Residency · Residency · Internal Medicine

Shock: Classification, Hemodynamic Profiles, and Vasopressor Selection

Introduction

Shock is a life-threatening state of circulatory failure resulting in inadequate tissue perfusion and cellular oxygen delivery. It is defined by the presence of hemodynamic instability, clinical signs of organ hypoperfusion, and biochemical evidence of cellular dysoxia (elevated lactate). Rapid identification of the shock type and initiation of targeted therapy are essential to prevent irreversible organ damage and death.

Classification of Shock

Distributive Shock (66% of Shock Cases)

• Septic shock: infection-induced vasodilation and capillary leak; most common type overall
• Anaphylactic shock: IgE-mediated or direct mast cell activation
• Neurogenic shock: loss of sympathetic tone (spinal cord injury above T6); bradycardia with hypotension
• Adrenal crisis: acute cortisol deficiency; refractory hypotension

Cardiogenic Shock (16%)

• Pump failure: acute MI (especially anterior STEMI), acute decompensated HF, myocarditis, takotsubo cardiomyopathy
• Mechanical complications: acute mitral regurgitation, ventricular septal rupture, free wall rupture
• Arrhythmias: sustained VT, complete heart block, rapid AF with reduced EF

Hypovolemic Shock (16%)

• Hemorrhagic: trauma, GI bleeding, ruptured AAA, postpartum hemorrhage
• Non-hemorrhagic: dehydration, burns, third-spacing, pancreatitis

Obstructive Shock (2%)

• Cardiac tamponade: pericardial effusion compressing chambers
• Tension pneumothorax: mediastinal shift with impaired venous return
• Massive pulmonary embolism: RV failure from acute pressure overload

Hemodynamic Profiles

• Cardiac output (CO): normal 4-8 L/min; cardiac index (CI) > 2.2 L/min/m2
• SVR: systemic vascular resistance; reflects afterload
• ScvO2: central venous oxygen saturation; low (< 70%) indicates increased extraction (inadequate delivery)
• Lactate: marker of anaerobic metabolism; serial trending more important than single value

Initial Assessment and Resuscitation

The First 10 Minutes

• Secure airway and breathing; initiate supplemental oxygen; prepare for intubation if needed
• Obtain large-bore IV access (two 18G or larger)
• Rapid bedside assessment: JVP/IVC ultrasound (volume status), lung exam (pulmonary edema), cardiac auscultation (murmurs, muffled sounds), extremity temperature (warm vs. cold)
• Point-of-care ultrasound (POCUS): cardiac function, IVC collapsibility, pleural/pericardial effusion, abdominal free fluid
• Labs: lactate, blood gas, CBC, BMP, coagulation studies, blood cultures (if infection suspected), troponin
• ECG and portable CXR

Fluid Resuscitation

• Distributive and hypovolemic shock: initial bolus of 30 mL/kg crystalloid (balanced solutions preferred: lactated Ringer's)
• Cardiogenic shock: cautious fluid administration; avoid volume overload
• Obstructive shock: treat the underlying cause (pericardiocentesis, needle decompression, thrombolysis/embolectomy)
• Assess fluid responsiveness before subsequent boluses: passive leg raise, pulse pressure variation, IVC variability, stroke volume variation

Vasopressor and Inotrope Selection

Norepinephrine

• First-line vasopressor for septic shock and most forms of distributive shock
• Predominantly alpha-1 agonist with moderate beta-1 activity
• Dose: 0.1-1 mcg/kg/min (up to 2 mcg/kg/min in severe cases)
• Superior to dopamine for septic shock (fewer arrhythmias, lower mortality in cardiogenic shock subgroup -- SOAP II trial)

Vasopressin

• Second-line agent in septic shock, added when norepinephrine is 0.25-0.5 mcg/kg/min
• V1 receptor agonist: non-catecholamine vasoconstrictor
• Fixed dose: 0.03-0.04 units/min (do not titrate as a sole vasopressor)
• VASST trial: vasopressin + norepinephrine reduced mortality in less severe septic shock subgroup
• May allow catecholamine dose reduction (catecholamine-sparing effect)

Epinephrine

• Alpha and beta-1/beta-2 agonist: increases MAP, heart rate, and cardiac output
• Second-line in septic shock; first-line in anaphylactic shock
• Dose: 0.01-0.5 mcg/kg/min
• Risk: tachyarrhythmias, mesenteric ischemia, increased lactate (aerobic glycolysis, not necessarily worsening tissue perfusion)

Phenylephrine

• Pure alpha-1 agonist: increases SVR without increasing heart rate
• Indicated when tachyarrhythmias limit norepinephrine use
• Avoid in cardiogenic shock (increases afterload without augmenting CO)

Dobutamine

• Beta-1 and beta-2 agonist: increases cardiac output with mild vasodilation
• First-line inotrope for cardiogenic shock (often combined with norepinephrine for MAP support)
• Dose: 2.5-20 mcg/kg/min
• Risk: hypotension (vasodilation), tachyarrhythmias

Milrinone

• PDE-3 inhibitor: inotrope and vasodilator (inodilator)
• Useful in RV failure, pulmonary hypertension, and post-cardiac surgery
• Renally cleared; prolonged effect after discontinuation
• Risk: hypotension, arrhythmias

Septic Shock: Specific Management

• Surviving Sepsis Campaign (SSC) 2021 bundles:
• Measure lactate; remeasure within 2-4 hours if elevated
• Obtain blood cultures before antibiotics
• Broad-spectrum antibiotics within 1 hour of recognition
• 30 mL/kg crystalloid for hypotension or lactate >= 4
• Vasopressors to maintain MAP >= 65 mmHg
• Stress-dose steroids: hydrocortisone 50 mg IV q6h if norepinephrine >= 0.25 mcg/kg/min for >= 4 hours (ADRENAL, APROCCHSS trials)
• Target MAP >= 65 mmHg (higher target of 80-85 not beneficial in SEPSISPAM trial; may benefit chronic hypertension patients)

Cardiogenic Shock: Specific Management

• Revascularization: emergent PCI for acute MI-related cardiogenic shock (SHOCK trial)
• Avoid excessive fluid; consider early vasopressor/inotrope support
• Mechanical circulatory support: intra-aortic balloon pump, Impella, ECMO for refractory cases
• Avoid high-dose vasopressors; they increase afterload and myocardial oxygen demand

Key Clinical Pearls

• Norepinephrine is the first-line vasopressor for nearly all shock states except anaphylaxis (epinephrine) and pure cardiogenic shock (dobutamine + norepinephrine)
• Always assess fluid responsiveness before giving additional fluid boluses; liberal fluids cause harm in cardiogenic shock and may not benefit in late sepsis
• A rising lactate despite hemodynamic targets being met should prompt reassessment of the diagnosis, source control adequacy, and microcirculatory perfusion
• Bedside POCUS can differentiate shock types within minutes and should be part of every shock evaluation

References

1. Evans L, Rhodes A, Alhazzani W, et al. Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock 2021. Crit Care Med. 2021;49(11):e1063-e1143.
2. De Backer D, Biston P, Devriendt J, et al. Comparison of Dopamine and Norepinephrine in the Treatment of Shock (SOAP II). N Engl J Med. 2010;362(9):779-789.
3. Hochman JS, Sleeper LA, Webb JG, et al. Early Revascularization in Acute Myocardial Infarction Complicated by Cardiogenic Shock (SHOCK). N Engl J Med. 1999;341(9):625-634.
4. Scheeren TWL, Bakker J, Kaufmann T, et al. Current Use of Vasopressors in Septic Shock. Ann Intensive Care. 2023;13(1):13.

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