7. Circulatory - Hyperemia/Congestion, Edema, Shock

What is the similarity and difference between hyperemia (active hyperemia) and congestion (passive hyperemia)?

- Similarity: Both are increased volume of blood in a tissue

- Difference: the difference is whether the blood is still in the vessels (hyperemia) or not (congestion)

Define hyperemia.

- Defined as an increased volume of blood in a tissue

What does hyperemia usually imply?

- Usually implies an active, arteriole or arteriolar-mediated process whereby dilated vessels bring increased blood flow to an area. This process may be called active hyperemia.

Explain when active hyperemia may be physiologic.

- Active hyperemia may be physiologic when there is increased blood flow to such sites as muscles during exercise, or the face when blushing occurs. At necropsy, some organs may appear hyperemia when blood pools after death.

What is the difference between hyperemia (Active hyperemia) and hemorrhage?

- Active hyperemia involves blood still in the vascular system and hemorrhage occurs when blood leaves the vascular system

Explain how tissues that are acutely hyperemic or congested look grossly.

- Red, wet, bloody.

Pathologic active hyperemia is most commonly due to _______________.

- Inflammation

Why is active hyperemia often associated with inflammation?

- The increased blood flow to an area that's needed to bring in inflammatory cells and inflammatory mediators. Inflamed tissues tend to be reddened and warm due to this process.

Describe how congestion causes tissues to appear/feel.

- Dark red or slightly purple, and tissues may feel slightly cool.

What are the most common causes of congestion?

- Local venous obstruction (acute or chronic)

- Failure of the heart to effectively pump blood through the venous system

When may acute local congestion occur. Provide an example.

- May occur in a tissue where a vein is thrombosed or otherwise obstructed (can also happen due to organ twisting like GDC or equine gut volvulus)

- An example is a type of equine colic where a section of intestine has torsed or twisted, causing strangulation from venous pressure

When might chronic local congestion occur? Provide examples.

- Occurs more slowly

- Examples are tumors or abscesses that slowly compress veins (collateral circulation can develop leading to no clinical signs). Another example is cirrhosis (fibrosis) of the liver, compressing the hepatic veins

What is chronic generalized congestion usually caused by?

1) Lesions in the heart leading to CHF

2) Primary lung disease leading to pulmonary hypertension

In heart disease, either the _____________ or the ______________ becomes congested depending on which side of the heart the lesion arises from.

- Lung

- Liver

What are common right-sided lesions of the heart leading to right-sided heart failure?

- Pulmonic stenosis or tricuspid insufficiency, causing blood to back up into the vena cava

Describe the pathogenesis and appearance of a liver with chronic passive congestion.

- There will be dilation of central veins, hypoxia, atrophy and loss of hepatocytes, and dilation of sinusoids. Because central veins and sinusoids are dilated and dark red, and parenchyma in the portal areas remain unaffected, this type of liver is described is a “nutmeg liver,” referring to its mottled gross appearance.

What are common left-sided lesions of the heart leading to right-sided heart failure?

- Aortic stenosis and mitral valve insufficiency cause blood to back up into the left atrium and pulmonary veins, leading to increased hydrostatic pressure in the lungs and they become heavy, wet, and edematous.

What is a common clinical sign of left-sided heart failure?

- A chronic, nonproductive cough

Describe the pathogenesis and appearance of a lung with chronic passive congestion.

- Capillaries become dilated, fluid leaks out (edema), and very small capillary ruptures may occur over time. The RBCs that are released in these micro-hemorrhages are phagocytized by macrophages that become stuffed with hemosiderin, which is a golden-brown pigment. These cells are called “heart-failure cells” when they occur in the lung.

What is a common description of the appearance of the liver when a patient is experiencing right-sided heart failure? What is commonly found in the lung of patients with left-sided heart failure?

- Nutmeg liver

- Heart failure cells

What is edema?

- A collection of excessive fluid in interstitial tissues or body cavities.

Define anasarca.

- Generalized, severe subcutaneous edema

Define hydrothorax.

- Edema of the pleural cavity

Define hydropericardium.

- Edema of the pericardial sac

Define hydroperitoneum.

- Edema of the peritoneal cavity (also known as ascites)

In general, what does edema result from?

- An imbalance between the hydrostatic and osmotic forces in the vasculature and the interstitium

What are the four primary mechanisms by which edema can occur?

1) Increased hydrostatic pressure

2) Reduced plasma osmotic pressure

3) Decreased lymphatic drainage

4) Increased permeability of vessels

Explain how increased hydrostatic pressure can cause edema. Provide examples.

- Due to increased volume of blood in the vessels, most often due to passive accumulation of blood (congestion) but may be increased flow of blood (hyperemia)

- Examples: CHF (edema may occurs primarily in the lungs, or become more generalized over time), venous thrombosis (usually localized edema), or sodium retention (induced by activation of RAAS)

Explain how reduced plasma osmotic pressure can cause edema. Provide examples.

- Due to hypoproteinemia (especially hypoalbuminemia)

- Examples: Parasitism (causes loss of blood through GI tract), protein loss through the kidneys (glomerulopathy, nephrotic syndrome), lack of adequate protein either because of poor diet or impaired absorption, severe liver disease/liver failure (lack of albumin production), severe burns (plasma exudate)

What are some example cases of reduced plasma osmotic pressure causing edema?

- Bottle jaw: Hypoproteinemia due to parasitism and gravity

- Bovine edema: Hypoalbuminemia due to Tansy Ragwort toxicity and liver failure

What are the three primary ways in which decreased lymphatic drainage causes edema?

- Inflammatory

- Neoplastic

- Fibrosis

- Note: All compress/obstruct the lymphatic vessel

How does increased permeability of vessels cause edema? Provide examples.

- Initial reaction to some stimuli (most often inflammation or immunologic stimuli)

- Examples: Inflammation, allergic reaction, appearance of edema)

Describe the gross appearance of edematous tissue.

- Edematous tissues look swollen, may feel slightly cool, and "pit" or dent when pressed with a finger. They are heavier than normal and ooze clear or pale, yellow fluid when cut.

Is edema generally a primary or secondary disease process?

- Secondary

What does the term shock basically imply?

- Peripheral circulatory failure with inadequate perfusion of tissue cells. Cellular hypoxia then causes metabolic derangements, cellular injury, and often leads to further circulatory failure

Are the changes associated with shock reversible?

- Up to a certain point, but at some point become irreversible leading to cell death and many times, the death of the patient.

What are clinical signs of shock?

- Hypotension

- Tachycardia

- Tachypnea

- Weak pulses

- Cool skin (hypothermia)

- Pale MMs

- Increased CRT

- Mental confusion (unresponsiveness)

What are the basic types of shock and how do they differ?

1) Hypovolemia shock

2) Cardiogenic shock

3) Blood maldistribution (anaphylactic shock, neurogenic shock, septic shock)

- The pathogenesis of shock becomes common at some point, but the initiating causes differ

Explain the pathogenesis of hypovolemic shock.

- Because of hemorrhage or fluid loss due to, for example, massive burns, vomiting, or diarrhea, blood volume is reduced.

This is associated with decreased vascular pressure and tissue hypoperfusion.

What are the body's compensatory mechanisms for hypovolemic shock?

- Vasoconstriction

- Fluid movement into the plasma

- RAAS (increase blood volume)

Explain when the body can compensate for hypovolemic shock and when it cannot.

- When the injury is mild, compensation can be successful and homeostasis is restored.

- When there is 35-45% loss of blood, blood pressure and cardiac output can fall dramatically.

What is the first and most important thing you can do as a clinician to reverse hypovolemic shock?

- IV catheterization and fluid therapy

Explain the pathogenesis of cardiogenic shock.

- This occurs in the failure of the heart ot successfully maintain blood flow. It can occur in myocardial infarction, pulmonary embolism, heart rupture, or cardiac tamponade (filling of the pericardium with blood)

What are the body's compensatory mechanisms for cardiogenic shock?

- The heart tries to compensate with increased heart contractility, stroke volume, and heart rate

What is blood maldistribution (as a cause of shock) characterized by?

- This is characterized by vasodilation (decreased vascular resistance) and pooling of blood in peripheral tissues. The actual blood volume doesn’t change, but the effective circulating blood volume is decreased because the vascular area is increased (a type of hypovolemia).

Anaphylactic shock occurs due to generalized type ___________ hypersensitivity.

- I

Explain what happens during anaphylactic shock. What is a common clinical sign?

- In this form of shock, there is systemic vasodilation due to release of mediators, including cytokines that are triggered by either an immunologic (IgE mediated) or non-immunologic (release of basophil and/or mast cell granule) mechanism.

- A significant drop in blood pressure is typical of this form of shock.

Explain what happens in neurogenic shock. What causes it?

- In this type of shock, the nervous system responds to certain types of stimuli by causing massive peripheral vasodilation.

- It may be caused by trauma to the nervous system, such as that caused by electrocution, lightning strike, fear or emotional stress. In wild animals, stress or capture and/or handling can be a cause.

What is the most common form of blood maldistribution shock?

- Septic shock

Explain what happens in septic shock.

- Peripheral vasodilation occurs, most often due to the presence of an endotoxin (LPS)

What are the stages of shock?

- Nonprogressive shock

- Progressive shock

- Irreversible shock

When does nonprogressive shock occur?

- Occurs when compensatory mechanisms kick in and homeostasis is restored.

When does progressive shock occur?

- Occurs when compensatory mechanisms are not adequate, tissues are not perfused, and cell injury progresses.

Explain what happens in irreversible shock.

- Metabolic acidosis occurs, metabolic and enzyme systems shut down, ischemia progresses, and multiple organ failure results

The failure of one organ system often leads to...

- The failure of another organ system/others

The lesions of shock depend on what factors?

- The duration and cause of the shock

What are some lesions of shock?

- Generalized congestion

- Pooling of blood

- Edema

- Hemorrhage

- Microthrombosis

- Cell degeneration

- Necrosis

What are the most common organs visibly affected by shock?

- Adrenal gland

- Kidneys

- Heart

- Brain

- GI tract

- Lungs