What is the SCAI (Society for Cardiovascular Angiography and Interventions) classification for shock and how should it be applied to an adult patient with signs of inadequate tissue perfusion?

SCAI Classification for Cardiogenic Shock

The SCAI (Society for Cardiovascular Angiography and Interventions) classification is a five-stage system (A through E) that risk-stratifies patients with cardiogenic shock based on physical examination findings, biochemical markers, and hemodynamics, with validated stepwise increases in mortality from Stage A to Stage E. 1, 2

The Five SCAI Stages

Stage A: At Risk

• Clinical Profile: Patients are normotensive with normal tissue perfusion, clear lung fields, and no signs of hypoperfusion 1, 3
• Hemodynamics: Normal cardiac index and blood pressure 3
• Typical Patients: Those with large acute myocardial infarction, prior infarction, or acute-on-chronic heart failure symptoms who have not yet developed shock 1
• Key Feature: No current signs or symptoms of shock, but at elevated risk for deterioration 1, 2

Stage B: Beginning Cardiogenic Shock

• Clinical Profile: Relative hypotension or tachycardia without evidence of end-organ hypoperfusion 1, 3, 2
• Distinguishing Feature: Normal lactate levels, adequate urine output, and preserved mental status despite hemodynamic abnormalities 3, 2
• Critical Point: This is the pre-hypoperfusion stage—patients show early warning signs but have not yet crossed into true shock 2

Stage C: Classic Cardiogenic Shock

• Clinical Profile: Hypoperfusion requiring intervention (inotropes, pressors, or mechanical support including ECMO) beyond volume resuscitation 1, 2
• Hemodynamic Criteria:
  • Systolic blood pressure <90 mmHg 3
  • Cardiac index <2.2 L/min/m² 3
  • Lactate >2 mmol/L 3
• Physical Findings: Cold extremities, oliguria, altered mental status, and relative hypotension 1, 3
• Key Threshold: Presence of tissue hypoperfusion distinguishes Stage C from Stage B 2

Stage D: Deteriorating/Doom

• Clinical Profile: Similar presentation to Stage C but worsening despite initial interventions 1, 2
• Temporal Criterion: Failure to respond after ≥30 minutes of observation and treatment 3, 2
• Management Implication: Initial therapy has failed; escalation to advanced mechanical circulatory support is typically required 3
• Prognosis: Significantly higher mortality than Stage C due to refractory nature 1, 4

Stage E: Extremis

• Clinical Profile: Cardiac arrest with ongoing CPR and/or ECMO support, accompanied by refractory hypotension and hypoperfusion 1, 3
• Hemodynamics: Cardiovascular collapse requiring multiple simultaneous interventions 1
• Mortality: In-hospital mortality approaches 50% or higher 3, 4
• Key Feature: Represents the most severe end of the shock spectrum with circulatory collapse 2

Hemodynamic Parameters for Classification

Cardiac Index Thresholds:

• Normal: 2.2-4.0 L/min/m² 3
• Shock: <2.2 L/min/m² 3
• Severe shock: <1.8 L/min/m² 3

Blood Pressure Criteria:

• Stage C and higher: Systolic BP <90 mmHg for ≥30 minutes or requiring vasopressors to maintain >90 mmHg 1, 3

Lactate as Perfusion Marker:

• Normal: <2 mmol/L 3
• Hypoperfusion: >2 mmol/L 3
• Severe hypoperfusion/poor prognosis: >7 mmol/L 5

Cardiac Power Output:

• Refractory shock threshold: <0.6 W (most critical marker for identifying need for mechanical support) 3

Filling Pressures:

• Pulmonary capillary wedge pressure >15 mmHg indicates left ventricular failure 3
• Central venous pressure >15 mmHg suggests right ventricular or biventricular involvement 3

Shock Phenotypes Within SCAI Stages

Left Ventricular-Dominant:

• PCWP >15 mmHg with CVP <15 mmHg 3
• Prominent pulmonary congestion with clear systemic venous system 3

Right Ventricular-Dominant:

• CVP >15 mmHg with PCWP <15 mmHg 3
• Jugular venous distension and hepatomegaly but clear lung fields 3

Biventricular:

• Both PCWP >15 mmHg and CVP >15 mmHg 3
• Combined pulmonary and systemic congestion; typically requires biventricular support 3

Cardiac Arrest Modifier

Critical Consideration: At every SCAI stage, the presence of cardiac arrest significantly increases mortality 1, 5

• Cardiac arrest is common across all stages and confers independent mortality risk 5
• Stage E specifically includes ongoing cardiac arrest requiring CPR 1
• Post-arrest patients who are comatose (Glasgow Coma Scale <8) should receive targeted temperature management 5

Validated Prognostic Value

Mortality Increases Stepwise:

• Multiple validation studies demonstrate progressive mortality increases from Stage A through E 1, 4
• The Mayo Clinic series of 10,004 patients showed stepwise mortality increases after multivariable adjustment 1
• The National Cardiogenic Shock Initiative demonstrated survival rates of 76% (Stage C), 76% (Stage D), and 58% (Stage E) 6

Reproducibility:

• Near-perfect inter-rater agreement (kappa = 0.975-0.985) between independent clinicians 6
• Classification remains relatively stable after the first 6-12 hours of admission 7

Practical Application Algorithm

Step 1: Initial Assessment

• Measure systolic blood pressure, heart rate, and calculate shock index (HR ÷ SBP) 8
• Assess for hypoperfusion: lactate level, urine output (<0.5 mL/kg/h), mental status, extremity temperature 1, 3
• Obtain cardiac index via echocardiography or pulmonary artery catheter if available 1, 3

Step 2: Assign SCAI Stage

• No hypotension or hypoperfusion but high-risk features → Stage A 1
• Hypotension or tachycardia WITHOUT hypoperfusion → Stage B 1, 2
• Hypoperfusion requiring pharmacologic/mechanical support → Stage C 1, 2
• Stage C features but worsening after 30 minutes of treatment → Stage D 1, 2
• Cardiac arrest with ongoing CPR/ECMO → Stage E 1, 2

Step 3: Identify Phenotype (Stages C-E)

• Obtain invasive hemodynamics via pulmonary artery catheter 1, 3
• Measure PCWP and CVP to determine LV-dominant, RV-dominant, or biventricular pattern 3
• Calculate cardiac power output if considering mechanical support 3

Step 4: Reassess Every 6-12 Hours

• Most patients reach their maximum stage within 6 hours of admission 7
• Stage progression or regression guides treatment escalation or de-escalation 7
• Patients in Stage E at 24 hours have <20% survival regardless of baseline stage 6

Common Pitfalls and Caveats

Pitfall 1: Assuming Normal Blood Pressure Excludes Shock

• Stage B patients may have near-normal systolic pressure despite early shock physiology 1
• "Preshock" patients with compensatory vasoconstriction can maintain BP >90 mmHg despite tissue hypoperfusion 1
• Always assess lactate and end-organ perfusion markers, not just blood pressure 3

Pitfall 2: Delayed Recognition of Stage Progression

• 78% of patients change SCAI stage within 6 hours of admission 7
• Failure to reassess frequently can miss deterioration from Stage C to D 7
• Implement continuous monitoring protocols rather than single time-point assessments 4

Pitfall 3: Confusing Distributive and Cardiogenic Shock

• Late-stage septic shock can develop myocardial depression but maintains decreased SVR (distributive pattern) 3
• Cardiogenic shock has elevated SVR as a compensatory mechanism 3
• Use hemodynamic profiling (cardiac index, SVR, PCWP) to differentiate when clinical picture is unclear 3

Pitfall 4: Underestimating Cardiac Arrest Impact

• Cardiac arrest at any stage dramatically worsens prognosis 1
• Do not assume electrolyte abnormalities caused the arrest; treat underlying shock 5
• Hypokalemia often results from cardiac arrest itself rather than being the primary cause 5

Pitfall 5: Inadequate Phenotyping

• Failing to distinguish LV-dominant, RV-dominant, and biventricular patterns leads to inappropriate mechanical support selection 3
• Biventricular shock requires different support strategies than isolated LV failure 3
• Obtain invasive hemodynamics early in Stage C or higher 1

Integration with Treatment Decisions

Stage A: Close monitoring, optimize medical therapy, address precipitating factors 1

Stage B: Initiate inotropes/pressors if hemodynamics worsen, prepare for potential escalation 1, 2

Stage C: Immediate coronary angiography if AMI-related (within 2 hours), consider mechanical support if cardiac power output <0.6 W 1, 3

Stage D: Escalate to advanced mechanical circulatory support (Impella, VA-ECMO) based on phenotype 3

Stage E: Ongoing CPR/ECMO, multidisciplinary shock team activation, consider palliative care consultation if futile 1