13.1 Shock States, Sepsis & Systemic Inflammatory Response

13.1 Shock States, Sepsis & Systemic Inflammatory Response

Shock is defined as a state of systemic cellular dysoxia resulting from an imbalance between oxygen delivery ($DO_2$) and oxygen demand ($VO_2$). When tissue perfusion falls below critical levels, cells switch to anaerobic metabolism, leading to cellular lactic acidosis, organ failure, and death if uncorrected. For USMLE Step 2 CK, mastering shock requires distinguishing the distinct hemodynamic profiles of the four major shock categories and applying time-sensitive clinical bundles for sepsis and septic shock.

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Hemodynamic Classification of Shock States

Shock is broadly categorized into four major etiologic classes based on primary intravascular and cardiac dynamics: hypovolemic, cardiogenic, obstructive, and distributive.

1. Hypovolemic Shock

Hypovolemic shock results from a critical loss of intravascular volume, either due to acute hemorrhage (trauma, GI bleeding, ruptured aneurysm) or non-hemorrhagic fluid loss (severe vomiting, diarrhea, burns, third-spacing).

• Primary disturbance: Decreased preload, reflected by a reduced Pulmonary Capillary Wedge Pressure (PCWP) or Central Venous Pressure (CVP).
• Compensatory response: Decreased stroke volume causes a drop in Cardiac Output (CO) / Cardiac Index (CI), triggering compensatory sympathetic activation that increases Systemic Vascular Resistance (SVR) via vasoconstriction to maintain blood pressure.
• Mixed Venous Oxygen Saturation ($SvO_2$): Decreased ($< 65%$) due to increased peripheral oxygen extraction from sluggish tissue perfusion.

2. Cardiogenic Shock

Cardiogenic shock stems from primary cardiac pump failure resulting in inadequate tissue perfusion despite adequate intravascular volume. Common causes include acute myocardial infarction (occupying $> 40%$ of left ventricular mass), acute mitral regurgitation secondary to papillary muscle rupture, acute ventricular septal rupture, and end-stage dilated cardiomyopathy.

• Primary disturbance: Severe impairment of myocardial contractility leading to decreased CO/CI ($< 2.2 \text{ L/min/m}^2$).
• Hemodynamic findings: Elevated PCWP ($> 18 \text{ mmHg}$) due to left ventricular end-diastolic fluid overload and backing up of pressure into the pulmonary vasculature, alongside reflex vasoconstriction causing high SVR ($> 1200 \text{ dynes}\cdot\text{sec/cm}^5$).
• $SvO_2$: Markedly decreased ($< 55%$) secondary to low tissue perfusion and maximal tissue oxygen extraction.

3. Obstructive Shock

Obstructive shock occurs when physical obstruction to cardiac inflow or outflow impairs ventricular filling or stroke volume. Causes include tension pneumothorax, cardiac tamponade, massive pulmonary embolism (PE), and restrictive pericarditis.

• Hemodynamic profile: Characterized by low CO/CI and elevated compensatory SVR.
• Preload parameters: Differ by etiology:
  • Cardiac Tamponade: Equalization of end-diastolic pressures across all cardiac chambers (CVP = Right Ventricular End-Diastolic Pressure = PCWP) with classic Beck triad (hypotension, jugular venous distension, muffled heart sounds) and pulsus paradoxus ($> 10 \text{ mmHg}$ drop in SBP during inspiration).
  • Massive PE: Markedly elevated CVP with normal or low PCWP (right ventricular failure without left atrial overload).
  • Tension Pneumothorax: Elevated CVP with decreased breath sounds and tracheal deviation away from the affected side.

4. Distributive Shock

Distributive shock is caused by profound systemic vasodilation and increased capillary permeability, leading to functional hypovolemia despite normal or elevated total intravascular volume. Causes include sepsis (most common), anaphylaxis, and neurogenic shock (spinal cord injury above T6 causing loss of sympathetic tone).

• Primary disturbance: Severe fall in SVR ($< 800 \text{ dynes}\cdot\text{sec/cm}^5$).
• Compensatory response: To maintain perfusion, cardiac output increases, creating a hyperdynamic state with elevated CO/CI ($> 4.0 \text{ L/min/m}^2$) and warm, flushed extremities.
• PCWP/CVP: Normal or low due to venous pooling and capillary leak.
• $SvO_2$: Uniquely elevated ($> 70%$) in early septic shock due to microvascular shunting and cellular inability to utilize oxygen (cytopathic hypoxia).

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Hemodynamic Parameters Comparison

The following table summarizes the diagnostic hemodynamic profiles measured via Pulmonary Artery Catheter (Swan-Ganz) placement on USMLE Step 2 CK:

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Sepsis-3 Definitions and Pathophysiology

The Sepsis-3 consensus guidelines redefined sepsis and septic shock, discarding the historical Systemic Inflammatory Response Syndrome (SIRS) criteria for official sepsis diagnosis due to lack of specificity.

Sepsis vs. Septic Shock

• Sepsis: Defined as a life-threatening organ dysfunction caused by a dysregulated host response to infection. Organ dysfunction is quantified as an acute change in total Sequential Organ Failure Assessment (SOFA) score of $\ge 2$ points attributable to the infection.
• Septic Shock: A subset of sepsis in which particularly profound circulatory, cellular, and metabolic abnormalities increase mortality. Diagnosed when patients satisfy sepsis criteria AND present with:
  1. Persistent hypotension requiring vasopressors to maintain a Mean Arterial Pressure (MAP) $\ge 65 \text{ mmHg}$, AND
  2. Serum lactate $> 2.0 \text{ mmol/L}$ ($> 18 \text{ mg/dL}$) despite adequate fluid resuscitation ($30 \text{ mL/kg}$ crystalloid).

Bedside Screening: qSOFA and SIRS

Outside the ICU, the quick SOFA (qSOFA) tool rapidly identifies patients at risk for deterioration. A score $\ge 2$ prompts immediate investigation for sepsis:

$\text{qSOFA Criteria (1 point each):}$

• Respiratory rate $\ge 22 \text{ breaths/min}$
• Altered mental status (Glasgow Coma Scale score $< 15$)
• Systolic blood pressure $\le 100 \text{ mmHg}$

(Note: Historical SIRS criteria required $\ge 2$ of: Temp $>38^\circ\text{C}$ or $<36^\circ\text{C}$; HR $>90$; RR $>20$ or $\text{PaCO}_2 <32 \text{ mmHg}$; WBC $>12,000$, $<4,000$, or $>10%$ bands).

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Diagnostic and Resuscitation Bundles

Management of septic shock is time-critical. The Surviving Sepsis Campaign emphasizes the 1-Hour Resuscitation Bundle:

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|                            1-HOUR SEPSIS BUNDLE ACTIONS                           |
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| 1. Measure serum lactate level (remeasure if initial lactate > 2 mmol/L).          |
| 2. Obtain blood cultures prior to administration of antibiotics.                  |
| 3. Administer empiric broad-spectrum IV antibiotics (e.g., Cefepime + Vancomycin). |
| 4. Rapidly infuse 30 mL/kg IV crystalloid for hypotension or lactate >= 4 mmol/L. |
| 5. Apply vasopressors (Norepinephrine) if MAP < 65 mmHg during/after fluid resus.|
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Vasopressor and Inotropic Management

• First-line Vasopressor: Norepinephrine (potent $\alpha_1 > \beta_1$ agonist) is the initial drug of choice to restore vascular tone without excessive tachycardia. Target MAP is $\ge 65 \text{ mmHg}$.
• Second-line Vasopressor: Vasopressin ($0.03 \text{ units/min}$) is added to norepinephrine to raise MAP or reduce norepinephrine dosage.
• Inotrope: Dobutamine ($\beta_1 > \beta_2$ agonist) is added if myocardial dysfunction persists (elevated PCWP with low CO) despite adequate volume and MAP.
• Corticosteroids: IV Hydrocortisone ($200 \text{ mg/day}$) is indicated ONLY in refractory septic shock where fluid resuscitation and vasopressors fail to restore hemodynamic stability.