries) are highly vulnerable, while those with dual supply or extensive anastomoses may be protected. The rate of occlusion is important because slowly developing obstruction allows collateral vessels to enlarge and compensate, while sudden occlusion does not permit this adaptation. Tissue vulnerability varies, with neurons surviving only minutes without oxygen while fibroblasts remain viable for hours. The oxygen content of the blood affects the margin of ischemia, with anemia or hypoxemia worsening outcomes.

White (anemic) infarcts occur in solid organs with end-arterial blood supplies, where arterial occlusion prevents blood from entering the infarcted tissue. The heart, kidney, and spleen classically develop pale infarcts because their architecture does not permit blood to enter from adjacent non-infarcted tissue. The infarcted tissue initially appears pale and slightly swollen, becoming progressively yellow and well-demarcated over days as necrotic tissue is broken down. Histologically, the ischemic tissue undergoes coagulative necrosis with preservation of tissue architecture and eventual fibrosis.

Red (hemorrhagic) infarcts develop when blood enters the area of ischemic necrosis, either from dual blood supplies, collateral flow, or venous congestion. The lungs have dual arterial supply from pulmonary and bronchial arteries, so pulmonary infarcts are typically hemorrhagic. Reperfusion of initially pale infarcts, as occurs with spontaneous or therapeutic thrombolysis, causes hemorrhage into the necrotic tissue. Venous occlusion, as in testicular or ovarian torsion or mesenteric vein thrombosis, causes hemorrhagic infarction because arterial blood continues to enter tissue whose venous drainage is blocked. Tissues with loose texture that can expand to accommodate blood, such as the lung and small intestine, tend to develop hemorrhagic infarcts.

<image>Panel A: Comparison of factors affecting infarction severity including nature of vascular supply (end-arterial versus collateral), rate of occlusion (acute versus gradual), tissue vulnerability (neurons versus fibroblasts), and blood oxygen content. Panel B: White (anemic) infarct of the spleen showing wedge-shaped pale area with apex at occluded artery and base at capsule, with corresponding histology showing coagulative necrosis. Panel C: Red (hemorrhagic) infarct of the lung showing dark blood-filled wedge-shaped area, with explanation of dual blood supply permitting hemorrhage. Panel D: Diagram explaining red versus white infarcts based on tissue architecture, blood supply characteristics, and mechanism of vascular occlusion.</image>

IX. Shock

Shock is defined as systemic hypoperfusion caused by reduced cardiac output or reduced effective circulating blood volume, leading to inadequate tissue oxygenation and cellular injury. The consequences of shock result from impaired oxygen and nutrient delivery to tissues and accumulation of metabolic waste products. If shock is not reversed, cellular injury becomes irreversible, leading to multi-organ failure and death. The classification of shock by etiology includes cardiogenic, hypovolemic, distributive, and obstructive types, each with distinct pathophysiology but converging on the final common pathway of tissue hypoperfusion.

Cardiogenic shock results from pump failure, with the heart unable to generate sufficient cardiac output to perfuse tissues despite adequate intravascular volume. The most common cause is extensive myocardial infarction, with shock occurring when approximately 40% of the left ventricular myocardium is dysfunctional. Other causes include severe valvular dysfunction, cardiac tamponade, and arrhythmias. Hypovolemic shock results from decreased blood or plasma volume, occurring with hemorrhage, severe burns, dehydration, or third-spacing of fluid. The reduction in venous return decreases cardiac filling and output. Distributive shock results from widespread vasodilation that reduces effective circulating volume despite normal total blood volume.

Septic shock is the most common and clinically important form of distributive shock, caused by systemic inflammatory responses to severe infection. Gram-negative bacterial infections causing endotoxin release are the most common cause, though gram-positive organisms, fungi, and viruses can also trigger septic shock. The pathophysiology involves a cytokine storm with massive release of TNF-alpha, IL-1, and other inflammatory mediators that cause widespread vasodilation, increased vascular permeability, and myocardial depression. Early septic shock is characterized by warm extremities and bounding pulses (warm shock) due to peripheral vasodilation, but progression leads to cold, clamped-down vasoconstriction (cold shock) as compensatory mechanisms fail.

Shock progresses through three stages with increasingly severe pathophysiology. The compensated stage involves neural and hormonal responses that maintain blood pressure through tachycardia, vasoconstriction, and fluid retention, preserving perfusion to vital organs at the expense of skin and splanchnic circulation. The progressive stage develops when compensatory mechanisms become overwhelmed, with tissue hypoperfusion causing metabolic acidosis from anaerobic metabolism, which further impairs cardiac function. The irreversible stage is marked by multi-organ failure, with acute tubular necrosis in the kidneys, shock lung (acute respiratory distress syndrome), hepatic ischemia, and intestinal mucosal injury that may permit bacterial translocation. At this stage, cellular damage is extensive and mortality approaches 100% despite treatment.

<image>Panel A: Classification of shock types showing cardiogenic (pump failure), hypovolemic (volume loss), distributive (vasodilation in septic, neurogenic, anaphylactic), and obstructive (PE, tamponade) with specific examples of each. Panel B: Pathophysiology of septic shock showing bacterial trigger, macrophage activation, cytokine release (TNF, IL-1), and downstream effects on vasodilation, permeability, and myocardial function. Panel C: Three stages of shock progression from compensated (tachycardia, vasoconstriction) through progressive (acidosis, deteriorating function) to irreversible (multi-organ failure). Panel D: Organ-specific manifestations of shock showing acute tubular necrosis, ARDS, hepatic centrilobular necrosis, and intestinal mucosal injury.</image>

X. Disseminated Intravascular Coagulation

Disseminated intravascular coagulation (DIC) is an acquired syndrome characterized by widespread activation of coagulation leading to intravascular fibrin formation and consumption of platelets and coagulation factors. The paradoxical clinical manifestations include both thrombosis with microvascular occlusion causing ischemic organ damage and bleeding due to consumption of hemostatic components. DIC is always secondary to an underlying condition and represents a final common pathway of multiple disease processes that cause systemic coagulation activation. The severity ranges from subclinical laboratory abnormalities to fulminant, life-threatening coagulopathy.

The pathophysiology of DIC centers on excessive tissue factor exposure and generation of thrombin activity that overwhelms natural anticoagulant mechanisms. Tissue factor released from damaged tissues, tumor cells, or activated monocytes initiates coagulation. The resulting widespread thrombin generation converts fibrinogen to fibrin, which deposits in the microvasculature of multiple organs. Simultaneously, platelets are consumed in the microthrombi, and clotting factors are depleted faster than they can be replaced. Secondary fibrinolysis generates fibrin degradation products that further impair hemostasis by interfering with fibrin polymerization and platelet function.

The causes of DIC span multiple categories of severe disease. Obstetric complications including placental abruption, retained dead fetus, and amniotic fluid embolism release thromboplastic substances that trigger DIC. Sepsis, particularly gram-negative septicemia, activates coagulation through endothelial injury and tissue factor expression by activated monocytes. Malignancy, especially mucin-secreting adenocarcinomas and acute promyelocytic leukemia, can cause chronic or acute DIC through tumor cell release of procoagulant substances. Massive tissue injury from trauma, burns, or surgery releases tissue factor. Hemolytic transfusion reactions cause DIC through release of phospholipids and tissue factor from lysed red cells.

The laboratory findings in DIC reflect the consumption of hemostatic components and activation of fibrinolysis. Thrombocytopenia develops as platelets are consumed in microthrombi. Prothrombin time (PT) and partial thromboplastin time (PTT) are prolonged due to consumption of clotting factors. Fibrinogen levels fall as fibrinogen is converted to fibrin and subsequently degraded. D-dimer, a specific fibrin degradation product, is elevated, indicating both fibrin formation and subsequent lysis. Microangiopathic hemolytic anemia with schistocytes on peripheral smear results from mechanical fragmentation of red blood cells passing through fibrin strands in small vessels. Treatment is directed at the underlying cause, with supportive measures including transfusion of platelets and clotting factors in actively bleeding patients.

<image>Panel A: Pathophysiology of DIC showing tissue factor exposure triggering coagulation cascade, widespread thrombin generation, microthrombus formation in small vessels, consumption of platelets and factors, and secondary fibrinolysis. Panel B: Causes of DIC categorized by mechanism including obstetric complications, sepsis, malignancy, massive trauma, and transfusion reactions with specific examples. Panel C: Laboratory findings in DIC showing decreased platelets and fibrinogen, prolonged PT/PTT, elevated D-dimer, and peripheral smear with schistocytes from microangiopathic hemolysis. Panel D: Clinical paradox of DIC showing simultaneous microvascular thrombosis causing organ ischemia and bleeding from consumption coagulopathy.</image>

Summary

• Edema results from imbalance of Starling forces, with transudate being protein-poor and exudate being protein-rich
• Congestion is passive blood accumulation; nutmeg liver results from right heart failure and pulmonary congestion from left heart failure
• Thrombosis is governed by Virchow's triad: endothelial injury, abnormal blood flow (stasis or turbulence), and hypercoagulability
• Arterial thrombi are platelet-rich and white; venous thrombi are RBC-rich and red
• Thromboembolism accounts for 99% of emboli; pulmonary embolism originates from leg DVT, systemic emboli from the heart
• Infarcts are white (anemic) in solid organs with end-arteries and red (hemorrhagic) with dual supply or venous occlusion
• Shock is classified as cardiogenic, hypovolemic, distributive (including septic), and obstructive
• Septic shock involves cytokine storm with progression from warm to cold shock
• DIC is a consumption coagulopathy with both thrombosis and bleeding due to widespread coagulation activation
• Multi-organ failure is the final common pathway of severe shock and DIC

Key Terms

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