Shock (circulatory) Study Guide

📖 Core Concepts Shock – inadequate tissue perfusion caused by circulatory failure → cellular hypoxia, metabolic acidosis. Types of shock – hypovolemic (low volume), cardiogenic, obstructive, distributive (septic, anaphylactic, neurogenic). Compensatory mechanisms – baroreceptor‑mediated catecholamine release, renin‑angiotensin‑aldosterone system, antidiuretic hormone → tachycardia, vasoconstriction, sodium/water retention. Key vitals – low systolic BP, narrowed pulse pressure, tachycardia (unless β‑blocker/athlete), cold extremities, oliguria. Shock index (SI) – $SI = \dfrac{\text{HR}}{\text{SBP}}$; values > 0.9 suggest significant hypovolemia. --- 📌 Must Remember Low BP + narrowed pulse pressure = red flag for shock. Elevated lactate → tissue hypoxia; treat aggressively. Fluid bolus: adults 1–2 L NS over 10 min (or 20 mL/kg in children). Urine output goal: > 0.5 mL kg⁻¹ h⁻¹. CVP target: 8–12 mm Hg; MAP target: 65–95 mm Hg. Permissive hypotension (SBP 70–90 mm Hg) only for penetrating torso hemorrhage. Vasopressors: norepinephrine first‑line; dopamine ↑ arrhythmia risk. Anaphylactic shock → immediate IM epinephrine. Sodium bicarbonate only if pH < 7.0. --- 🔄 Key Processes Cellular hypoxia → lactic acidosis ↓ perfusion → anaerobic glycolysis → ↑ lactate → metabolic acidosis. Compensatory cascade Baroreceptors → ↑ epinephrine (↑ HR) + norepinephrine (vasoconstriction). Renin–angiotensin activation → angiotensin II (vasoconstriction, aldosterone). ADH release → water reabsorption, ↑ CVP. Fluid resuscitation algorithm Rapid crystalloid bolus → reassess MAP, CVP, urine output. If Hb < 100 g/L after fluids → transfuse packed RBCs. Persistent hypotension → start vasopressor (norepinephrine). Vasopressor selection Norepinephrine: ↑ MAP, minimal tachycardia. Phenylephrine: pure α‑agonist (↑ SVR, can ↓ HR). Dobutamine: β‑agonist for low cardiac output with adequate BP. --- 🔍 Key Comparisons Hypovolemic vs. Cardiogenic Shock Pre‑load: ↓ (hypovolemic) vs. ↓ (cardiogenic) due to pump failure. JVP: flat/low (hypovolemic) vs. distended (cardiogenic). Pulmonary findings: clear lungs (hypovolemic) vs. crackles, congestion (cardiogenic). Obstructive vs. Cardiogenic Shock Cause: physical blockage (tamponade, PE) vs. myocardial pump failure. Imaging: ultrasound shows pericardial fluid or RV strain in obstructive. Septic vs. Anaphylactic Shock (Distributive) Mediator: cytokines/histamine (septic) vs. massive histamine/IgE (anaphylactic). Skin: warm, flushed (septic) vs. hives, itching, swelling (anaphylactic). Norepinephrine vs. Dopamine Arrhythmia risk: higher with dopamine. α/β balance: norepinephrine → strong α, modest β; dopamine → dose‑dependent α/β. --- ⚠️ Common Misunderstandings “All shock patients are tachycardic.” β‑blockers, athletes, intra‑abdominal bleed can mask tachycardia. “Give as much fluid as possible.” Over‑resuscitation worsens pulmonary edema; use MAP, CVP, urine output to guide. “Any vasopressor works equally.” Choice depends on underlying physiology; norepinephrine is first‑line for most. “Sodium bicarbonate corrects lactic acidosis.” It only helps when pH < 7.0; otherwise it may increase CO₂ load. --- 🧠 Mental Models / Intuition “Perfusion = Pressure × Flow.” If pressure falls (BP ↓) or flow is blocked (tamponade, PE), perfusion drops → shock. “The 4‑C’s of Shock” – Circulation (BP, pulse pressure), Capillary refill, Confusion, Cold extremities. Spotting all 4 suggests advanced shock. Shock Index Rule: HR ≈ SBP → SI ≈ 1. When HR > SBP, think severe hypovolemia. --- 🚩 Exceptions & Edge Cases Permissive hypotension is contraindicated in traumatic brain injury or spinal cord injury (risk of cerebral hypoperfusion). Neurogenic shock → bradycardia (loss of sympathetic tone) – opposite of typical tachycardia. Severe hypothyroidism can mimic cardiogenic shock (low output, bradycardia) despite normal volume. --- 📍 When to Use Which | Situation | Preferred Fluid | Preferred Vasopressor | Rationale | |-----------|----------------|----------------------|-----------| | General hypovolemic shock | Crystalloid (NS or balanced) 20 mL/kg | Norepinephrine if MAP < 65 mm Hg after fluids | Rapid volume replacement; norepinephrine restores SVR. | | Penetrating torso hemorrhage | Crystalloid bolus → permissive hypotension (SBP 70–90 mm Hg) | Norepinephrine only after bleeding control | Limits further bleeding while maintaining vital organ perfusion. | | Cardiogenic shock with low MAP | Limited fluids; consider early inotrope | Dobutamine (β‑agonist) ± norepinephrine | Improves contractility without excessive preload. | | Anaphylactic shock | Small crystalloid bolus if needed | IM epinephrine 0.3 mg (1 mg/mL) → IV norepinephrine if refractory | Epinephrine reverses bronchospasm, vasodilation; norepinephrine maintains MAP. | | Septic shock | Balanced crystalloids; early goal‑directed resuscitation | Norepinephrine first‑line | Targets vasodilation‑driven hypotension. | | Obstructive shock (tamponade) | Minimal fluid; urgent pericardiocentesis | Norepinephrine if MAP remains low after decompression | Removing obstruction is definitive; vasopressor bridges until flow restored. | --- 👀 Patterns to Recognize Narrow pulse pressure + cool, clammy skin → early hypoperfusion. Distended neck veins + muffled heart sounds → cardiac tamponade (obstructive). Rapid shallow breathing + metabolic acidosis → hypovolemic (sympathetic drive). Widespread vasodilation + warm skin → distributive (septic or anaphylactic). Elevated lactate >2 mmol/L in any shock type → need aggressive resuscitation. --- 🗂️ Exam Traps “Give massive fluids to any shock patient.” Trap: Over‑resuscitation in cardiogenic/obstructive shock worsens pulmonary edema. “Dopamine is the best vasopressor for all shock.” Trap: Higher arrhythmia risk; norepinephrine has better outcomes. “Normal heart rate rules out shock.” Trap: β‑blocker use, athletes, or intra‑abdominal bleed can mask tachycardia. “All distributive shock is septic.” Trap: Anaphylactic and neurogenic subtypes have distinct triggers and treatments. “Sodium bicarbonate corrects any low pH.” Trap: Only indicated when pH < 7.0; otherwise may cause CO₂ retention. ---