Pre-operative workup

• Medical Hx and clinical exam
• Bloods
  • FBC, U+E, LFTs, ABGs
• ECG
• Echo
• Pulmonary Function Tests – FEV1/FVC
• Basic exercise tolerance
  • Distance on flat, ? 2 flights of stairs
• CPET testing if borderline

Metabolic Equivalents

• Overview
  • Metabolic equivalents (METs) are a standardized way to measure exercise intensity and energy expenditure.
  • One MET represents the amount of oxygen consumed at rest
  • METs increase with higher activity levels.
• Definition of METs
  • 1 MET = 3.5 mL O₂/kg/min
  • This is the amount of oxygen consumed per kilogram of body weight per minute at rest.
  • Exercise capacity is often reported in METs, making it easier to compare across different populations.
• METs and Cardiopulmonary Exercise Testing (CPEX)
  • A. VO₂ Max and METs Conversion
  • 1 MET = 3.5 mL O₂/kg/min
  • VO₂ max (mL/kg/min) ÷ 3.5 = Peak METs Achieved
    • For example
      • If a patient has a VO₂ max of 35 mL/kg/min, then:
      • 35 ÷ 3.5 = 10 METs
• METs as a Measure of Functional Capacity
  • METs provide a simple way to assess exercise tolerance in clinical and perioperative settings.
  • Higher METs correlate with better cardiovascular fitness.
  • Low METs indicate impaired aerobic capacity (e.g., heart failure, pulmonary disease).
• METs and Perioperative Risk Stratification
  • In preoperative assessment, METs are used to predict surgical risk based on functional capacity.
  • Functional Capacity and Surgical Risk
    • METs Achieved Functional Status Surgical Risk

    • 10 METs Excellent fitness Low risk for most surgeries

    • 4–10 METs Moderate fitness Generally safe for major surgery

    • <4 METs Poor fitness High surgical risk; further evaluation needed
  • ≥4 METs is often considered the threshold for safe surgery.
  • If a patient cannot achieve 4 METs, additional CPEX testing, cardiac stress tests, or prehabilitation may be needed.
• Estimating METs Clinically (Without CPEX)
  • Functional capacity can be estimated based on patient-reported activities:
    • Activity - Approximate METs
    • Sitting quietly - 1 MET
    • Walking slowly (~2 km/h) - 2 METs
    • Climbing a flight of stairs - 4–5 METs
    • Brisk walking (~6 km/h) - 6 METs
    • Jogging (~8 km/h) - 8 METs
    • Running (~10 km/h) - 10 METs
    • Competitive sports (e.g., soccer) - 12–15 METs
  • If a patient cannot walk up a flight of stairs or carry groceries, they likely have low METs (<4) and may need further evaluation.

Cardiopulmonary Exercise Testing

Clinical Applications

• Perioperative Risk Assessment: Predicts surgical outcomes (e.g., major abdominal, cardiothoracic, vascular surgery).
• Heart Failure & Cardiovascular Disease: Differentiates cardiac vs. respiratory limitations.
• Respiratory Disease: Assesses lung disease severity (e.g., COPD, ILD, pulmonary hypertension).
• Exercise Intolerance & Unexplained Dyspnea: Identifies whether cardiac, pulmonary, or metabolic issues limit exercise.
• Athletic Performance Evaluation: Measures maximal oxygen uptake (VO₂ max) for training optimization.

Test Protocol

• Conducted on a treadmill or cycle ergometer (bike).
• The patient wears a face mask or mouthpiece connected to a metabolic cart that analyzes oxygen (O₂) and carbon dioxide (CO₂) exchange.
• ECG leads are attached for continuous heart monitoring.
• Blood pressure and pulse oximetry are measured throughout.
• The test begins at a low intensity and gradually increases in difficulty (ramp protocol) until the patient reaches their limit (symptom-limited maximal effort).

Key Parameters Measured

Aerobic Capacity and Oxygen Consumption

• VO₂ Max (Maximum Oxygen Consumption)
  • The gold standard for cardiorespiratory fitness.
  • Represents the highest oxygen uptake achieved during maximal effort.
  • Normal values: ~30–50 mL/kg/min (varies by age, sex, and fitness level).
  • Low VO₂ max suggests cardiac, pulmonary, or mitochondrial dysfunction.
• Anaerobic Threshold (AT) / Ventilatory Threshold (VT1)
  • The point where lactate accumulates, causing a rise in ventilation disproportionate to oxygen uptake.
  • Key in perioperative risk stratification: AT <11 mL/kg/min suggests high surgical risk.

Ventilatory Efficiency & Breathing Patterns

• VE/VCO₂ Slope (Ventilatory Efficiency)
  • Measures how effectively CO₂ is cleared during exercise.
  • Elevated in heart failure & pulmonary hypertension.
• Breathing Reserve (BR)
  • Percentage of maximal voluntary ventilation (MVV) used during peak exercise.
  • Low BR (<15%) suggests ventilatory limitation (e.g., COPD, restrictive lung disease).

Cardiovascular Markers

• Heart Rate (HR) Response
  • Chronotropic incompetence (failure to increase HR adequately) may indicate cardiac dysfunction.
• O₂ Pulse (VO₂/HR)
  • A surrogate for stroke volume.
  • Low values suggest impaired cardiac output.

Metabolic Data

• Respiratory Exchange Ratio (RER = VCO₂/VO₂)
  • Indicates effort level (RER >1.1 confirms maximal effort).
• Lactate Threshold
  • Helps assess metabolic function and endurance capacity.