What is shock?

What is Shock?

Shock is a state of acute circulatory failure resulting in inadequate delivery of oxygen and nutrients to tissues, leading to cellular dysfunction, metabolic abnormalities, and end-organ damage that can progress to death if untreated. 1, 2

Core Pathophysiology

Shock fundamentally represents a mismatch between oxygen delivery and cellular metabolic demands, characterized by:

• Widespread tissue hypoperfusion causing cellular hypoxia and anaerobic metabolism 3, 4
• Lactic acidosis accumulation as a marker of inadequate tissue perfusion and anaerobic metabolism 3, 4
• Progressive organ dysfunction that becomes irreversible if prolonged 3, 2

The circulatory failure stems from derangements in one or more of four fundamental mechanisms: intravascular volume, myocardial function, systemic vascular resistance, or blood flow distribution 3.

Four Primary Categories of Shock

1. Hypovolemic Shock

• Results from inadequate intravascular volume due to hemorrhage, fluid losses, or dehydration 1, 3

2. Cardiogenic Shock

• Defined as the heart's inability to maintain effective cardiac output commensurate with the body's metabolic demands due to primary cardiac pathology 5
• Most commonly caused by acute myocardial infarction, but also valvular disease, cardiomyopathy, pericardial disease, or arrhythmia 5
• Characterized by hypotension, tachycardia, peripheral vasoconstriction, pulmonary/systemic venous congestion, decreased urine output, altered mental status, and acute liver or kidney injury 5

3. Distributive Shock

• Septic shock is the most clinically significant subtype, defined by the Society of Critical Care Medicine as requiring vasopressor therapy to maintain mean arterial pressure ≥65 mmHg AND serum lactate >2 mmol/L (>18 mg/dL) despite adequate fluid resuscitation 6, 7
• Involves profound vasodilation, increased vascular permeability, and microcirculatory dysfunction 6, 7
• Hospital mortality exceeds 40% when both vasopressor requirement and elevated lactate are present 6

4. Obstructive Shock

• Caused by mechanical obstruction to cardiac output (e.g., pulmonary embolism, tension pneumothorax, cardiac tamponade) 1, 3

Clinical Recognition

Shock manifests through multiple organ systems:

• Cardiovascular: Hypotension (systolic BP <90 mmHg for >30 minutes), tachycardia, weak pulses 5
• Neurologic: Altered mental status, confusion, obtundation 5
• Renal: Oliguria, acute kidney injury 5
• Metabolic: Elevated lactate levels, metabolic acidosis 6, 3
• Skin: Cool, clammy extremities with peripheral vasoconstriction (except distributive shock) 5

Critical Diagnostic Pitfalls

• Failing to measure serum lactate, which is essential for diagnosing septic shock and assessing tissue perfusion 6, 7
• Assuming adequate perfusion based on normal blood pressure in patients receiving vasopressors—perfusion abnormalities may persist despite normalized BP 6, 7
• In elderly patients, attributing altered mental status to dementia rather than investigating for shock, as mental status changes may be the primary presenting sign 8
• Underestimating severity in elderly patients who may have less dramatic vital sign abnormalities despite significant organ dysfunction 8

Compensatory Mechanisms

The body attempts to preserve vital organ perfusion through:

• Sympathoadrenal activation increasing heart rate, contractility, and peripheral vasoconstriction 3, 4
• Neurohumoral responses to maintain cardiac output and blood pressure 4
• These compensatory mechanisms may initially mask the severity of shock, delaying recognition 3