6.1 Shock Classification, Cellular Pathophysiology & Biomarkers

ABIM Critical Care Medicine Study Guide Exam

Track your progress as you read

Finished sections get a check mark.

1 Chapter 1: Exam Orientation: Format, Blueprint & Study Strategy

1.1 ABIM Critical Care Medicine Exam: Format, Blueprint & Study Strategy

2 Chapter 2: Acute Respiratory Failure & ARDS

2.1 ARDS Definition, Pathophysiology & Phenotypes 2.2 Lung-Protective Mechanical Ventilation 2.3 Refractory ARDS: Prone Positioning, Neuromuscular Blockade & VV-ECMO

3 Chapter 3: Obstructive Airways, Pulmonary Vascular & Pleural Emergencies

3.1 Severe Asthma & COPD Exacerbations in the ICU 3.2 Massive & Submassive Pulmonary Embolism 3.3 Pulmonary Hypertension & Right Ventricular Failure in the ICU 3.4 Massive Hemoptysis, Complex Pleural Effusions & Thoracic Drainage

4 Chapter 4: Airway, Parenchymal & Nonthrombotic Pulmonary Emergencies

4.1 Airway Management, Upper-Airway Obstruction & Artificial-Airway Complications 4.2 Diffuse Alveolar Hemorrhage, Interstitial Lung Disease, Pulmonary Vasculitis & Noncardiogenic Edema 4.3 Severe Community-Acquired & Viral Pneumonia 4.4 Nonthrombotic Embolism (Air, Fat, Amniotic Fluid, Tumor, Septic) & Acute Chest Syndrome

5 Chapter 5: Mechanical Ventilation Modes, Dyssynchrony & Liberation

5.1 Mechanical Ventilator Modes & Advanced Breath Delivery 5.2 Patient-Ventilator Dyssynchrony & Waveform Analysis 5.3 Ventilator Liberation, Spontaneous Breathing Trials & Post-Extubation Support

6 Chapter 6: Shock Syndromes & Hemodynamic Principles

6.1 Shock Classification, Cellular Pathophysiology & Biomarkers 6.2 Hemodynamic Monitoring: Arterial Lines, CVP & Pulmonary Artery Catheter 6.3 Dynamic Assessment of Fluid Responsiveness & Resuscitation Targets

7 Chapter 7: Vasoactive Therapies & Mechanical Circulatory Support

7.1 Vasopressors & Inotropes: Pharmacology & Clinical Titration 7.2 Temporary Mechanical Circulatory Support: IABP, Impella & TandemHeart 7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia 7.4 Post-Cardiac Arrest Care, Targeted Temperature Management & Neuroprognostication

8 Chapter 8: Acute Kidney Injury & Renal Replacement Therapy

8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation 8.2 Continuous Renal Replacement Therapy (CRRT) vs Intermittent Hemodialysis 8.3 Indications & Timing for RRT in Critical Illness: AKIKI & STARRT-AKI

9 Chapter 9: Electrolyte, Acid-Base & Endocrine Emergencies

9.1 Severe Dysnatremias: Hyponatremia, Hypernatremia & Osmotic Demyelination 9.2 Potassium, Calcium, Magnesium & Phosphate Disorders 9.3 Complex & Mixed Acid-Base Disorders: Boston vs Copenhagen vs Stewart 9.4 Endocrine Crises: DKA, HHS, Thyroid Storm, Myxedema Coma & Adrenal Crisis

10 Chapter 10: Glycemic Control, Hyperammonemia, Pituitary & Adrenal Disorders

10.1 ICU Glycemic Control & Severe Hypoglycemia 10.2 Non-Hepatic Hyperammonemia & Metabolic Encephalopathy 10.3 Pituitary Emergencies, Adrenal Excess & Critical Illness-Related Corticosteroid Insufficiency

11 Chapter 11: Acute Coronary Syndromes & Cardiac Arrhythmias

11.1 Acute Coronary Syndromes & Mechanical Complications of Myocardial Infarction 11.2 Critical Tachyarrhythmias: Atrial Fibrillation with RVR, VT & VF 11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing

12 Chapter 12: Acute Heart Failure, Valvular & Pericardial Emergencies

12.1 Acute Decompensated Heart Failure & Cardiogenic Pulmonary Edema 12.2 Acute Valvular Regurgitation & Prosthetic Valve Emergencies 12.3 Pericardial Tamponade & Acute Aortic Syndromes

13 Chapter 13: Myocardial Disease, Hypertensive & Cardiac Device Emergencies

13.1 Fulminant Myocarditis, Stress & Peripartum Cardiomyopathy, and Hypertrophic Cardiomyopathy 13.2 Hypertensive Emergencies & Ruptured Abdominal Aortic Aneurysm 13.3 Implanted Cardiac Devices, LVAD Emergencies & Electrical Storm

14 Chapter 14: Sepsis & Systemic Infectious Diseases

14.1 Sepsis-3 Definitions & Surviving Sepsis Campaign 2026 Guidelines 14.2 Refractory Septic Shock & Adjunctive Therapies 14.3 Multidrug-Resistant Pathogens & Antimicrobial Stewardship in the ICU

15 Chapter 15: ICU-Acquired & Severe Focal Infections

15.1 Hospital-Acquired & Ventilator-Associated Pneumonia 15.2 Central Line-Associated Bloodstream Infections & Catheter-Associated UTIs 15.3 Intra-Abdominal Sepsis, Necrotizing Soft Tissue Infections & CNS Infections

16 Chapter 16: Focal, Special-Host & Systemic Infections

16.1 CNS Infections Beyond Bacterial Meningitis: Encephalitis, Brain Abscess & Epidural Abscess 16.2 Infections in the Immunocompromised Host, Tropical & Bioterror Infections 16.3 Infective Endocarditis, Device Infections & Other Focal Cardiovascular Infections 16.4 Antimicrobial Pharmacokinetics, Allergy, Toxicity & Infection Prevention 16.5 Gastrointestinal & Hepatobiliary Infections: Fulminant C. difficile, Typhlitis, CMV Colitis & Abscesses

17 Chapter 17: Neurocritical Care & Coma

17.1 Coma, Altered Mental Status & Acute Delirium in the ICU 17.2 Status Epilepticus & Non-Convulsive Status Epilepticus 17.3 Acute Ischemic Stroke & Intracranial Hemorrhage

18 Chapter 18: Cerebrovascular Syndromes, Encephalopathy & Withdrawal States

18.1 Cerebral Venous Sinus Thrombosis, PRES & Reversible Cerebral Vasoconstriction Syndrome 18.2 Alcohol & Sedative Withdrawal, Wernicke Encephalopathy & Drug-Induced Encephalopathy

19 Chapter 19: Intracranial Hypertension & Neuromuscular Weakness

19.1 Intracranial Pressure Monitoring, Cerebral Perfusion & Tiered ICP Management 19.2 Acute Neuromuscular Respiratory Failure, ICU-Acquired Weakness & Brain Death Determination

20 Chapter 20: Traumatic Brain Injury, Spinal Cord Injury & Toxin-Mediated Paralysis

20.1 Traumatic Brain Injury Management & Paroxysmal Sympathetic Hyperactivity 20.2 Acute Spinal Cord Injury, Neurogenic Shock & Autonomic Dysreflexia 20.3 Toxin-Mediated Paralysis & Neuromuscular Crises: Botulism, Tetanus, Tick Paralysis & Myasthenic Crisis

21 Chapter 21: Gastrointestinal, Hepatic & Abdominal Emergencies

21.1 Acute Upper & Lower Gastrointestinal Bleeding in the ICU 21.2 Acute Liver Failure & Severe Acute Pancreatitis 21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

22 Chapter 22: Luminal GI Emergencies & Complications of Cirrhosis

22.1 Esophageal, Gastric & Bowel Emergencies: Perforation, Obstruction, Ogilvie Syndrome & Colonic Ischemia 22.2 Complications of Cirrhosis: Alcohol-Associated Hepatitis, Hepatorenal Syndrome & Pulmonary Vascular Syndromes

23 Chapter 23: Trauma, Burns & Perioperative Critical Care

23.1 Polytrauma Resuscitation, Damage Control & Massive Transfusion 23.2 Severe Burn Injury, Inhalation Injury & Environmental Emergencies 23.3 Perioperative Critical Care for High-Risk Surgical Patients

24 Chapter 24: Thoracic Trauma, Environmental Injury & Obstetric Critical Care

24.1 Thoracic Trauma: Flail Chest, Pulmonary Contusion, Great Vessel & Tracheobronchial Injury 24.2 Environmental Injury: Drowning, Diving, Altitude, Heat, Cold, Electrical & Radiation 24.3 Obstetric & Genitourinary Emergencies and the Physiology of Pregnancy

25 Chapter 25: Transplantation, Donor Management & Post-Cardiac Surgery Care

25.1 Solid Organ Transplant Recipients & Deceased Donor Management 25.2 Post-Cardiac Surgery Critical Care: Bleeding, Tamponade, Low Output & Vasoplegia

26 Chapter 26: Hematologic Emergencies, Toxicology, Critical Ultrasound & ICU Liberation

26.1 Hematologic Emergencies in the ICU: HIT, DIC, TMA & Anticoagulant Reversal 26.2 Critical Care Toxicology, Toxidromes & Specific Antidotes 26.3 Critical Care Point-of-Care Ultrasound (POCUS) 26.4 ICU Liberation, Analgosedation, Goals of Care & Critical Care Ethics

27 Chapter 27: Transfusion Medicine, Hematologic & Oncologic Disorders

27.1 Transfusion Medicine: Products, Thresholds, Reactions & Apheresis 27.2 Red and White Cell Disorders, Hemoglobinopathies, Factor Deficiencies & Hypercoagulable States 27.3 Oncologic Emergencies, Hematopoietic Cell Transplant Complications & Cellular Therapy Toxicity

28 Chapter 28: Clinical Pharmacology, Adverse Drug Reactions & Specific Poisonings

28.1 Pharmacokinetic Principles, Dose Adjustment & Drug Interactions in Critical Illness 28.2 Adverse Drug Reactions: Anaphylaxis, Severe Cutaneous Reactions & Hyperthermic Syndromes 28.3 Specific Poisonings: Sodium-Channel Blockers, Lithium, Valproate, Theophylline, Iron, Sulfonylureas & Methemoglobinemia

29 Chapter 29: Ethics, Research Methods, Administration & Abdominal Ultrasound

29.1 Advance Care Planning, Nonbeneficial Treatment, Informed Consent & Confidentiality 29.2 Critical Care Research Methods, Statistics, IRB Oversight & Quality Improvement 29.3 Abdominal, Vascular & Procedural Point-of-Care Ultrasound

6.1 Shock Classification, Cellular Pathophysiology & Biomarkers

Key Takeaways Explain This Topic AI

Shock is defined as life-threatening circulatory collapse where systemic oxygen delivery (DO2) fails to meet cellular metabolic oxygen demand (VO2), precipitating cellular dysoxia, anaerobic glycolysis, and progressive multi-organ failure.

The four classic physiological shock categories are hypovolemic (decreased preload, elevated SVR), cardiogenic (primary pump failure, elevated filling pressures, elevated SVR), distributive (profound systemic vasodilation, depressed SVR, normal or high cardiac output initially), and obstructive (extracardiac flow impediment with elevated systemic or pulmonary filling pressures).

Hyperlactatemia reflects both tissue hypoperfusion with anaerobic glycolysis (Type A lactic acidosis) and non-hypoperfusion mechanisms (Type B lactic acidosis) such as beta-2 adrenergic stimulation by endogenous or exogenous catecholamines, impaired hepatic clearance, thiamine deficiency, and toxic mitochondrial decoupling.

Endothelial glycocalyx degradation and microvascular derecruitment create a functional uncoupling between macro-hemodynamics and microvascular perfusion, explaining persistent cellular ischemia despite normal blood pressure and cardiac output.

Cardiac tamponade produces hemodynamic equalization of end-diastolic pressures across the right atrium, right ventricle, pulmonary artery, and pulmonary capillary wedge position, accompanied by pulsus paradoxus (> 10 mmHg) and an attenuated or absent y-descent.

Last updated: October 2026

Definition and Cellular Dysoxia

Circulatory shock is defined as a life-threatening, generalized maldistribution of blood flow resulting in the failure of cellular oxygen delivery ( DO 2 \text{DO}_2 DO 2 ​ ) to satisfy cellular metabolic oxygen consumption ( VO 2 \text{VO}_2 VO 2 ​ ). The cardinal pathophysiological consequence of shock is cellular dysoxia—a state in which mitochondrial oxidative phosphorylation is constrained by inadequate oxygen availability, triggering energetic failure, cell injury, and multi-organ dysfunction syndrome (MODS).

Under basal physiological conditions, global oxygen delivery exceeds oxygen consumption by a factor of three to four:

DO 2 = CO × CaO 2 = CO × ( 1.34 × Hb × SaO 2 + 0.003 × PaO 2 ) \text{DO}_2 = \text{CO} \times \text{CaO}_2 = \text{CO} \times \left( 1.34 \times \text{Hb} \times \text{SaO}_2 + 0.003 \times \text{PaO}_2 \right) DO 2 ​ = CO × CaO 2 ​ = CO × ( 1.34 × Hb × SaO 2 ​ + 0.003 × PaO 2 ​ )

VO 2 = CO × ( CaO 2 − CvO 2 ) \text{VO}_2 = \text{CO} \times \left( \text{CaO}_2 - \text{CvO}_2 \right) VO 2 ​ = CO × ( CaO 2 ​ − CvO 2 ​ )

Where CO \text{CO} CO is cardiac output, CaO 2 \text{CaO}_2 CaO 2 ​ is arterial oxygen content, CvO 2 \text{CvO}_2 CvO 2 ​ is mixed venous oxygen content, Hb \text{Hb} Hb is hemoglobin concentration, and SaO 2 \text{SaO}_2 SaO 2 ​ is arterial oxygen saturation. In healthy resting adults, DO 2 \text{DO}_2 DO 2 ​ is approximately 1000 mL/min 1000\text{ mL/min} 1000 mL/min ( 500 − 600 mL/min/m 2 500-600\text{ mL/min/m}^2 500 − 600 mL/min/m 2 ) and VO 2 \text{VO}_2 VO 2 ​ is approximately 250 mL/min 250\text{ mL/min} 250 mL/min ( 110 − 160 mL/min/m 2 110-160\text{ mL/min/m}^2 110 − 160 mL/min/m 2 ), yielding a normal oxygen extraction ratio ( O 2 ER = VO 2 / DO 2 \text{O}_2\text{ER} = \text{VO}_2 / \text{DO}_2 O 2 ​ ER = VO 2 ​ / DO 2 ​ ) of 25 % 25\% 25% to 30 % 30\% 30% .

When DO 2 \text{DO}_2 DO 2 ​ declines (due to hypovolemia, myocardial depression, or severe anemia), tissues maintain constant oxygen consumption by progressively increasing the oxygen extraction ratio up to 60 − 70 % 60-70\% 60 − 70% . However, when DO 2 \text{DO}_2 DO 2 ​ falls below a critical threshold (termed the critical oxygen delivery, DO 2 crit \text{DO}_{2\text{crit}} DO 2 crit ​ ), tissue extraction capacity becomes exhausted. Beyond this tipping point, cellular oxygen consumption becomes supply-dependent, intracellular adenosine triphosphate (ATP) production plummets, and cells transition to anaerobic glycolysis.

At the cellular level, the loss of mitochondrial ATP generation impairs membrane ion homeostatic pumps, most notably the Na + / K + \text{Na}^+/\text{K}^+ Na + / K + -ATPase and the Ca 2 + \text{Ca}^{2+} Ca 2 + -ATPase. Intracellular sodium and water accumulation induce cytotoxic edema and cellular swelling, while cytosolic calcium overload activates endogenous proteases, phospholipases, and endonucleases. The mitochondrial permeability transition pore (mPTP) opens, collapsing the mitochondrial membrane potential, releasing cytochrome c into the cytosol, and initiating programmed apoptotic and necrotic cell death pathways.

The Four Classic Shock Categories

Shock is clinically categorized into four fundamental pathophysiological phenotypes: hypovolemic, cardiogenic, distributive, and obstructive shock. While patient presentations in the intensive care unit frequently exhibit mixed shock features (such as septic shock complicated by hypovolemia and septic cardiomyopathy), mastering the pure profiles provides the foundation for hemodynamic diagnosis.

1. Hypovolemic Shock

Hypovolemic shock results from a critical loss of effective circulating intravascular volume, reducing venous return (mean systemic filling pressure) to the right atrium and diminishing end-diastolic ventricular filling.

Hemorrhagic Etiologies : Blunt or penetrating trauma, gastrointestinal bleeding, ruptured abdominal aortic aneurysm, retroperitoneal hemorrhage, ruptured ectopic pregnancy, or surgical bleeding.

Non-Hemorrhagic Etiologies : Severe gastrointestinal volume loss (cholera, intractable vomiting, profuse diarrhea), renal wasting (diabetic ketoacidosis, hyperosmolar hyperglycemic state, high-dose diuretic therapy, salt-wasting nephropathies), severe burns (thermal injury causing diffuse capillary extravasation), and internal fluid sequestration/third-spacing (acute pancreatitis, bowel obstruction, crush injury).

Hemodynamic Profile : Central venous pressure (CVP) and pulmonary capillary wedge pressure (PCWP) are markedly reduced. Stroke volume and cardiac output are severely depressed. In response to arterial baroreceptor de-activation, massive sympathetic discharge drives compensatory arterial vasoconstriction, resulting in elevated systemic vascular resistance ( S V R > 1200 dynes ⋅ sec ⋅ cm − 5 SVR > 1200\text{ dynes}\cdot\text{sec}\cdot\text{cm}^{-5} S V R > 1200 dynes ⋅ sec ⋅ cm − 5 ) and narrow pulse pressure ( < 30 mmHg < 30\text{ mmHg} < 30 mmHg ). Extremities are typically cool, clammy, and show delayed capillary refill ( > 3 seconds > 3\text{ seconds} > 3 seconds ).

2. Cardiogenic Shock

Cardiogenic shock is characterized by primary circulatory pump failure resulting in inadequate cardiac output despite adequate or elevated ventricular intravascular filling pressures.

Myopathic Etiologies : Acute myocardial infarction involving > 40 % > 40\% > 40% of left ventricular myocardium, acute ischemic stunning, fulminant viral or autoimmune myocarditis, end-stage dilated cardiomyopathy, stress (Takotsubo) cardiomyopathy, or severe post-cardiac arrest myocardial stunning.

Arrhythmogenic Etiologies : Sustained ventricular tachycardia, ventricular fibrillation, refractory rapid atrial fibrillation/flutter in non-compliant ventricles, or high-grade atrioventricular block and severe bradyarrhythmias.

Mechanical Etiologies : Acute mitral regurgitation secondary to papillary muscle rupture or chordal rupture, acute ventricular septal defect (VSD), free wall rupture, severe acute aortic insufficiency (due to infective endocarditis or acute aortic dissection), or critical aortic stenosis.

Hemodynamic Profile : Cardiac index is severely depressed ( C I < 2.2 L/min/m 2 CI < 2.2\text{ L/min/m}^2 C I < 2.2 L/min/m 2 , or < 1.8 L/min/m 2 < 1.8\text{ L/min/m}^2 < 1.8 L/min/m 2 without support). Ventricular filling pressures are pathologically elevated ( P C W P > 15 − 18 mmHg PCWP > 15-18\text{ mmHg} PC W P > 15 − 18 mmHg and C V P > 10 − 12 mmHg CVP > 10-12\text{ mmHg} C V P > 10 − 12 mmHg ). SVR is compensatorily elevated. Cardiac power output ( C P O = [ M A P × C O ] / 451 CPO = [MAP \times CO] / 451 CPO = [ M A P × CO ] /451 ), a robust prognostic metric, drops below 0.6 W 0.6\text{ W} 0.6 W . Pulmonary venous congestion produces tachypnea, diffuse rales, and hypoxemia.

3. Distributive Shock

Distributive shock is characterized by extensive systemic vasodilation, peripheral arterial and venous pooling, and widespread microvascular endothelial barrier breakdown, resulting in a severe reduction in systemic vascular resistance.

Septic Shock : Dysregulated host response to infection characterized by profound vasodilation mediated by inducible nitric oxide synthase (iNOS), endothelial activation, and systemic inflammation.

Anaphylactic Shock : IgE-mediated (or non-IgE direct mast cell degranulation) release of histamine, tryptase, leukotrienes, and platelet-activating factor, triggering catastrophic arteriolar dilation, profound venous pooling, bronchospasm, and rapid transudation of intravascular fluid into interstitial spaces.

Neurogenic Shock : Interruption of sympathetic vasomotor outflow following traumatic spinal cord injury (typically at or above the T6 level) or high spinal anesthesia. Marked by the classic hemodynamic triad of hypotension, relative bradycardia (loss of cardiac accelerator fibers from T1-T4 preventing compensatory tachycardia), and warm, dry peripheries.

Endocrine Shock : Acute adrenocortical insufficiency (Addisonian crisis, Waterhouse-Friderichsen syndrome) with absolute or relative lack of cortisol, causing loss of vascular responsiveness to endogenous catecholamines and down-regulation of alpha-1 receptors; or severe myxedema coma.

Hemodynamic Profile : Systemic vascular resistance is severely depressed ( S V R < 700 − 800 dynes ⋅ sec ⋅ cm − 5 SVR < 700-800\text{ dynes}\cdot\text{sec}\cdot\text{cm}^{-5} S V R < 700 − 800 dynes ⋅ sec ⋅ cm − 5 ). In the early hyperdynamic phase ("warm shock"), cardiac output and cardiac index are elevated ( C I > 4.0 L/min/m 2 CI > 4.0\text{ L/min/m}^2 C I > 4.0 L/min/m 2 ) with a widened pulse pressure ( > 40 − 50 mmHg > 40-50\text{ mmHg} > 40 − 50 mmHg ) and bounding peripheral pulses. Central filling pressures (CVP, PCWP) are normal or low due to increased venous capacitance and capillary leakage.

4. Obstructive Shock

Obstructive shock results from physical, extracardiac impedance to the flow of blood into or out of the heart or great vessels, leading to profound reduction in stroke volume.

Massive Pulmonary Embolism : Sudden mechanical occlusion of > 50 % > 50\% > 50% of the pulmonary arterial bed, abruptly escalating right ventricular afterload. The thin-walled right ventricle dilates acutely, the interventricular septum shifts leftward during diastole (septal flattening or D-shaped left ventricle on echocardiography), and LV diastolic filling is mechanically compromised (ventricular interdependence). Hemodynamics show high CVP, severely elevated pulmonary artery pressures, low PCWP, and depressed cardiac output.

Cardiac Tamponade : Accumulation of pericardial fluid under tension exceeding intracardiac chamber pressures. Compression of all four cardiac chambers restricts diastolic filling. Hemodynamics reveal equalization of end-diastolic pressures: right atrial pressure, RV end-diastolic pressure, pulmonary artery diastolic pressure, and PCWP equalize within 1 − 3 mmHg 1-3\text{ mmHg} 1 − 3 mmHg . Diagnostic physical signs include Beck's triad (hypotension, jugular venous distension, muffled heart sounds) and pulsus paradoxus (an inspiratory decrease in systolic arterial pressure > 10 mmHg > 10\text{ mmHg} > 10 mmHg ).

Tension Pneumothorax : Progressive accumulation of air in the pleural space creates high intrapleural positive pressure, causing ipsilateral lung collapse, compression of the superior and inferior vena cava, shift of the mediastinum to the contralateral hemithorax, and cessation of venous return. Exam shows unilateral absent breath sounds, hyperresonance to percussion, tracheal deviation away from the affected side, and marked jugular venous distension.

Hemodynamic Profiles Across Shock Categories

Shock Category Primary Insult CVP (RAP) PCWP Cardiac Output (CO/CI) Systemic Vascular Resistance (SVR) Mixed Venous Saturation ( S v O 2 SvO_2 S v O 2 ​ ) Pulse Pressure & Exam Hallmarks

Hypovolemic Loss of intravascular volume Decreased ( < 4 mmHg < 4\text{ mmHg} < 4 mmHg ) Decreased ( < 6 mmHg < 6\text{ mmHg} < 6 mmHg ) Decreased Elevated ( > 1200 > 1200 > 1200 ) Decreased ( < 65 % < 65\% < 65% ) Narrow pulse pressure, flat neck veins, cool extremities, delayed capillary refill

Cardiogenic Primary pump failure Elevated ( > 10 mmHg > 10\text{ mmHg} > 10 mmHg ) Elevated ( > 18 mmHg > 18\text{ mmHg} > 18 mmHg ) Severely decreased ( C I < 2.2 CI < 2.2 C I < 2.2 ) Elevated ( > 1200 > 1200 > 1200 ) Decreased ( < 60 % < 60\% < 60% ) Narrow pulse pressure, elevated JVD, S3 gallop, diffuse pulmonary rales, oliguria

Distributive (Early/Warm) Vasomotor tone loss, vasoplegia Low or Normal Low or Normal Normal to Elevated ( C I > 3.5 CI > 3.5 C I > 3.5 ) Markedly decreased ( < 700 < 700 < 700 ) Normal to Elevated ( > 75 % > 75\% > 75% ) Wide pulse pressure, brisk capillary refill, warm flushed skin, bounding peripheral pulses

Distributive (Late/Refractory) Capillary leak, septic cardiomyopathy Low or Variable Normal to Elevated Decreased Depressed to Normal Variable to Low Mottling, poor perfusion, mixed cellular and vascular collapse

Obstructive: Tamponade Pericardial chamber constraint Markedly elevated Markedly elevated Decreased Elevated Decreased ( < 65 % < 65\% < 65% ) Pulsus paradoxus > 10 mmHg > 10\text{ mmHg} > 10 mmHg , equalized diastolic pressures, absent y y y -descent, distant heart sounds

Obstructive: Massive PE Pulmonary arterial occlusion Markedly elevated Low or Normal Decreased Elevated Decreased ( < 60 % < 60\% < 60% ) RV heave, loud P 2 P_2 P 2 ​ , McConnell's sign, acute tricuspid regurgitation jet, hypoxia

Obstructive: Tension Pneumo Pleural tension, caval compression Markedly elevated Low or Normal Severely decreased Elevated Decreased ( < 60 % < 60\% < 60% ) Tracheal deviation, unilateral hyperresonance, absent unilateral breath sounds

Note

In cardiac tamponade, the pericardial fluid compresses the heart throughout the cardiac cycle, so total intrapericardial volume is essentially fixed. Venous return to the atria occurs mainly during ventricular systole, when ejection makes room in the pericardial sac, so the right atrial waveform shows a preserved or prominent x x x -descent; there is no rapid early-diastolic ventricular filling, so the y y y -descent is attenuated or absent. In contrast, in constrictive pericarditis early diastolic filling is unimpeded and abnormally rapid until the rigid pericardium halts it abruptly in mid-to-late diastole, producing a prominent, steep y y y -descent and the "dip-and-plateau" ("square root") ventricular tracing.

Cellular Metabolic Crisis: Lactate Pathophysiology

Serum lactate is the primary clinical biomarker used to gauge resuscitation and cellular metabolic stress. Normal physiological serum lactate concentration is less than 1.0 mmol/L 1.0\text{ mmol/L} 1.0 mmol/L in non-stressed individuals, with levels > 2.0 mmol/L > 2.0\text{ mmol/L} > 2.0 mmol/L defining hyperlactatemia in the critically ill.

Historically categorized by Cohen and Woods in 1976, lactic acidosis is divided into Type A and Type B: Lactic Acidosis

• │
• ┌──────────────────────────┴──────────────────────────┐
• ▼ ▼
• Type A Type B
• (Tissue Hypoxia & Hypoperfusion) (Non-Hypoperfusion Etiologies)
• Cardiogenic Shock • Type B1: Liver failure, malignancy
• Hypovolemic Shock • Type B2: Epinephrine, metformin, linezolid
• Obstructive Shock • Type B3: Thiamine deficiency, inborn errors
• Mesenteric / Limb Ischemia
• Severe Carbon Monoxide Toxicity
• Type A Lactic Acidosis (Tissue Hypoxia and Hypoperfusion)

Type A lactic acidosis occurs when cellular oxygen supply falls below DO 2 crit \text{DO}_{2\text{crit}} DO 2 crit ​ . In the absence of oxygen to serve as the terminal electron acceptor in the mitochondrial electron transport chain, oxidative phosphorylation halts, and electrons accumulate in NADH, increasing the intracellular NADH / NAD + \text{NADH}/\text{NAD}^+ NADH / NAD + ratio. To sustain anaerobic ATP production via glycolysis, the cytosolic enzyme lactate dehydrogenase (LDH) reduces pyruvate to L-lactate, regenerating NAD + \text{NAD}^+ NAD + :

Pyruvate + NADH + H + ⇌ LDH L-Lactate + NAD + \text{Pyruvate} + \text{NADH} + \text{H}^+ \xrightleftharpoons{\text{LDH}} \text{L-Lactate} + \text{NAD}^+ Pyruvate + NADH + H + LDH ​ L-Lactate + NAD +

While anaerobic glycolysis yields only 2 moles of ATP per mole of glucose (compared to 36 moles generated by complete aerobic oxidation), it preserves immediate survival. Clinical causes of Type A lactic acidosis include all forms of hypoperfusion shock, localized tissue ischemia (acute mesenteric infarction, compartment syndrome, ischemic bowel, limb gangrene), severe carbon monoxide toxicity, and methemoglobinemia.

Type B Lactic Acidosis (Non-Hypoperfusion Mechanisms)

Type B lactic acidosis develops in the absence of overt systemic hypoperfusion or cellular hypoxia and is subdivided into three categories:

Type B1 (Underlying Systemic Illness) : Severe hepatic insufficiency or cirrhosis diminishes the metabolic clearance of lactate via the Cori cycle (gluconeogenesis in hepatocytes clears 70 % 70\% 70% of circulating lactate; the renal cortex clears 20 − 30 % 20-30\% 20 − 30% ). Severe hematologic malignancies (lymphoma, leukemia) generate high lactate via the Warburg effect—aerobic glycolysis driven by upregulated hexokinase and glucose transporter activity in neoplastic cells.

Type B2 (Drugs, Toxins, and Medications) :

Epinephrine Infusion : Epinephrine is a potent agonist of skeletal muscle β 2 \beta_2 β 2 ​ -adrenergic receptors. β 2 \beta_2 β 2 ​ activation stimulates membrane-bound adenylate cyclase, elevating intracellular cyclic AMP (cAMP). This initiates a phosphorylation cascade that accelerates glycogen phosphorylase and phosphofructokinase, resulting in intense glycogenolysis and glycolysis. Pyruvate production outstrips the maximum catalytic capacity of mitochondrial pyruvate dehydrogenase (PDH), shunting the excess pyruvate to lactate. This "aerobic hyperlactatemia" occurs despite normal or elevated cardiac output, normal tissue oxygen delivery, and improving organ perfusion metrics.

Metformin Toxicity : Metformin inhibits complex I of the mitochondrial respiratory chain and blocks hepatic lactate gluconeogenesis. In the presence of acute kidney injury or severe dehydration, toxic accumulation triggers severe Type B lactic acidosis with an extraordinarily high anion gap.

Linezolid and Nucleoside Reverse Transcriptase Inhibitors (NRTIs) : Linezolid binds to bacterial 50S ribosomal subunits but shares cross-reactivity with human mitochondrial ribosomes, disrupting mitochondrial protein synthesis and inducing lactic acidosis after prolonged exposure.

Propofol Infusion Syndrome (PRIS) : High-dose propofol ( > 4 − 5 mg/kg/h > 4-5\text{ mg/kg/h} > 4 − 5 mg/kg/h for > 48 hours > 48\text{ hours} > 48 hours ) impairs mitochondrial entry of long-chain fatty acids and uncouples oxidative phosphorylation, presenting with severe metabolic acidosis, rhabdomyolysis, hyperkalemia, hepatomegaly, and refractory myocardial failure.

Type B3 (Inborn Errors of Metabolism and Nutritional Deficiencies) : Thiamine (vitamin B 1 B_1 B 1 ​ ) deficiency impairs the activity of thiamine pyrophosphate (TPP)-dependent enzymes, including the pyruvate dehydrogenase multienzyme complex and alpha-ketoglutarate dehydrogenase. When PDH cannot convert pyruvate into acetyl-CoA to enter the Krebs cycle, pyruvate is diverted into lactate. This can manifest as acute high-output shock with severe lactic acidosis ("wet beriberi" or Shoshin beriberi) in malnourished patients, alcohol use disorder, or following bariatric surgery.

Tip

When evaluating persistent hyperlactatemia in a patient receiving an epinephrine infusion whose vital signs, urine output, and central venous oxygen saturation ( ScvO 2 > 70 % \text{ScvO}_2 > 70\% ScvO 2 ​ > 70% ) indicate restored perfusion, avoid reflexively infusing massive fluid boluses. The elevated lactate is driven predominantly by β 2 \beta_2 β 2 ​ -receptor-mediated aerobic glycolysis (Type B2), not tissue dysoxia.

Microvascular Dysfunction, Glycocalyx Shedding & Macro-Micro Uncoupling

One of the most consequential insights in modern shock physiology is the dissociation or "uncoupling" between systemic macro-hemodynamics and microvascular tissue perfusion, particularly in septic and distributive shock.

The microcirculation consists of arterioles, capillaries (diameter < 10 μ m < 10\ \mu\text{m} < 10 μ m ), and post-capillary venules. The luminal surface of healthy vascular endothelium is coated by the endothelial glycocalyx , a dense, gel-like layer 0.5 0.5 0.5 to 3.0 μ m 3.0\ \mu\text{m} 3.0 μ m thick composed of membrane-bound proteoglycans (syndecans, glypicans), glycosaminoglycans (heparan sulfate, chondroitin sulfate), and adsorbed plasma proteins (albumin, antithrombin). The intact glycocalyx performs vital homeostatic functions:

Maintains the vascular permeability barrier, restricting the transvascular movement of albumin and fluid.

Transduces fluid shear stress into endothelial nitric oxide (eNOS) production, maintaining homeostatic vasodilation.

Prevents pathological leukocyte marginization and microvascular platelet aggregation.

In severe shock states (particularly sepsis, major trauma, ischemia-reperfusion, and acute hyperglycemia), inflammatory cytokines (TNF- α \alpha α , IL-1 β \beta β ), reactive oxygen species, and bacterial endotoxin activate endogenous shedding enzymes (heparanases, hyaluronidases, and matrix metalloproteinases). Circulating levels of syndecan-1 and heparan sulfate surge, serving as biomarkers of severe glycocalyx degradation. Furthermore, iatrogenic hypervolemia and rapid crystalloid infusion cause atrial natriuretic peptide (ANP) release, which directly accelerates glycocalyx shedding.

Glycocalyx degradation induces severe pathophysiological derangements:

Increased Microvascular Permeability : Capillary leak causes fluid extravasation into the interstitium, generating tissue edema that increases the diffusion distance between capillaries and parenchymal cells.

Microthrombi and Capillary Plugging : Loss of glycocalyx-bound antithrombin promotes localized microvascular thrombosis, while activated, rigid neutrophils obstruct capillary lumens.

Loss of Autoregulation and Heterogeneous Perfusion : Direct visualization of the sublingual microcirculation using handheld sidestream dark-field (SDF) or incident dark-field (IDF) imaging reveals severe microcirculatory heterogeneity. Functional capillary density decreases; hyperemic, fast-flowing capillaries coexist directly adjacent to plugged or non-flowing microvessels.

As a consequence of this microvascular uncoupling, achieving standard macro-hemodynamic targets—such as a mean arterial pressure (MAP) ≥ 65 mmHg \ge 65\text{ mmHg} ≥ 65 mmHg or a normal cardiac index—does not ensure restored organ parenchymal oxygenation. Clinicians must combine macrocirculatory metrics with dynamic indicators of tissue perfusion, including capillary refill time, serum lactate clearance kinetics, and central venous oxygen saturation.