How is a cardiopulmonary exercise test (CPET) interpreted?

Interpretation of Cardiopulmonary Exercise Test (CPET)

Cardiopulmonary exercise testing should be interpreted using an integrative approach that examines interrelationships between multiple physiological parameters rather than relying on any single measurement, as exercise limitation is typically multifactorial. 1

Key Components of CPET Interpretation

1. Pre-Test Considerations

• Determine precise reason(s) for CPET (exercise intolerance, preoperative evaluation, disability assessment)
• Review clinical information including:
  • Medical history and physical examination
  • Pulmonary function tests (PFTs)
  • Chest X-ray and ECG
  • Other relevant testing (echocardiography, bronchial provocation)
  • Medications (especially β-blockers which affect heart rate response)
  • Skeletal abnormalities that may limit exercise
  • Laboratory data (hemoglobin, carboxyhemoglobin)

2. Quality Assessment

• Evaluate overall test quality and subject effort
• Document reasons for exercise cessation
• Verify achievement of maximal effort (RER >1.10, heart rate >85% predicted)

3. Core Parameters Analysis

A. Aerobic Capacity

• Peak VO₂ (absolute value and % predicted)
  • Primary indicator of functional capacity
  • Reduced in cardiac, pulmonary, vascular disease, deconditioning

B. Cardiovascular Function

• Heart rate response (peak HR, chronotropic index)
• O₂ pulse (VO₂/HR) - surrogate for stroke volume
• Blood pressure response
• VO₂/Work rate relationship - slope normally 8.5-11 mL/min/watt

C. Ventilatory Function

• Peak ventilation (VE) relative to MVV (normally <70-80%)
• Breathing reserve (MVV-peak VE)
• Breathing pattern (respiratory rate, tidal volume)

D. Gas Exchange

• VE/VCO₂ slope - ventilatory efficiency
• PaO₂, P(A-a)O₂, VD/VT - if arterial sampling performed
• SpO₂ - oxygen desaturation
• End-tidal PCO₂ and PO₂

E. Metabolic Function

• Anaerobic threshold (AT)
  • Determined by V-slope method, ventilatory equivalents
  • Normally occurs at 50-60% of predicted VO₂max
• Respiratory exchange ratio (RER)

Integrative Interpretation Strategy

Step 1: Assess Exercise Capacity

• Determine if peak VO₂ is normal, mildly, moderately, or severely reduced
• Compare with appropriate reference values for age, sex, height, weight

Step 2: Identify Limiting Factors

Ask these key questions:

1. Does cardiovascular function contribute to exercise limitation?

   • Abnormal HR response, O₂ pulse, BP
   • Early plateau in VO₂

2. Does ventilatory function contribute to exercise limitation?

   • Low breathing reserve (<15%)
   • Abnormal breathing pattern
   • Flow limitation during exercise

3. Does pulmonary gas exchange contribute to exercise limitation?

   • Desaturation during exercise
   • Increased VE/VCO₂
   • Widened P(A-a)O₂

4. Is there premature metabolic acidosis?

   • Early AT
   • Steep VE/VCO₂ slope

Step 3: Pattern Recognition

Compare findings to characteristic patterns seen in:

• Cardiovascular diseases (heart failure, coronary artery disease)
• Pulmonary diseases (COPD, ILD, pulmonary vascular disease)
• Deconditioning
• Obesity
• Neuromuscular disorders

Common Pitfalls and Caveats

• Overreliance on single parameters: Exercise limitation is typically multifactorial; avoid algorithms based on single measurements 1
• Failure to consider multiple diseases: Patients often have coexisting conditions
• Inadequate effort: Submaximal tests limit interpretation
• Technical issues: Equipment calibration, mask leaks can affect results
• Inappropriate reference values: Use population-appropriate norms
• Medication effects: β-blockers limit heart rate response; diuretics affect metabolic parameters

Data Presentation Format

• Include both tabular and graphical display of data
• Present key parameters at peak exercise and as % predicted
• Include trending data from rest through exercise
• Essential graphs:
  • VO₂ vs. work rate
  • VE vs. VCO₂
  • Heart rate vs. VO₂
  • VE/VO₂ and VE/VCO₂ vs. time or VO₂
  • SpO₂ vs. work rate or VO₂

Safety Considerations

Before performing CPET, screen for contraindications 1:

Absolute contraindications:

• Acute myocardial infarction (3-5 days)
• Unstable angina
• Uncontrolled arrhythmias causing symptoms
• Active endocarditis/myocarditis/pericarditis
• Symptomatic severe aortic stenosis
• Uncontrolled heart failure
• Acute pulmonary embolus
• Acute non-cardiac disorders affecting exercise performance

Relative contraindications:

• Left main coronary stenosis
• Moderate stenotic valvular heart disease
• Severe untreated hypertension
• Significant pulmonary hypertension
• Tachy/bradyarrhythmias

Remember that CPET interpretation requires integration of multiple physiological parameters and should be correlated with the patient's clinical presentation. The patterns-based approach is flexible but requires clinical validation for specific disease entities.