How do you interpret a cardiopulmonary stress test?

Interpreting a Cardiopulmonary Exercise Test

Cardiopulmonary exercise testing (CPET) is the gold standard for differentiating between cardiac, pulmonary, and other causes of exertional shortness of breath, providing objective measurements of exercise capacity and physiologic limitations. 1

Key Parameters to Evaluate

Primary Measurements

• Peak Oxygen Consumption (VO₂max): Measures maximal aerobic capacity
• Anaerobic Threshold (AT): Point where metabolic demand exceeds aerobic energy production
• Ventilatory Efficiency (VE/VCO₂): Reflects ventilation-perfusion matching
• Oxygen Pulse (O₂ pulse): Surrogate for stroke volume (VO₂/HR)
• Work Rate-VO₂ Relationship: Reflects exercise efficiency
• Breathing Reserve: Difference between maximal voluntary ventilation and peak exercise ventilation

Secondary Parameters

• Heart Rate Response: Chronotropic competence
• Blood Pressure Response: Normal, hypertensive, or hypotensive
• Oxygen Saturation: Evidence of exercise-induced hypoxemia
• Respiratory Exchange Ratio (RER): Indicator of effort adequacy (>1.10 indicates maximal effort)

Systematic Interpretation Approach

1. Assess Test Quality and Effort

   • Verify adequate effort (RER >1.10)
   • Ensure appropriate protocol selection (8-12 minute test duration ideal) 2
   • Check for technical issues or artifacts

2. Evaluate Exercise Capacity

   • Determine VO₂max (L/min and mL/kg/min) and compare to predicted values
   • Assess peak work rate achieved (watts or METs)
   • Analyze AT (normal: 50-60% of predicted VO₂max)

3. Identify Exercise Limitation

   • Cardiovascular limitation: Reduced O₂ pulse, early plateau in O₂ pulse-VO₂ relationship, abnormal HR response, reduced AT 2
   • Pulmonary limitation: Reduced breathing reserve (<15%), O₂ desaturation, increased VE/VCO₂ slope
   • Peripheral limitation: Normal cardiac and ventilatory responses with reduced AT
   • Deconditioning: Reduced VO₂max with normal physiologic responses

4. Analyze Exercise Patterns

   • Evaluate trending phenomena throughout exercise (submaximal to peak)
   • Assess graphical relationships (VE vs. VCO₂, VO₂ vs. work rate) 2

Specific Pattern Recognition

Cardiac Limitation Patterns

• Early plateau in O₂ pulse
• Flattened VO₂-work rate relationship
• Reduced AT (<40% predicted)
• Increased VE/VCO₂ slope (>34)
• Possible ECG changes or arrhythmias

Pulmonary Limitation Patterns

• Reduced breathing reserve (<15%)
• Exercise-induced hypoxemia (>4% drop in SpO₂)
• Increased dead space ventilation (high VD/VT)
• Ventilatory limitation (peak VE approaching or exceeding MVV)

Combined Cardiac-Pulmonary Patterns

• Features of both limitations present
• Often seen in heart failure, pulmonary hypertension

Peripheral Limitation/Deconditioning

• Normal cardiac and ventilatory responses
• Early onset of metabolic acidosis
• Reduced VO₂max and AT

Indications for Test Termination

CPET should be terminated for any of the following 2:

• Decrease in ventricular rate with increasing workload with symptoms of insufficient cardiac output
• Failure of heart rate to increase with exercise with symptoms
• Progressive fall in systolic blood pressure with increasing workload
• Severe hypertension (>250 mmHg systolic or >125 mmHg diastolic)
• Intolerable dyspnea
• Symptomatic tachycardia
• Progressive fall in oxygen saturation to <90% or a 10-point drop from resting saturation with symptoms

Clinical Application

The interpretation should conclude with:

1. Quantification of exercise capacity (normal, mild, moderate, or severe impairment)
2. Primary limitation (cardiac, pulmonary, peripheral, or deconditioning)
3. Specific pathophysiologic mechanisms contributing to exercise limitation
4. Clinical correlation with patient symptoms and presentation

Common Pitfalls to Avoid

• Submaximal effort: Tests with RER <1.0 may miss significant pathology 1
• Inappropriate protocol selection: Too aggressive or too gentle protocols can affect interpretation
• Over-reliance on single parameters: Integrate multiple variables for accurate interpretation
• Failure to consider medications: Beta-blockers affect heart rate response
• Misinterpreting ventilatory limitation: Distinguish between deconditioning and true pulmonary disease

CPET provides valuable diagnostic information beyond standard exercise testing by identifying the physiologic mechanisms limiting exercise capacity, which is crucial for clinical decision-making regarding treatment options and prognosis in patients with unexplained dyspnea, heart failure, or suspected cardiopulmonary disease.