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Cardiopulmonary Exercise Testing (CPET) — Full Details for MD Exams (15 Marks)

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Definition & Overview

• Lungs (gas diffusion/ventilation)
• Cardiovascular system (cardiac output, O₂ transport)
• Skeletal muscle (O₂ extraction, aerobic metabolism)
• Cellular pathways (mitochondrial oxidative phosphorylation)

Braunwald's Heart Disease — "CPET is the reference standard test for assessment of exercise intolerance and dyspnea."

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Fick's Principle (Physiologic Basis)

• CO = cardiac output
• CaO₂ = arterial O₂ content
• CvO₂ = venous O₂ content

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Equipment & Protocol

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Key Variables Measured in CPET

1. Peak VO₂ (Peak Oxygen Uptake)

• Most important CPET parameter
• Expressed as mL/kg/min or % predicted
• Reflects maximal aerobic capacity; limited by cardiac output primarily
• Normal: >84% predicted (or >20 mL/kg/min roughly in healthy adults)
• Severely reduced: <10–12 mL/kg/min → poor prognosis in HF; listed for transplant
• Heart failure classification (Weber):

2. Anaerobic Threshold (AT) / Ventilatory Threshold (VT)

• The point at which aerobic metabolism is supplemented by anaerobic metabolism
• Lactic acid accumulates → stimulates extra ventilation → CO₂ rises faster than VO₂
• Detected by V-slope method (VCO₂ vs VO₂ plot — inflection point) or ventilatory equivalents method
• Normal AT: >40% of predicted VO₂max
• Low AT = early anaerobic metabolism = poor cardiovascular reserve
• AT is effort-independent → more reliable than peak VO₂ in submaximal tests

3. VE/VCO₂ Slope (Ventilatory Efficiency)

• Ratio of minute ventilation (VE) to CO₂ output during progressive exercise
• Normal: <30
• Elevated (>34–36): ventilation–perfusion (V/Q) mismatch → indicates pulmonary vascular disease, heart failure, or pulmonary hypertension
• Strongest prognostic marker in heart failure — better than peak VO₂ in some studies
• >45: severe impairment, very poor prognosis

4. Oxygen Uptake Efficiency Slope (OUES)

• Derived from VO₂ vs log(VE) relationship
• Submaximal index — effort independent
• Reduced in cardiac and pulmonary disease

5. Breathing Reserve (BR)

• Normal: >15 L/min or >20–30% of MVV at peak exercise
• Low BR (<15%): ventilatory limitation → suggests pulmonary disease (COPD, asthma, ILD)
• High BR preserved: cardiac or vascular cause of exercise limitation

6. Respiratory Exchange Ratio (RER)

• At rest: ~0.80–0.85
• RER ≥1.10–1.15 at peak exercise → confirms maximal effort (test validity)
• Used to confirm whether patient gave adequate effort

7. O₂ Pulse (VO₂/HR)

• By Fick: O₂ pulse = stroke volume × arteriovenous O₂ difference
• Surrogate for stroke volume
• Normal: >80% predicted
• Reduced O₂ pulse = reduced SV → suggests cardiac limitation (ischemia, cardiomyopathy, HF)
• Plateau of O₂ pulse during exercise = early sign of myocardial ischemia

8. Heart Rate Reserve (HRR)

• Normal: <15 bpm remaining (i.e., near-maximal HR achieved)
• Elevated HRR (>15 bpm): chronotropic incompetence or submaximal effort
• Chronotropic incompetence: failure to achieve 80% of predicted max HR at peak exercise → suggests sinus node disease, beta-blocker use, severe HF

9. Exercise Oscillatory Ventilation (EOV)

• Cyclic fluctuation of VE at rest and low-level exercise (amplitude >15%, duration >60% of exercise time)
• Marker of Cheyne-Stokes respiration pattern
• Seen in advanced heart failure; strong independent predictor of mortality

10. End-Tidal CO₂ (PETCO₂)

• At rest normal: 36–42 mmHg
• Should rise with exercise in healthy subjects
• Low resting PETCO₂ (<36 mmHg) that fails to rise → pulmonary vascular disease/PH
• Decreasing PETCO₂ with exercise = high dead space → pulmonary arterial hypertension

11. SpO₂ / Exercise-Induced Desaturation

• SpO₂ should remain ≥95% throughout
• Drop of >4% or fall below 88% → exercise-induced desaturation → pulmonary limitation (ILD, severe COPD, pulmonary vascular disease)

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CPET Patterns — Differential Diagnosis

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Clinical Applications

1. Unexplained Dyspnea

• CPET is the investigation of choice to identify the etiology (cardiac vs. pulmonary vs. vascular vs. deconditioning vs. psychogenic)

2. Heart Failure

• Risk stratification and transplant listing:
  • Peak VO₂ <12 mL/kg/min (on beta-blockers) or <14 mL/kg/min (without) → consider cardiac transplant listing
  • VE/VCO₂ slope >35 → poor prognosis
  • EOV presence → high mortality risk
• Guides cardiac rehabilitation intensity
• Monitors response to therapy

3. Pre-operative Risk Assessment

• High-risk surgery (thoracic, abdominal, vascular):
  • AT <11 mL/kg/min → high surgical risk
  • AT <8 mL/kg/min → very high risk
• Used before lung resection surgery to predict post-operative lung function reserve

4. Pulmonary Hypertension (PAH)

• VE/VCO₂ >45, low PETCO₂, low peak VO₂, low AT → classic PAH pattern
• Prognostic and follow-up tool

5. COPD and Interstitial Lung Disease

• Identifies ventilatory vs. cardiovascular predominant limitation
• Guides pulmonary rehabilitation prescription
• SpO₂ desaturation → supplemental O₂ prescription

6. Cardiac Rehabilitation & Exercise Prescription

• AT-based training ensures safe, submaximal aerobic exercise prescription

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Contraindications

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Criteria for Test Termination

• ST depression >2 mm or elevation
• Sustained VT
• Drop in SBP >10 mmHg from baseline with increasing workload
• Severe angina (≥3/4)
• Signs of poor perfusion (cyanosis, pallor)
• Technical failure

• ST or QRS changes
• Worsening chest pain
• Fatigue, wheezing, leg cramps
• BP >250/115 mmHg
• RER ≥1.15 with submaximal appearance

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9-Panel Wassermann Plot

1. VO₂ and VCO₂ vs. time
2. HR and O₂ pulse vs. VO₂
3. VE vs. VCO₂ (V-slope)
4. VE vs. time
5. VE/VO₂ and VE/VCO₂ vs. time
6. PETCO₂ and PETO₂ vs. time
7. RR and VT vs. VE
8. HR vs. VO₂
9. Workload vs. time

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Summary Box — Examiner's Key Points

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