What is the purpose and protocol for a treadmill test in patients with suspected coronary artery disease?

Treadmill Test for Suspected Coronary Artery Disease

Primary Purpose

The treadmill test serves two critical purposes in patients with suspected coronary artery disease: risk stratification for future cardiac events (including mortality) and diagnosis of obstructive CAD, with exercise capacity being one of the strongest independent predictors of long-term survival. 1

Patient Selection Criteria

Ideal Candidates

• Intermediate pre-test probability of CAD (15-65%) 2
• Normal baseline ECG without ST-segment abnormalities 2
• Physically capable of achieving at least moderate physical functioning (performing household, yard, or recreational work and most activities of daily living) 1
• Not taking digoxin (interferes with ST-segment interpretation) 1, 3
• Able to achieve at least 85% of maximum predicted heart rate 2

Absolute Contraindications Based on Baseline ECG

• Left bundle branch block 1, 3
• Electronically paced ventricular rhythm 1
• Wolff-Parkinson-White syndrome 1, 2
• >1 mm ST depression at rest 1, 2
• ST elevation at baseline 4

These patients require stress imaging (echocardiography or nuclear perfusion) instead of standard exercise ECG. 1, 4

Standard Protocol

Exercise Protocol

• Bruce protocol is the standard approach, initiating exercise at 3.2 to 4.7 METs and increasing by several METs every 2-3 minutes 1
• Goal is maximal volitional exertion, not just 85% of age-predicted heart rate (which has high variability and should not be used as a termination criterion) 1
• Most activities of daily living require 4-5 METs, so patients unable to perform these activities will likely be unable to achieve maximal exercise 1

Key Parameters Monitored

• Exercise capacity in METs (strongest prognostic indicator) 2, 3
• ST-segment changes (≥1 mm horizontal or downsloping depression indicates ischemia) 1
• Blood pressure response (decrease >10 mmHg from rest to peak indicates high risk) 2, 3
• Angina symptoms during exercise 1
• Heart rate recovery (≤12 bpm decrease at 1 minute post-exercise indicates high risk) 3
• Ventricular arrhythmias 1, 3

Risk Stratification Using Duke Treadmill Score

The Duke Treadmill Score is the most validated and generalizable method for assessing risk and prognosis. 1

Calculation Formula

Duke Treadmill Score = Exercise time (minutes) - (5 × ST deviation in mm) - (4 × angina index) 1, 2, 3

Angina index: 0 = no angina, 1 = non-limiting angina, 2 = exercise-limiting angina 5

Risk Categories and Prognosis

• Low risk (score ≥5): 4-year survival 99%, annual mortality 0.25% 1, 5
• Moderate risk (score -10 to +4): Intermediate prognosis 2, 3
• High risk (score ≤-10 or ≤-11): 4-year survival 79%, annual mortality 5% 1, 5

Low-risk scores generally do not require further imaging studies. 2 Intermediate or high-risk scores warrant consideration of stress imaging or invasive coronary angiography. 2

High-Risk Findings

ECG Criteria

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

Functional Criteria

• Exercise capacity <5 METs or <100% age-predicted METs 2, 3
• Failure to attain at least 90% of age-predicted maximal heart rate (in absence of beta blockers) 1
• Exercise duration <6 minutes on Bruce protocol 1

Hemodynamic Criteria

• Decrease in systolic blood pressure >10 mmHg from rest to peak exercise 1, 2, 3
• Complex ventricular ectopy 1

Prognostic Value Beyond Diagnosis

Exercise capacity alone is one of the strongest indicators of long-term risk (including death) for both men and women with suspected and known CAD. 1, 2, 3 This prognostic ability is independent of and incremental to coronary anatomy and left ventricular ejection fraction. 6

Among asymptomatic populations with coronary risk factors, a positive exercise ECG is associated with 15-fold higher incidence of future coronary events in men and 5-fold higher in women. 1

Safety Profile

Expected complication rates are 0-6 deaths or cardiac arrests per 10,000 tests and 2-10 myocardial infarctions per 10,000 tests. 2 The supervising physician must be trained in advanced cardiopulmonary resuscitation, with a defibrillator and emergency medications immediately available. 2

Sex-Specific Considerations

Women have lower diagnostic accuracy with standard ECG criteria alone, but integration of the Duke Treadmill Score improves accuracy and performs equally well in both sexes. 1, 3 For women with normal resting ECG and good exercise tolerance, routine exercise treadmill test remains the recommended initial test. 1

Common Pitfalls to Avoid

• Do not use 85% age-predicted heart rate as a criterion to terminate the test (high variability among individuals of identical age) 1
• Do not perform standard treadmill testing in patients taking digoxin (interferes with ST-segment interpretation) 1, 3
• Do not interpret as negative if submaximal effort achieved (results in indeterminate estimation of CAD; consider pharmacological stress imaging instead) 1
• Do not perform standard treadmill testing in patients with baseline ECG abnormalities (left bundle branch block, paced rhythm, >1 mm ST depression, ST elevation) 1, 4