Understanding Shock: Types, Causes, and Effects

• Shock is circulatory failure impairing tissue perfusion and leading to cellular hypoxia
• Untreated shock can lead to multiple organ failure and death

Types of Shock

• Cardiogenic shock is pump failure
  • Low cardiac output means tissues and organs receive insufficient oxygen and nutrients
  • Etiology includes myocardial infarction, ventricular rupture, arrhythmias, cardiac tamponade, pulmonary embolism
  • The body compensates by increasing the heart rate (tachycardia) and vascular resistance, but these mechanisms eventually fail
• Hypovolemic shock is low blood volume
  • A loss of blood or fluid reduces preload
  • This leads to low stroke volume and decreased cardiac output
  • Compensatory mechanisms (vasoconstriction, tachycardia) attempt to maintain blood pressure
  • Etiology can be hemorrhagic (blood loss) or non-hemorrhagic (fluid loss)
• Septic shock is infection-induced vasodilation and inflammation
  • Infection triggers a systemic inflammatory response (SIRS)
  • Vasodilation and endothelial dysfunction lead to leaky blood vessels, causing fluid loss into tissues
  • Widespread coagulation activation (DIC) results in microthrombi formation
  • Tissue hypoxia leads to multiple organ failure if untreated
  • Etiology can be overwhelming microbial infection, severe trauma or burns, or pancreatitis

Classifications of Shock

• Distributive shock involves severe vasodilation and blood pooling, where blood vessels lose their tone
• Septic shock is caused by infection-induced SIRS
  • High cardiac output with low vascular resistance leads to hypoperfusion
• Anaphylactic shock is caused by severe IgE-mediated hypersensitivity reaction
  • Massive histamine release causes vasodilation, airway constriction, and fluid leakage, leading to cardiovascular collapse and respiratory distress
• Neurogenic shock is caused by spinal cord injury or brain trauma, disrupting autonomic regulation
  • Loss of sympathetic tone leads to unopposed parasympathetic activity, causing severe hypotension and bradycardia
• Endocrine shock is caused by endocrine failure
  • Adrenal insufficiency (lack of cortisol) leads to poor stress response
  • Myxedema coma (severe hypothyroidism) leads to low metabolism and vasodilation
• Hypovolemic shock involves loss of circulating volume
  • Hemorrhagic hypovolemic shock is caused by blood loss
  • Reduced vascular volume leads to inadequate cardiac output
  • Non-hemorrhagic hypovolemic shock is caused by GI losses, renal losses, skin losses, or third-space losses
• Cardiogenic shock involves intracardiac causes
  • Acute myocardial infarction causes cardiac muscle damage
  • Myocarditis causes inflammation of the heart muscle
  • Arrhythmias causes abnormal rhythms, reducing output
  • Valvular heart disease creates mechanical obstruction of blood flow
• Obstructive shock involves extra cardiac causes
  • Pulmonary embolism causes a clot that blocks pulmonary circulation, increasing right heart strain
  • Tension pneumothorax causes a collapsed lung that compresses heart and vessels
  • Cardiac tamponade is fluid accumulation restricting heart movement

Pathogenesis of Septic Shock

• Microorganisms trigger inflammatory cells, causing a cytokine storm
• Endothelial activation and injury increase vascular permeability which causes fluid leaks out of vessels
• Damaged endothelial tissue activates the coagulation cascade, resulting in DIC
• Uncontrolled clotting leads to microvascular thrombosis, causing tissue ischemia and organ dysfunction
• Cytokines and reactive oxygen species released by leukocytes damage the mitochondria.
  • Impaired oxidative phosphorylation leads to cells not being able to generate enough ATP, reflecting metabolic abnormalities
  • Mitochondrial dysfunction leads to cellular energy failure due to a shift toward anaerobic metabolism which produces lactic acid and causes metabolic acidosis
  • failure to generate ATP also causes sodium-potassium pump dysfunction, leading to cellular swelling and apoptosis
• These combined effects lead to multiple organ dysfunction syndrome (MODS) that is the failure of lungs, kidneys, heart, and brain
• Irreversible stage leads to death due to cardiogenic shock and end-organ failure

Stages of Shock

• Initial stage: oxygen delivery slightly decreases, but compensatory mechanisms prevent major symptoms
  • Cells still rely on aerobic metabolism, but oxygen extraction increases to compensate
  • ATP production remains sufficient for function
  • Clinical signs are often subtle or absent, as vital organs still receive adequate perfusion
• Non-progressive stage (compensated shock): the body detects low blood pressure or reduced perfusion and activates additional compensatory mechanisms
  • Activation of baroreceptors increases the heart rate and vasoconstriction
  • Baroreceptors stimulate the sympathetic nervous system, causing increased heart rate (tachycardia), which aims to raise blood pressure and increases myocardial contractility
  • RAAS activation: kidneys retain sodium and water to increase blood pressure
  • Low renal perfusion pressure stimulates the renin-angiotensin-aldosterone system (RAAS)
  • Hypothalamus detects hypovolemia, causing ADH (vasopressin) release from the posterior pituitary, which leads to an increase in water reabsorption in the kidneys and vasoconstriction
• Progressive stage (decompensated shock): hypoxia worsens, leading to anaerobic metabolism and lactic acidosis, while vasodilation causes worsening hypotension
• Irreversible stage: cellular death leads to multiple organ failure and death

Morphology of Shock (Tissue Changes Due to Hypoxia)

• Microvascular thrombosis (clots in small vessels)
• Fibrin thrombi in kidneys, leading to acute kidney injury
• Adrenocortical lipid depletion, leading to adrenal gland dysfunction
• Diffuse alveolar damage, leading to pulmonary dysfunction and acute respiratory distress syndrome (ARDS)

Clinical Features of Shock

• Hypotension (low blood pressure): blood flow cannot sustain organ function
• Weak, rapid pulse: compensation for low cardiac output
• Cold, clammy, cyanotic skin: vasoconstriction to preserve vital organs
• Cardiac, cerebral, and pulmonary dysfunction: altered mental status, shortness of breath
• Worsening renal function: reduced urine output (oliguria)