APACHE II (Acute Physiology and Chronic Health Evaluation II)

The APACHE II (Acute Physiology and Chronic Health Evaluation II) score is a widely used severity-of-illness scoring system designed for adult patients in intensive care units (ICUs). Developed in 1985, it quantifies disease severity and predicts hospital mortality risk based on physiological measurements, age, and chronic health status. The score is calculated within the first 24 hours of ICU admission and is applicable across a broad range of adult critical care conditions. It is a cornerstone tool for risk stratification, quality assessment, and research in ICUs.

Purpose

• Clinical: Assesses illness severity to guide resource allocation and inform prognosis discussions.
• Quality Benchmarking: Enables comparison of ICU performance by adjusting for patient risk profiles.
• Research: Standardizes risk adjustment in clinical trials and observational studies.
• Prognostic: Estimates hospital mortality risk, though not intended for individual patient-level decisions.

Components of APACHE II Score

The APACHE II score (range: 0–71) is calculated using three components:

1. Acute Physiology Score (APS) (0–60 points):
   • Comprises 12 physiological variables, each scored from 0 to 4 based on deviation from normal:
     • Temperature (°C)
     • Mean arterial pressure (mmHg)
     • Heart rate (beats/min)
     • Respiratory rate (breaths/min)
     • Oxygenation (PaO₂ or A-aDO₂, depending on FiO₂)
     • Arterial pH
     • Serum sodium (mmol/L)
     • Serum potassium (mmol/L)
     • Serum creatinine (mg/dL)
     • Hematocrit (%)
     • White blood cell count (×10³/mm³)
     • Glasgow Coma Scale (GCS)
   • The worst value for each variable within the first 24 hours is used.
2. Age Points (0–6 points):
   • <45 years: 0 points
   • 45–54 years: 2 points
   • 55–64 years: 3 points
   • 65–74 years: 5 points
   • ≥75 years: 6 points
3. Chronic Health Points (0–5 points):
   • 2 points for elective postoperative patients with severe chronic conditions.
   • 5 points for nonoperative or emergency postoperative patients with severe chronic conditions (e.g., liver cirrhosis, severe COPD, dialysis-dependent renal failure, immunosuppression, or NYHA Class IV heart failure).

The total score is the sum of these components. Higher scores indicate greater illness severity and mortality risk. The score is combined with a diagnostic category (e.g., sepsis, trauma) to estimate mortality probability using established equations.

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Scoring Example

• A 60-year-old patient (3 age points) with severe COPD (5 chronic health points) and the following worst physiological values:
  • Temperature 38.5°C (1 point)
  • Mean arterial pressure 50 mmHg (4 points)
  • Heart rate 140 beats/min (3 points)
  • Other variables contributing 10 points total.
• Total APACHE II score = 3 + 5 + 1 + 4 + 3 + 10 = 26.
• This score, with a diagnosis like septic shock, may predict a mortality risk of ~50–60% (depending on calibration).

Clinical Application

1. Risk Stratification: Identifies high-risk patients for intensified monitoring or interventions.
2. Prognostic Guidance: Provides objective data for discussions with families, though clinical judgment remains paramount.
3. Quality Improvement: Adjusts mortality rates for illness severity, allowing fair comparison of ICU outcomes across institutions.
4. Resource Allocation: Informs decisions about staffing, equipment, or bed management in resource-constrained settings.

Advantages

• Comprehensive: Incorporates multiple physiological parameters, age, and comorbidities.
• Validated: Extensively studied and applicable across diverse adult ICU populations.
• Standardized: Facilitates benchmarking and research with a universal framework.
• Prognostic Accuracy: Reasonably predicts group-level mortality when calibrated to specific populations.

Limitations

• Adult Focus: Not validated for pediatric or neonatal populations, where tools like CRIB II or PRISM III are used.
• Time-Sensitive: Relies on worst values in the first 24 hours, which may miss later clinical changes.
• Calibration: Mortality predictions may vary by region, ICU type, or advances in care, requiring periodic recalibration.
• Not Individualized: Designed for cohort risk, not precise individual prognosis.
• Data Burden: Requires collection of 12 physiological variables, which may be incomplete in some settings.

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Implementation

• Data Collection: Ensure accurate measurement of physiological variables, ideally using standardized protocols (e.g., arterial blood gas for pH, consistent GCS assessment).
• Calculation: Use validated APACHE II scoring tables or software for precision. Online calculators are widely available.
• Interpretation: Combine score with clinical context and diagnosis. Avoid using APACHE II alone for treatment decisions.
• Training: ICU staff should be trained in consistent data collection and score interpretation to minimize variability.

Comparison with Other Scores

• APACHE III/IV: Updated versions with more variables and refined predictions but less widely used due to complexity.
• SOFA (Sequential Organ Failure Assessment): Focuses on organ dysfunction over time, more dynamic than APACHE II.
• SAPS II (Simplified Acute Physiology Score II): Similar to APACHE II but uses fewer variables and different weighting.
• MPM (Mortality Probability Model): Emphasizes admission characteristics over physiological data.

APACHE II remains popular due to its balance of comprehensiveness and practicality.

Relevance to Non-Adult ICUs

• Pediatric ICUs: APACHE II is not suitable; use PRISM III or PIM3 (Pediatric Index of Mortality 3).
• Neonatal ICUs: Inappropriate due to unique neonatal physiology; use CRIB II, SNAP-II, or SNAPPE-II.
• Mixed ICUs: May be applied to adult patients only, with separate tools for pediatric/neonatal cases.

Conclusion

The APACHE II score is a robust and widely adopted tool for assessing illness severity and predicting mortality in adult ICU patients. Its comprehensive approach, incorporating physiological, age, and chronic health data, makes it valuable for risk stratification, quality benchmarking, and research. Medical professionals should use APACHE II as part of a broader clinical assessment, ensuring accurate data collection and context-specific interpretation. While not applicable to pediatric or neonatal populations, it remains a gold standard in adult critical care when appropriately implemented.