What is the purpose and protocol for a treadmill test in assessing heart function?

Treadmill Test for Assessing Heart Function

Primary Purpose

The treadmill test (exercise ECG) serves to diagnose coronary artery disease, assess functional capacity, stratify cardiovascular risk, and guide therapeutic decisions in patients with suspected or known cardiac disease. 1

Core Indications

• Diagnostic evaluation of patients with chest pain who have normal resting ECG and are not taking digoxin, as this population achieves nearly equivalent accuracy to imaging modalities for detecting severe coronary disease 1, 2
• Risk stratification through exercise capacity measurement, which remains one of the strongest independent predictors of mortality in both men and women 1, 3
• Functional assessment in heart failure patients to measure peak oxygen consumption and guide treatment decisions, particularly for transplant evaluation 1
• Evaluation of exertional dyspnea when resting echocardiography shows grade 1 diastolic dysfunction with normal filling pressures at rest 1

Standard Protocol Components

Equipment Setup

• Treadmill must have variable speed and grade capability with accurate calibration 1
• Front and side rails for stability, though patients should avoid tight gripping to prevent workload reduction 1
• ECG monitoring, blood pressure measurement equipment, and advanced CPR capabilities must be immediately available 1

Protocol Selection

The optimal protocol should be individualized to achieve 8-12 minutes total duration with stage increments of approximately 1 MET (metabolic equivalent). 1

Standard Bruce Protocol:

• Starts at 1.7 mph at 10% grade (approximately 5 METs) 1
• Increases by several METs every 3 minutes 1
• Best suited for patients with good functional capacity 1
• Disadvantages include large interstage increments that may force premature termination 1

Modified Bruce Protocol:

• Includes initial stages at 1.7 mph at 0% and 5% grades 1
• Appropriate for patients with compromised exercise capacity 1

Ramp Protocol:

• Continuously increases workload in small increments (10-60 second intervals) 1
• Allows precise targeting of 8-12 minute test duration 1
• Provides linear heart rate and oxygen uptake responses, improving gas exchange interpretation 1
• Particularly recommended for heart failure patients 1

Test Execution

Patient must achieve maximal volitional effort; 85% of age-predicted maximum heart rate should NOT be used as a termination criterion. 1

Key measurements during testing:

• Continuous ECG monitoring for ST-segment changes 3
• Blood pressure response at each stage 3
• Heart rate response and chronotropic competence 3
• Symptoms (chest pain, dyspnea) with angina index scoring 1, 3
• Total exercise time and METs achieved 3

Recovery phase:

• Heart rate recovery measured at 1 minute post-exercise (≤12 bpm decrease indicates high risk) 3
• ST-segment changes monitored for at least 5 minutes 3

Interpretation Framework

Exercise Capacity Assessment

• High-risk threshold: <5 METs or <100% age-predicted METs 3
• Exercise capacity alone provides powerful prognostic information independent of ECG changes 1
• Patients achieving 9 minutes on Bruce protocol have extremely low subsequent cardiac event rates (1.6% at 4 years) regardless of symptoms or ECG changes 4

Duke Treadmill Score Calculation

Formula: Exercise time (minutes) - (5 × ST deviation in mm) - (4 × angina index) 1, 3

Risk stratification:

• Low risk: Score ≥5 3
• Moderate risk: Score -10 to +4 3
• High risk: Score ≤-11 3

High-Risk ECG Findings

• ST-segment depression ≥2 mm at any time 3
• ST-segment depression ≥1 mm at <5 METs or persisting >5 minutes into recovery 3
• ST-segment elevation ≥2 mm 3

High-Risk Hemodynamic Findings

• Systolic blood pressure decrease >10 mmHg from rest to peak exercise 3
• Chronotropic incompetence (inability to achieve adequate heart rate response) 3

Critical Contraindications

Absolute contraindications requiring alternative testing:

• Left bundle branch block 1, 3

• 1 mm ST depression at rest 3

• Electronically paced rhythm 1, 3
• Pre-excitation syndrome (WPW) 3
• Digoxin use (interferes with ST-segment interpretation) 1, 3

Clinical contraindications:

• Inability to perform moderate physical functioning (≥4-5 METs in daily activities) 1
• Disabling comorbidities: severe obesity, peripheral arterial disease, COPD, orthopedic limitations 1

When NOT to Perform Treadmill Testing

Patients with completely normal diastolic function at rest (septal e' >7 cm/sec, lateral e' >10 cm/sec) do not require stress testing as they are highly unlikely to develop diastolic dysfunction with exercise 1

Patients with established elevated LV filling pressures at rest should not undergo stress testing as cardiac etiology is already confirmed 1

Asymptomatic low- or intermediate-risk adults should not undergo stress echocardiography for cardiovascular risk assessment (Class III recommendation) 1

Sex-Specific Considerations

Women have lower diagnostic accuracy with standard ECG criteria alone, but integration of Duke Treadmill Score improves accuracy. 3 In female patients with equivocal or positive ETT, obstructive CAD prevalence is significantly lower (11-16%) compared to males (nearly 50% with positive ETT), suggesting lower threshold for anatomic imaging confirmation 5

Common Pitfalls

• Submaximal effort resulting in false-negative results when patient fails to achieve adequate workload despite normal heart rate response 1, 3
• Overreliance on ST-segment changes alone without incorporating exercise capacity, Duke Treadmill Score, and hemodynamic response 1, 3
• Misclassification of equivocal results as negative: 20% of equivocal ETTs show obstructive CAD on subsequent imaging 5
• Inappropriate testing in patients with baseline ECG abnormalities leading to uninterpretable results 1, 3

Mandatory Reporting Elements

Every treadmill test interpretation must document: 3

• Exercise capacity in METs
• ST-segment response with magnitude and timing
• Duke Treadmill Score
• Chronotropic response
• Heart rate recovery at 1 minute
• Symptoms during exercise
• Any arrhythmias observed