Shock — Cardiogenic, Distributive, Hypovolemic

• Cardiogenic Shock Mechanism: Reduced cardiac contractility (systolic dysfunction) or filling impairment (diastolic dysfunction) causes decreased stroke volume (SV) and cardiac output (CO), despite compensatory tachycardia and vasoconstriction. The Frank-Starling mechanism becomes unfavorable as left ventricular (LV) diastolic pressure increases, resulting in pulmonary and systemic venous congestion. In acute MI with large infarcts (typically >40% LV mass), loss of contractile tissue overwhelms compensatory mechanisms. Beta-adrenergic stimulation increases myocardial oxygen demand in an already-ischemic myocardium, worsening the supply-demand mismatch and perpetuating cellular injury. Neurohormonal activation (sympathetic nervous system, renin-angiotensin-aldosterone system) increases systemic vascular resistance (SVR) in an attempt to maintain blood pressure, but this increases cardiac afterload and further reduces forward flow—a counterproductive "vicious cycle." Pulmonary edema develops from hydrostatic pressure transmission backward to pulmonary capillaries, impairing gas exchange and worsening hypoxemia.

• Hypovolemic Shock Mechanism: Blood volume loss (acute hemorrhage typically >20% blood volume or >1 L in adults, or severe fluid loss from diarrhea, burns, or third-spacing) reduces venous return and preload. Decreased preload diminishes stroke volume via the Frank-Starling mechanism, triggering profound compensatory vasoconstriction through baroreceptor reflexes and catecholamine release. Initially, systemic vascular resistance (SVR) increases dramatically to maintain mean arterial pressure (MAP) despite reduced CO, creating a "warm, well-perfused" early phase. However, intense vasoconstriction causes tissue hypoperfusion, lactate accumulation, and metabolic acidosis. Prolonged vasoconstriction also triggers capillary injury from hypoxia-reperfusion mechanisms, activating endothelial damage and increasing vascular permeability. This "second hit" transitions shock from reversible (compensated) to irreversible (decompensated) phases, where fluid resuscitation alone becomes insufficient without addressing underlying bleeding or ongoing fluid losses.

• Distributive (Septic) Shock Mechanism: Infection-triggered release of bacterial lipopolysaccharides (LPS) and damage-associated molecular patterns (DAMPs) activates pattern recognition receptors (TLRs, NOD-like receptors) on macrophages and dendritic cells. This triggers a cytokine cascade—IL-6, TNF-α, IL-1β—that induces inducible nitric oxide synthase (iNOS) in vascular endothelium and smooth muscle. Massive nitric oxide (NO) production causes profound vasodilation and loss of vascular tone, resulting in distributive shock with decreased SVR and increased venous capacitance. Despite compensatory increases in CO (through tachycardia and increased contractility), maldistribution of blood flow creates regional hypoperfusion and shunting away from critical organs. Endothelial dysfunction increases capillary permeability through loss of adherens junctions (involving VE-cadherin), allowing fluid and proteins to leak into interstitium—causing hypovolemia despite increased total body water. Mitochondrial dysfunction from oxidative stress and impaired oxygen utilization prevents tissues from extracting and using oxygen even when oxygen delivery is adequate, a state termed cytopathic hypoxia. Coagulopathy develops through activation of tissue factor and thrombin generation, with simultaneous fibrinolysis (DIC), leading to microvascular thrombosis, organ ischemia, and bleeding complications.

Cardiogenic Shock

• Acute Myocardial Infarction (most common cause, 80% of cases): Large anterior or inferior wall MI with significant loss of ventricular function (typically >40% LV involvement) causes acute pump failure. Mechanical complications include acute mitral regurgitation (papillary muscle rupture), ventricular septal defect (VSD), and free wall rupture. Right ventricular infarction, while typically inferior, can severely impair RV contractility and reduce LV preload (RV dysfunction is preload-dependent), precipitating shock disproportionately to LV involvement. Post-MI Takotsubo cardiomyopathy (stress-induced) with apical ballooning and severely reduced ejection fraction can present with fulminant shock.

• Decompensated Heart Failure: Acute exacerbation of chronic systolic or diastolic heart failure, often triggered by infection, arrhythmia, renal insufficiency, medication non-adherence, or ischemia. Acute diastolic dysfunction from severe hypertensive emergency or restrictive physiology (tamponade, constrictive pericarditis) reduces ventricular filling and forward flow.

• Arrhythmias: Sustained ventricular tachycardia or atrial fibrillation with rapid ventricular response eliminating the atrial kick can precipitate shock, particularly in patients with baseline reduced ejection fraction. Severe bradycardia (<40 bpm) reduces CO despite increased stroke volume compensation.

• Valvular Emergencies: Acute severe aortic regurgitation (from endocarditis, aortic dissection, or mechanical failure) causes acute LV volume overload; acute mitral stenosis (rheumatic, prosthetic thrombosis) limits filling; prosthetic valve thrombosis causes sudden pump failure.

• Fulminant Myocarditis: Viral (enterovirus, adenovirus, EBV), autoimmune, or toxic (chemotherapy) myocarditis with acute inflammation and extensive myocardial necrosis can present with severe systolic dysfunction and shock even in young patients without prior heart disease.

• Massive Pulmonary Embolism: Acute increase in RV afterload from pulmonary vascular obstruction causes acute RV dilation, septal shift, LV compression, and reduced LV preload—manifesting as cardiogenic shock with right heart strain despite normal LV contractility. Produces elevated central venous pressure (CVP) despite low CO.

• Risk factors for development: Advanced age, diabetes, anterior MI location, prior MI, female gender, multivessel coronary disease.

Hypovolemic Shock

• Hemorrhagic Shock (most common overall cause globally): Acute blood loss >20% blood volume (>1 L in 70-kg adult) overwhelms compensatory mechanisms. Trauma remains the leading cause in developed nations; gastrointestinal bleeding (peptic ulcer, varices, Mallory-Weiss tears) is the most common non-traumatic cause. Intra-abdominal bleeding (ruptured AAA, splenic/hepatic injury), intrathoracic bleeding (hemothorax), and pelvic fractures with retroperitoneal hemorrhage are high-mortality sources.

• Non-Hemorrhagic Hypovolemic Shock: Severe dehydration from diarrhea (cholera, infectious gastroenteritis), vomiting, inadequate fluid intake, or diuretic overuse. Burn shock (thermal, electrical, chemical injuries >20% TBSA) causes massive third-spacing of fluid into edematous tissue and EVL with ongoing plasma losses. Pancreatitis (acute hemorrhagic) produces severe interstitial edema and third-spacing. Bowel obstruction causes sequestration of 2–8 L of fluid in bowel lumen and mesentery, effectively creating hypovolemia.

• Risk factors amplifying severity: Extremes of age (neonates, elderly with reduced physiologic reserve), pre-existing anemia (reduces oxygen-carrying capacity despite given blood loss), cardiovascular disease limiting compensatory tachycardia/vasoconstriction, medication effects (beta-blockers blunt compensatory tachycardia, ACE inhibitors impair vasoconstriction).

Distributive Shock

• Sepsis (80–90% of distributive shock cases): Infection with systemic inflammatory response syndrome (SIRS) criteria. Gram-negative bacteria (E. coli, Klebsiella, Pseudomonas—LPS producers) account for ~60%, gram-positive (Staph aureus, Strep pneumoniae) ~25%, fungi ~10%. Septic shock specifically denotes sepsis with refractory hypotension (MAP <65 mmHg despite fluid resuscitation) requiring vasopressor support. Common sources: pneumonia (most frequent), urinary tract (especially with obstructing stones/catheters), intra-abdominal (perforated viscus, appendicitis, diverticulitis), skin/soft tissue (necrotizing fasciitis, diabetic foot), bloodstream (central lines, endocarditis).

• Anaphylaxis: IgE-mediated mast cell and basophil degranulation releasing histamine, tryptase, and other mediators. Triggers: medications (penicillins, NSAIDs, ACE inhibitors), foods (peanuts, shellfish, tree nuts), insect stings (hymenoptera), latex, contrast agents. Shock develops within seconds to minutes of exposure.

• Neurogenic Shock: Acute spinal cord injury (typically T6 level or above), particularly in acute spinal cord transection or severe trauma, causes loss of sympathetic tone with profound vasodilation and bradycardia (unusual combination in shock). Unopposed parasympathetic effect causes both decreased SVR and decreased HR, distinguishing it from other distributive forms. Occurs in ~5–10% of spinal cord injuries; usually self-limited if cord swelling resolves.

• Toxin-Mediated Shock: Toxic shock syndrome (TSST-1 from Staph aureus or SPE from Strep pyogenes), organophosphate poisoning, certain chemotherapy agents, and drug reactions produce vasodilation and increased vascular permeability mimicking septic shock.

• Risk factors: Immunocompromise (HIV/AIDS, chemotherapy, diabetes), extremes of age, malignancy, asplenia, indwelling devices, prior antibiotic exposure (C. difficile), recent surgery/hospitalization.

Cardiogenic Shock

• Hypotension and Altered Mental Status: Systolic BP typically <90 mmHg despite compensatory vasoconstriction, reflecting severely reduced cardiac output. Cerebral hypoperfusion causes confusion, lethargy, or coma. Peripheral cyanosis from decreased cutaneous perfusion appears early.

• Dyspnea and Pulmonary Edema: Acute dyspnea at rest progressing to orthopnea and paroxysmal nocturnal dyspnea reflects pulmonary venous congestion from elevated LV diastolic pressure. Crackles (rales) on auscultation are nearly universal; pink frothy sputum indicates pulmonary edema with fluid in airways. Reduced oxygen saturation on pulse oximetry reflects ventilation-perfusion mismatch and intrapulmonary shunting.

• Cold Extremities and Poor Perfusion: Compensatory peripheral vasoconstriction produces cool, clammy skin with delayed capillary refill (>2 seconds); mottled appearance indicates microcirculatory compromise. Oliguria (<0.5 mL/kg/hr) develops from renal hypoperfusion and angiotensin II-mediated vasoconstriction of afferent arterioles.

• Elevated Jugular Venous Pressure (JVP): Unlike hypovolemic shock, cardiogenic shock presents with elevated JVP (>8 cmH₂O) due to impaired venous return into a failing right ventricle. Prominent v waves appear with acute mitral regurgitation; hepatomegaly with pulsatile hepatic congestion reflects right heart failure.

• Cardiac Auscultation Findings: New or worsening murmurs suggest mechanical complications: holosystolic murmur at apex (acute MR from papillary muscle rupture), harsh systolic murmur at left sternal border (VSD), or tachycardia with S3 gallop from rapid ventricular filling into a failing, dilated ventricle.

• Mechanical Complications of MI: Sudden severe dyspnea with acute pulmonary edema and shock 2–7 days post-MI suggests VSD (audible murmur, left-to-right shunt on cardiac catheterization); acute severe dyspnea with signs of mitral regurgitation suggests papillary muscle rupture; sudden electromechanical dissociation with muffled heart sounds, elevated JVP, and pulsus paradoxus suggests free wall rupture with tamponade.

Hypovolemic Shock

• Hypotension: Systolic BP <90 mmHg (in adults) develops acutely after significant blood loss; however, compensatory mechanisms may maintain near-normal BP early, making shock underappreciated. "Shock index" (heart rate ÷ systolic BP) >0.9 indicates early shock despite "normal" BP. Orthostatic vital signs (BP drop >20 mmHg systolic or >10 mmHg diastolic, HR increase >20 with standing) indicate mild-moderate hypovolemia.

• Tachycardia: Heart rate >100 bpm reflects maximal sympathetic activation; absence of tachycardia in a hypovolemic patient (due to beta-blocker use or intrinsic bradycardia) is particularly ominous and indicates severe shock. Compensatory tachycardia maintains CO despite reduced stroke volume until blood loss exceeds ~30% total volume.

• Cool, Clammy Skin and Vasoconstriction: Profound peripheral vasoconstriction produces pale, mottled skin with delayed capillary refill. Unlike cardiogenic shock, skin remains warm initially because vasoconstriction preserves core body temperature; only in decompensated shock does skin become cold. Absence of pulses in extremities indicates severe shock with profound vasoconstriction.

• Altered Mental Status: Restlessness, anxiety, and confusion develop from cerebral hypoperfusion and lactate-induced metabolic acidosis. Coma indicates profound shock (MAP persistently <50 mmHg).

• Low Jugular Venous Pressure: Unlike cardiogenic shock, JVP remains low or undetectable (<2 cmH₂O) due to hypovolemia. Flat neck veins, even when supine, indicate severe volume depletion. This finding distinguishes hypovolemic from cardiogenic shock at bedside.

• Oliguria and Dark Urine: Urine output <0.5 mL/kg/hr reflects renal hypoperfusion. Acute kidney injury develops from hypoperfusion-induced acute tubular necrosis (ATN), particularly after 4–6 hours of sustained shock. Urine appears dark concentrated due to dehydration.

• Bleeding or Fluid Loss Sites: External hemorrhage is obvious, but occult bleeding is frequently missed:

Initial evaluation (all shock)

• Serum lactate: the single most useful screening and prognostic marker of anaerobic metabolism. Sepsis-3 and the Surviving Sepsis Campaign define lactate >2 mmol/L persisting after adequate fluid resuscitation as a defining feature of septic shock. Failure of lactate to clear with therapy predicts mortality better than the initial value.
• Blood gas and chemistry: anion-gap metabolic acidosis, base deficit, and rising creatinine mark hypoperfusion. Central venous oxygen saturation (ScvO₂) is low (<70%) in cardiogenic and hypovolemic shock (high extraction from low flow) and normal or high in distributive shock (impaired extraction, cytopathic hypoxia).
• Focused bedside ultrasound (RUSH/POCUS): the fastest way to separate categories — hyperdynamic small LV with collapsible IVC (hypovolemic), dilated hypocontractile LV with plethoric IVC (cardiogenic), RV strain and McConnell sign (massive PE), pericardial effusion with RA/RV collapse (tamponade).
• Etiology-directed testing: ECG and troponin for MI; two sets of blood cultures before antibiotics plus source imaging for sepsis; type-and-cross, CBC, FAST exam for hemorrhage. Serum tryptase supports anaphylaxis retrospectively.

Confirmatory hemodynamics

• Pulmonary artery catheterization remains the reference standard when the profile is ambiguous or mixed. Cardiogenic: cardiac index reduced, pulmonary capillary wedge pressure elevated, systemic vascular resistance high. Hypovolemic: low wedge, low index, high SVR. Distributive: low SVR with normal or high cardiac index and low wedge.

Named criteria

• SHOCK trial criteria for cardiogenic shock: sustained hypotension (SBP <90 mmHg) with end-organ hypoperfusion and a depressed cardiac index despite adequate filling pressures.
• SCAI SHOCK stages A–E (at risk, beginning, classic, deteriorating, extremis) grade cardiogenic shock severity.
• Sepsis-3/SOFA defines sepsis; qSOFA is a screening prompt, not a diagnostic rule.
• ATLS hemorrhage classes I–IV grade blood loss; hypotension appears only at class III (~30% volume loss).

Distributive — septic

• Crystalloid resuscitation: Surviving Sepsis Campaign recommends at least 30 mL/kg balanced crystalloid (lactated Ringer's, Plasma-Lyte) within the first 3 hours, then fluid-responsiveness-guided boluses.
• Broad-spectrum antibiotics: within 1 hour for septic shock, after cultures, with rapid source control (drainage, debridement).
• Vasopressor — norepinephrine first-line; add vasopressin as second agent, then epinephrine. Hydrocortisone for pressor-refractory shock.
• Contraindicated: hydroxyethyl starches and routine albumin as initial fluid; dopamine (more arrhythmias, SOAP II).

Distributive — anaphylactic/neurogenic

• Epinephrine 0.3 mg IM into the anterolateral thigh, repeated every 5–15 minutes, is first-line per the AAAAI/ACAAI practice parameter. H1/H2 blockers and glucocorticoids are adjuncts only and never delay epinephrine.
• Neurogenic: fluids plus a vasopressor with chronotropic support (norepinephrine); atropine or pacing for symptomatic bradycardia.

Hypovolemic/hemorrhagic

• Hemorrhage control first (pressure, tourniquet, endoscopy, angioembolization, operation) — ATLS.
• Balanced blood product transfusion in ~1:1:1 ratio via massive transfusion protocol rather than large-volume crystalloid; tranexamic acid early in trauma. Correct hypocalcemia from citrate.

Cardiogenic

• Reperfusion is definitive: ACC/AHA supports emergent revascularization (PCI of the culprit vessel; CABG for unsuitable anatomy) in MI-associated shock, regardless of time from symptom onset.
• Norepinephrine is the preferred initial vasopressor; add an inotrope (dobutamine, or milrinone if beta-blocked) for low cardiac output.
• Mechanical circulatory support (Impella, VA-ECMO) for refractory shock; routine intra-aortic balloon pump is not recommended after IABP-SHOCK II.
• Contraindicated: beta blockers, nondihydropyridine calcium channel blockers, and negative inotropes in acute shock; nitrates and diuretics in preload-dependent RV infarction (give fluids instead).

Complications of shock itself

• Multi-organ dysfunction syndrome (MODS): cumulative hypoperfusion and inflammatory injury; rising SOFA score across organ systems. The final common pathway of shock death.
• Acute kidney injury from acute tubular necrosis: sustained renal hypoperfusion; oliguria with muddy brown granular casts and a rising creatinine. Hyperkalemia and refractory acidosis are dialysis emergencies.
• **Ischemic hepatitis (shock liver)**: centrilobular zone 3 necrosis; transaminases in the thousands peaking within 1–3 days and falling rapidly, with an elevated LDH — distinguishes it from viral hepatitis.
• ARDS: inflammatory alveolar-capillary leak; bilateral infiltrates with hypoxemia not explained by cardiac failure. Requires low-tidal-volume ventilation.
• Mesenteric ischemia and stress-related mucosal bleeding: splanchnic vasoconstriction; pain out of proportion to exam, then peritonitis or hematemesis — surgical emergency.
• DIC: tissue-factor-driven consumption; thrombocytopenia, prolonged PT/PTT, low fibrinogen, elevated D-dimer with simultaneous bleeding and microthrombosis.
• Irreversible shock: vasoplegia unresponsive to catecholamines from ATP-sensitive potassium channel opening and vasopressin depletion.

Complications of therapy

• Vasopressor-induced digital and mesenteric ischemia: alpha-1 vasoconstriction; dusky fingertips or new abdominal pain. Extravasation causes local necrosis — treat with phentolamine infiltration.
• Tachyarrhythmias: dobutamine, epinephrine, and especially dopamine increase myocardial oxygen demand; new atrial fibrillation or VT signals it.
• Fluid overload: over-resuscitation produces pulmonary edema, abdominal compartment syndrome, and worse outcomes; positive fluid balance with worsening oxygenation is the clue.
• Hyperchloremic non-anion-gap acidosis from large-volume normal saline.
• Transfusion complications: TRALI (hypoxemia within 6 hours, non-cardiogenic edema), TACO, citrate-induced hypocalcemia, hypothermia, and dilutional coagulopathy — the lethal triad of hypothermia, acidosis, and coagulopathy in trauma.
• Device complications: Impella hemolysis and limb ischemia; VA-ECMO LV distension and differential hypoxemia (Harlequin/north–south syndrome).

• The four-variable hemodynamic grid is the highest-yield fact: cardiogenic = ↓CO, ↑PCWP, ↑SVR, ↓ScvO₂; hypovolemic = ↓CO, ↓PCWP, ↑SVR, ↓ScvO₂; distributive = ↑CO, ↓PCWP, ↓SVR, ↑ScvO₂. Low SVR with warm extremities means distributive until proven otherwise.
• JVP is the bedside discriminator: elevated in cardiogenic and obstructive shock, flat in hypovolemic and distributive. Beck triad (hypotension, JVD, muffled heart sounds) points to tamponade; JVD plus clear lungs plus inferior MI points to RV infarction — give fluids, and never nitroglycerin or morphine.
• Anaphylaxis: the single best next step is epinephrine 0.3 mg IM in the anterolateral thigh, before IV access, antihistamines, or steroids. The classic distractor is diphenhydramine or IV corticosteroid as "first-line."
• Neurogenic shock is the one shock with hypotension plus bradycardia plus warm, dry skin (lost sympathetic outflow above T6). Hemorrhagic shock in a trauma patient is tachycardic — do not attribute hypotension to the cord until bleeding is excluded.
• Norepinephrine is the first-line vasopressor in both septic and cardiogenic shock; vasopressin is the standard add-on in sepsis. Dopamine is a distractor (arrhythmias, SOAP II), and phenylephrine's pure alpha effect is generally avoided.
• In MI-associated cardiogenic shock, emergent revascularization is definitive therapy regardless of time since symptom onset (SHOCK trial). Routine intra-aortic balloon pump does not improve mortality (IABP-SHOCK II).
• A normal blood pressure does not exclude shock. Young trauma patients compensate until roughly 30% volume loss (ATLS class III); a shock index >0.9 and rising lactate unmask it. Beta blockade blunts the compensatory tachycardia — absence of tachycardia is ominous, not reassuring.
• Sepsis-3: septic shock requires vasopressors to maintain MAP ≥65 mmHg plus lactate >2 mmol/L after adequate fluids — not SIRS criteria, which are obsolete for the definition.