7.3 Hemodynamic Disorders, Thrombosis, Embolism & Shock

7.3 Hemodynamic Disorders, Thrombosis, Embolism & Shock

Independent study guide by OpenExamPrep.

Core Examination Pearl: Board examiners heavily test the diagnostic distinctions between transudates and exudates, the molecular genetics of inherited thrombophilias (Factor V Leiden resistance to APC, prothrombin mutation, warfarin skin necrosis mechanism), the clinical triad of fat embolism following long bone fractures, the pathophysiology of atheroembolism causing 'blue toe syndrome', and the classic hemodynamic profiles differentiating hypovolemic, cardiogenic, and septic shock.

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1. Edema & Starling Forces

Edema represents an abnormal expansion of the interstitial fluid volume within tissues. Fluid movement between the vascular and extravascular compartments is governed by Starling forces:

$\text{Net Fluid Movement} = K_f \cdot [(P_c - P_i) - \sigma \cdot (\pi_c - \pi_i)]$

Where $P_c$ is capillary hydrostatic pressure, $P_i$ is interstitial hydrostatic pressure, $\pi_c$ is capillary plasma colloid oncotic pressure, $\pi_i$ is interstitial oncotic pressure, $K_f$ is capillary filtration coefficient, and $\sigma$ is the reflection coefficient.

Primary Pathophysiological Mechanisms of Edema

1. Increased Capillary Hydrostatic Pressure (Pc):
   • Systemic: Impaired venous return in Congestive Heart Failure (CHF). Decreased cardiac output reduces renal blood flow, stimulating the Renin-Angiotensin-Aldosterone System (RAAS) to retain sodium and water, expanding plasma volume and exacerbating hydrostatic capillary hypertension (bilateral dependent lower extremity pitting edema).
   • Localized: Impaired venous outflow due to Deep Vein Thrombosis (DVT), chronic venous insufficiency (damaged venous valves), external mass compression, or prolonged dependent positioning.
2. Decreased Plasma Colloid Oncotic Pressure (πc):
   • Driven by hypoalbuminemia (serum albumin <2.5–3.0 g/dL). Because albumin generates ~80% of total plasma colloid osmotic pressure, loss of albumin reduces the intravascular drawing force, allowing fluid to filter into tissues.
   • Etiologies: Nephrotic syndrome (urinary protein loss >3.5 g/24 h due to glomerular podocyte effacement or basement membrane damage), cirrhosis of the liver (decreased hepatic albumin synthesis), protein-losing enteropathy, and severe protein malnutrition (Kwashiorkor).
3. Lymphatic Obstruction (Lymphedema):
   • Obstruction of regional lymphatic drainage prevents clearance of interstitial fluid and macromolecules.
   • Etiologies: Parasitic infection by Wuchereria bancrofti (filariasis / elephantiasis), surgical lymphadenectomy (e.g., axillary or groin dissection), therapeutic radiation, malignant obstruction (peau d'orange in breast cancer), or non-filarial geochemical lymphedema of the feet (podoconiosis from volcanic clay soil exposure).
   • Clinical Hallmark: Non-pitting, woody, indurated swelling with thickened hyperkeratotic skin and a positive Stemmer's sign (inability to pinch a fold of skin at the base of the second toe).
4. Sodium and Water Retention:
   • Acute or chronic renal failure, excessive salt intake with renal insufficiency, or secondary hyperaldosteronism expansion of extracellular volume.
5. Increased Microvascular Permeability:
   • Acute inflammation, thermal burns, sepsis, allergic reactions, and trauma cause widening of postcapillary venular interendothelial junctions.

Transudate vs. Exudate Differential

+-----------------------------------------------------------------------------------------+
|                         TRANSUDATE vs. EXUDATE COMPARATIVE PROFILE                      |
+-----------------------+-----------------------------+-----------------------------------+
| Parameter             | Transudate                  | Exudate                           |
+-----------------------+-----------------------------+-----------------------------------+
| Primary Etiology      | Hydrostatic / Oncotic       | Increased vascular permeability   |
|                       | pressure imbalance          | driven by inflammation / infection|
| Endothelial Barrier   | **Intact & Normal**         | **Disrupted & Permeable**         |
| Protein Content       | **Low** (<3.0 g/dL)         | **High** (>3.0 g/dL)              |
| Specific Gravity      | **Low** (<1.012)            | **High** (>1.020)                 |
| Effusion/Serum Protein| **< 0.5**                   | **> 0.5** (Light's criteria)      |
| Effusion/Serum LDH    | **< 0.6**                   | **> 0.6** (Light's criteria)      |
| Cellular Infiltrate   | Hypocellular (few mesothelial| Dense cellularity (neutrophils in |
|                       | cells or mononuclear cells) | acute; lymphocytes in chronic)    |
| Fibrin Clot Formation | Absent (lacks fibrinogen)   | Spontaneous clotting common       |
| Common Clinical Sites | CHF dependent ankle edema,  | Diabetic foot abscess, cellulitis |
|                       | cirrhosis ascites, nephrosis| septic arthritis, empyema         |
+-----------------------+-----------------------------+-----------------------------------+

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2. Thrombosis & Virchow's Triad

Thrombosis is the pathological formation of an intravascular solid or semi-solid aggregate of blood elements (platelets, fibrin, red blood cells) within an uninjured vessel, or inappropriate thrombotic occlusion of a vessel following minor injury. Thrombogenesis is governed by Virchow's Triad:

+-----------------------------------------------------------------------------------------+
|                              VIRCHOW'S TRIAD OF THROMBOSIS                              |
+-----------------------------------------------------------------------------------------+
|  1. ENDOTHELIAL INJURY      • Atherosclerotic plaque rupture, vasculitis, hypertension  |
|                             • Exposes subendothelial collagen & von Willebrand factor   |
|                             • Downregulates thrombomodulin, tPA, and prostacyclin       |
+-----------------------------+-----------------------------------------------------------+
|  2. ALTERATIONS IN FLOW     • Stasis (deep veins of calf, bed rest, casting, paralysis)|
|     (Stasis & Turbulence)   • Turbulence (arterial bifurcations, aneurysms, stenosis)   |
+-----------------------------+-----------------------------------------------------------+
|  3. HYPERCOAGULABILITY      • Inherited: Factor V Leiden, Prothrombin G20210A, PC/PS def|
|     (Thrombophilia)         • Acquired: Antiphospholipid syndrome, malignancy, OCPs     |
+-----------------------------------------------------------------------------------------+

1. Endothelial Injury

• The single most critical factor in arterial and cardiac thrombogenesis. Endothelial denudation exposes underlying thrombogenic subendothelial extracellular matrix, specifically von Willebrand factor (vWF) and fibrillar Type I/III collagen, triggering platelet adhesion and activation.
• Endothelial injury downregulates endogenous anticoagulants (thrombomodulin, heparan sulfate, tissue factor pathway inhibitor [TFPI]) and diminishes synthesis of platelet inhibitors (prostacyclin PGI2 and nitric oxide).
• Etiologies: Rupture of an unstable atherosclerotic plaque in femoral/popliteal arteries, hemodynamic shear stress from severe hypertension, vasculitis, bacterial endotoxemia, hyperhomocysteinemia, and cigarette smoke toxins.

2. Alterations in Normal Blood Flow (Stasis & Turbulence)

• Normal Laminar Flow: Blood cells travel in the central axial stream, separated from the vascular endothelium by a slow-moving clear zone of plasma.
• Turbulence: Chaotic flow with vortices and local countercurrents. Causes physical endothelial injury, promotes endothelial gene activation, and creates localized pockets of stasis. Primary driver of arterial and cardiac thrombi (e.g., at arterial bifurcations, aneurysms, and over ulcerated plaques).
• Stasis: Sluggish, decelerated blood flow. Primary driver of venous thrombi. Stasis allows platelets to fall out of the axial stream and contact endothelium, prevents dilution and hepatic clearance of activated clotting factors, and impairs fresh inflow of clotting inhibitors.
• Podiatric Board Context: Prolonged immobilization following lower extremity orthopedic surgery, non-weight-bearing cast or boot immobilization for ankle/Achilles injuries, long-distance air travel, and varicose veins.

3. Hypercoagulability (Thrombophilia)

Hypercoagulability represents any disorder of the blood that predisposes to thrombosis. Classified into primary (inherited) and secondary (acquired) states:

Arterial vs. Venous Thrombi

+-----------------------------------------------------------------------------------------+
|                           ARTERIAL vs. VENOUS THROMBI COMPARISON                        |
+-----------------------+-----------------------------+-----------------------------------+
| Characteristic        | Arterial Thrombi            | Venous Thrombi (Phlebothrombosis) |
+-----------------------+-----------------------------+-----------------------------------+
| Flow Environment      | High shear, rapid flow      | Low shear, stasis, sluggish flow  |
| Primary Site          | Coronary, cerebral, femoral | Deep calf veins (soleal, tibial), |
|                       | arteries; over plaques      | popliteal, femoral, iliac veins   |
| Composition           | **Platelet-rich ('white')** | **Fibrin- and RBC-rich ('red')**  |
| Direction of Growth   | **Retrograde** (grows back  | **Anterograde** (grows in         |
|                       | toward the heart)           | direction of venous blood flow)   |
| Lines of Zahn         | **Prominent and distinct**  | Subtle or poorly developed        |
| Primary Consequence   | Ischemia, distal tissue     | Edema, congestion, embolization   |
|                       | infarction (gangrene)       | to pulmonary arteries (PE)        |
+-----------------------+-----------------------------+-----------------------------------+

The Significance of Lines of Zahn

• Under microscopic examination, antemortem thrombi formed within flowing blood demonstrate alternating laminations called Lines of Zahn:
  • Pale layers: Platelets mixed with fibrin meshwork.
  • Darker layers: Enmeshed red blood cells and leukocytes.
• Crucial Medicolegal Board Distinction: Lines of Zahn indicate that a thrombus was formed in flowing blood prior to death. In contrast, postmortem clots formed after circulation ceases are gelatinous, soft, lack Lines of Zahn, and separate by gravity into a dependent dark-red 'currant jelly' portion and a supernatant yellow 'chicken fat' layer, lying non-adherent within the vascular lumen.

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3. Embolism & Infarction

An embolus is a detached intravascular solid, liquid, or gaseous mass that is carried by the blood to a site distant from its point of origin, where it lodges in a vessel lumen and produces tissue ischemia or infarction.

1. Thromboembolism (Venous & Arterial)

• Pulmonary Thromboembolism (PE):
  • In >95% of cases, pulmonary emboli originate from Deep Vein Thromboses (DVTs) of the lower extremity proximal veins (popliteal, femoral, and iliac veins). Thrombi restricted strictly to the calf veins (soleal plexus, posterior tibial veins) have a lower risk of pulmonary embolization unless they propagate proximally above the knee.
  • Emboli travel up the inferior vena cava, pass through the right atrium and right ventricle, and lodge in the pulmonary arterial tree.
  • Saddle Embolus: A massive embolus that straddles the bifurcation of the main pulmonary artery, abruptly blocking right ventricular outflow. Causes acute cor pulmonale (acute right ventricular dilation and failure), electromechanical dissociation, and sudden death.
  • Pulmonary Infarction: Because the lung possesses a dual blood supply (pulmonary arteries and bronchial arteries), smaller peripheral pulmonary emboli cause infarction only if bronchial circulation or cardiac function is already compromised. Manifests as a wedge-shaped, pleura-based hemorrhagic infarct accompanied by pleuritic chest pain, hemoptysis, and a friction rub.
• Paradoxical Embolism:
  • A venous thrombus originates in the lower extremity deep veins but crosses into the systemic arterial circulation via an intracardiac right-to-left shunt, classically a patent foramen ovale (PFO) or an atrial septal defect (ASD), triggered by transiently elevated right atrial pressure (e.g., Valsalva maneuver, coughing).
  • The embolus bypasses the pulmonary filter and enters the systemic arterial tree, producing an embolic stroke, mesenteric ischemia, or acute lower extremity arterial occlusion (cold, pale, pulseless leg; 'blue toe').

2. Fat Embolism Syndrome (FES)

• Etiology: Follows traumatic fractures of marrow-containing long bones (especially the femur and tibia) or major pelvic fractures, as well as severe orthopedic procedures (total knee/hip arthroplasty). Ruptured intramedullary veins allow fat globules from disrupted bone marrow to enter the venous circulation.
• Pathogenesis: Mechanical microvascular occlusion of pulmonary and cerebral capillary beds by fat droplets, compounded by biochemical injury: circulating endothelial lipases hydrolyze neutral fat into toxic free fatty acids (FFAs), inducing widespread endothelial injury, capillary leakage, and acute lung injury.
• Clinical Presentation (The Classic Triad): Typically emerges after a latent asymptomatic period of 24 to 72 hours post-injury:
  1. Respiratory Failure: Sudden hypoxemia, dyspnea, tachypnea, and ARDS.
  2. Neurological Symptoms: Acute confusion, agitation, delirium, stupor, or coma (due to cerebral microemboli).
  3. Petechial Rash (Pathognomonic): A non-palpable petechial eruption localized in a 'vest' distribution across the anterior chest, neck, axillae, and conjunctiva, caused by microvascular fat embolization and thrombocytopenia.
• Laboratory Findings: Thrombocytopenia, anemia, fat globules visible on oil red O stain of urine or sputum.

3. Atheroembolism (Cholesterol Crystal Embolization)

• Pathophysiology: Dislodgement of atheromatous debris from ulcerated plaques in the abdominal aorta, iliac, or femoral arteries into small distal digital arteries (100–200 μm diameter).
• Iatrogenic Triggers: Frequently occurs following vascular interventions: arterial catheterization, endovascular aneurysm repair, peripheral angiography, vascular surgery, or initiation of thrombolytic therapy or anticoagulation.
• Podiatric Clinical Manifestations:
  • 'Blue Toe Syndrome': Sudden onset of painful, cyanotic, violaceous discoloration of one or more toes in the presence of palpable, intact pedal pulses (dorsalis pedis and posterior tibial pulses remain strong because proximal large conduct vessels are open; occlusion is strictly in terminal digital arterioles).
  • Livedo Reticularis: Lacy, purplish, reticulated vascular mottling of the foot and lower leg.
  • Microscopic ulcerations, digital gangrene, and acute kidney injury.
• Histopathology (Gold Standard): Microscopic examination of a digital or skin biopsy demonstrates pathognomonic biconvex, needle-shaped or spindle-shaped ghost clefts (representing cholesterol crystals dissolved and washed out during routine paraffin tissue processing) within the lumens of small arterioles, surrounded by a foreign body giant cell reaction and luminal thrombosis.

4. White (Anemic) vs. Red (Hemorrhagic) Infarction

An infarct is an area of ischemic necrosis caused by occlusion of either the arterial supply or the venous drainage in a particular tissue.

• White (Pale / Anemic) Infarction:
  • Occurs with arterial occlusion in solid compact organs with end-arterial circulations (organs where collateral blood flow is minimal or absent).
  • Solid organ architecture limits the amount of hemorrhage that can seep into the necrotic area from neighboring viable beds.
  • Classic Organs: Heart, Kidney, Spleen.
  • Appearance: Wedge-shaped, pale, sharply demarcated zone with apex pointing toward the occluded vessel and base at the organ capsule, undergoing coagulative necrosis.
• Red (Hemorrhagic) Infarction:
  • Occurs when blood continues to pool, leak, or recirculate into the necrotic tissue territory.
  • Characteristic Settings:
    1. Dual Blood Supply: Organs possessing two independent arterial systems. Occlusion of one arterial supply causes ischemia, while blood from the second system extravasates into the damaged, necrotic capillary beds (Lung [pulmonary and bronchial arteries]; Liver [hepatic artery and portal vein]; Small Intestine [extensive arcade collaterals]).
    2. Venous Occlusion / Strangulation: Venous outflow is blocked while arterial inflow continues under pressure, causing intense vascular congestion, rupture, and infarction (Testicular torsion, Ovarian torsion, strangulated bowel hernia).
    3. Loose Spongy Tissues: Loose stroma allows blood to collect extensively (Lung).
    4. Reperfusion Injury: Restoration of arterial flow (following thrombolysis, mechanical embolectomy, or angioplasty) into a necrotic vascular bed with damaged capillaries, flooding the dead tissue with red blood cells and generating reactive oxygen species.

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4. Shock: Systemic Circulatory Collapse

Shock is a state of profound systemic tissue hypoperfusion resulting from either impaired cardiac pump function, massive reduction in effective circulating blood volume, or widespread loss of vascular tone. It leads to widespread cellular hypoxia, failure of aerobic metabolism, lactic acidosis, and irreversible multiorgan damage.

+-----------------------------------------------------------------------------------------+
|                        HEMODYNAMIC CLASSIFICATION OF SHOCK                              |
+-----------------------+---------+---------+---------+---------+-------------------------+
| Shock Category        | Cardiac | Central | PCWP    | Systemic| Skin / Extremity        |
|                       | Output  | Venous  | (Left   | Vascular| Clinical Presentation   |
|                       | (CO)    | Pressure| Preload)| Resist. |                         |
+-----------------------+---------+---------+---------+---------+-------------------------+
| **Hypovolemic**       | Low     | Low     | Low     | High    | Cold, clammy, pale      |
| (Hemorrhage, burns)   | (v)     | (v)     | (v)     | (^)     | (peripheral vasoconstr) |
+-----------------------+---------+---------+---------+---------+-------------------------+
| **Cardiogenic**       | Low     | High    | High    | High    | Cold, clammy, cyanotic, |
| (Massive MI, HF)      | (v)     | (^)     | (^)     | (^)     | pulmonary rales, S3     |
+-----------------------+---------+---------+---------+---------+-------------------------+
| **Distributive:**     |         |         |         |         |                         |
| • **Septic (Early)**  | **High**| Normal  | Low or  | **Low** | **Warm, flushed, pink** |
|   (LPS, iNOS, TNF)    | (^)     | or Low  | Normal  | (vv)    | (vasodilation, wide PP) |
| • **Septic (Late)**   | Low (v) | High (^) | Normal  | High (^) | Cold, mottled, cyanotic |
| • **Anaphylactic**    | Low (v) | Low (v) | Low (v) | Low (vv)| Urticaria, wheezing     |
| • **Neurogenic**      | Low (v) | Low (v) | Low (v) | Low (vv)| Warm, dry; **BRADYCARDIA|
+-----------------------+---------+---------+---------+---------+-------------------------+
| **Obstructive**       | Low     | **High**| Variable| High    | Distended neck veins    |
| (Tamponade, massive PE| (v)     | (^^)    |         | (^)     | (JVD), pulsus paradoxus |
+-----------------------+---------+---------+---------+---------+-------------------------+

Clinical Classifications of Shock

1. Hypovolemic Shock:
   • Etiologies: Severe acute hemorrhage (major trauma, ruptured abdominal aortic aneurysm), severe cutaneous burns (massive plasma loss), persistent severe vomiting/diarrhea, or dehydration.
   • Pathophysiology: Primary deficit is a marked loss of intravascular blood volume, causing decreased venous return (preload), reduced end-diastolic volume, and decreased cardiac output. Baroreceptors stimulate massive compensatory sympathetic discharge, releasing norepinephrine and activating the RAAS, provoking intense systemic vasoconstriction to redirect blood to the brain and heart (high SVR, cold/clammy skin).
2. Cardiogenic Shock:
   • Etiologies: Massive myocardial infarction (loss of >40% of left ventricular muscle), acute ventricular septal rupture, severe papillary muscle rupture with acute mitral regurgitation, end-stage dilated cardiomyopathy, or sustained ventricular arrhythmias.
   • Pathophysiology: Primary pump failure. The damaged ventricle fails to eject forward flow (low CO), causing retrograde blood damming into the pulmonary circulation (markedly elevated PCWP >18 mmHg) and pulmonary edema. Compensatory peripheral vasoconstriction increases afterload (high SVR).
3. Distributive (Vasodilatory) Shock:
   • Characterized by profound loss of systemic vascular tone, causing pathological peripheral vasodilation, arteriovenous shunting, and widespread pooling of blood in venous capacitance vessels.
   • Septic Shock: Most common distributive shock. Triggered by microbial endotoxins (e.g., Lipopolysaccharide [LPS] from Gram-negative bacteria) binding to Toll-Like Receptor 4 (TLR4) / CD14 on macrophages and endothelial cells.
     • Massive release of pro-inflammatory cytokines: TNF-alpha, IL-1, IL-6, and Interferon-gamma.
     • Upregulation of Inducible Nitric Oxide Synthase (iNOS), generating torrential amounts of nitric oxide (NO) that provoke profound, refractory arteriolar smooth muscle relaxation (markedly reduced SVR).
     • Early (Warm / Hyperdynamic) Phase: Cardiac output is reflexively increased (high CO), stroke volume is maintained, and peripheral extremities are warm, flushed, and pink with bounding pulses and wide pulse pressures.
     • Late (Cold / Hypodynamic) Phase: Myocardial depression (induced by high TNF-alpha and lactic acidosis) and endothelial fluid leakage drop cardiac output (low CO), transitioning extremities to cold, cyanotic, and mottled.
   • Neurogenic Shock: Sudden interruption of sympathetic vasomotor innervation due to severe high spinal cord transection (above T6) or high spinal anesthesia. Widespread loss of sympathetic tone causes massive venous and arteriolar pooling (low SVR). Unlike all other shock variants (which feature compensatory tachycardia), neurogenic shock presents with the classic combination of hypotension accompanied by paradoxical bradycardia (due to unopposed vagal parasympathetic cardiac tone) and warm, dry skin.
   • Anaphylactic Shock: Severe systemic Type I IgE-mediated hypersensitivity. Widespread mast cell degranulation releases massive histamine, leukotrienes, and PAF, causing systemic vasodilation (low SVR), increased capillary permeability, and acute upper airway laryngeal edema with bronchospasm.
4. Obstructive Shock:
   • Mechanical obstruction to cardiac filling or forward outflow. Classic etiologies include cardiac tamponade (fluid accumulation in pericardial sac compressing ventricles; Beck's triad: hypotension, muffled heart tones, JVD), tension pneumothorax (mediastinal shift kinking the vena cava), and massive saddle pulmonary embolism. Characterized by low CO, compensatory high SVR, and severely elevated central venous pressure (CVP) with prominent jugular venous distention (JVD).

The Three Sequential Stages of Shock

1. Non-Progressive (Compensatory) Stage:
   • Compensatory neurohormonal reflex mechanisms are activated: carotid and aortic baroreceptor reflexes fire, catecholamines are released, the RAAS is activated, and vasopressin (ADH) is secreted.
   • Net effect: Tachycardia, peripheral vasoconstriction, and renal fluid conservation maintain cardiac output and blood pressure to vital organs (brain and coronary vessels) while sacrificing cutaneous, splanchnic, and renal perfusion.
2. Progressive Stage:
   • Compensatory mechanisms are overwhelmed. Persistent systemic hypoperfusion leads to widespread tissue hypoxia. Cells shift to anaerobic glycolysis, producing massive lactic acidosis.
   • Lactic acidosis blunts myocardial contractility and dilates precapillary sphincters, causing blood to pool in microvascular capillary beds. Endothelial injury promotes fluid leakage, hemoconcentration, and microthrombosis, worsening hypoperfusion in a vicious cycle. Manifests with confusion, oliguria, dyspnea, and metabolic acidosis.
3. Irreversible Stage:
   • Cellular injury becomes widespread and permanent. Severe ATP depletion causes lysosomal membranes to rupture, releasing acid hydrolases into the cytoplasm. Myocardial contractility fails completely.
   • Renal tubular necrosis produces complete anuria; ischemic bowel allows intestinal bacteria and endotoxins to enter the systemic circulation, superimposing septic shock; disseminated intravascular coagulation (DIC) exhausts clotting factors; multiorgan system failure (MOSF) ensures fatal outcome even if hemodynamic parameters are artificially restored.