The Significance of the ST Segment in Exercise ECG: A Key Indicator for Myocardial Ischemia Screening in Endurance Athletes

Why Endurance Athletes Should Pay Attention to the ST Segment

Endurance athletes over 35 (cyclists, runners, triathletes) face the combined pressure of an “aging cardiovascular system plus high-intensity exercise,” putting them at significantly higher risk of myocardial ischemia or arrhythmia than the general population. Research shows:

• Among marathon runners over 40, roughly 5-8% show abnormalities on ECG testing
• The prevalence of exercise-related sudden cardiac death during races is 1 in 50,000 to 1 in 200,000
• Exercise ECG (stress ECG) is the most cost-effective tool for screening myocardial ischemia

Among all the indicators involved, the ST segment is the most critical.

What Is the ST Segment

A complete heartbeat on an ECG consists of:

• The P wave (atrial depolarization)
• The QRS complex (ventricular depolarization)
• The T wave (ventricular repolarization)
• The ST segment: the flat portion between the end of the QRS complex (the J point) and the start of the T wave

A normal ST segment should be level with the baseline (the PR segment); a slight deviation of less than 0.5mm is within normal range.

ST-Segment Depression

Clinical Significance

ST-segment depression of 1mm or more (horizontal or downsloping) generally indicates:

• Myocardial ischemia: Coronary artery narrowing, where the heart’s oxygen demand increases during exercise but blood supply cannot keep up
• Myocardial hypertrophy: Left ventricular hypertrophy (a common variant in athletes’ hearts)
• Electrolyte imbalance: Hypokalemia
• Medication effects: Digitalis drugs

ST-Segment Depression During Exercise

The most dangerous warning signs are:

• Depression that appears once heart rate rises to 80-85% of max HR during exercise
• Depression magnitude greater than 2mm
• Depression that persists for more than 5 minutes after exercise ends
• Accompanied by chest tightness, chest pain, or shortness of breath

This typically indicates “exercise-induced myocardial ischemia” and requires immediate further evaluation (coronary angiography or nuclear myocardial perfusion imaging).

The Possibility of False Positives

Not all ST-segment depression is pathological. Possible non-pathological causes include:

• Hyperventilation
• Anxiety, excessive caffeine intake
• Effects of the menstrual cycle in women
• Medications (beta-blockers, digitalis)

The false-positive rate can reach 20-30%, so comprehensive clinical judgment is needed.

ST-Segment Elevation

Clinical Significance

ST-segment elevation of 1mm or more is a more serious warning sign, indicating:

• Acute myocardial infarction (STEMI): Complete occlusion of a coronary artery
• Pericarditis: Inflammation of the pericardium
• Early repolarization syndrome: A benign variant commonly seen in young athletes
• Left ventricular aneurysm

ST-Segment Elevation During Exercise

If ST-segment elevation greater than 2mm appears during or immediately after exercise:

• Acute myocardial infarction should be strongly suspected
• Exercise must be stopped immediately
• The person should be sent for emergency medical care with blood troponin testing
• Cardiac catheterization intervention may be needed within 24 hours

The Exercise ECG (Stress Test) Procedure

Indications

• First-time participation in an endurance event at age 35 or older
• A family history of cardiovascular disease
• Exercise-related chest tightness or shortness of breath
• Poorly controlled hypertension, diabetes, or hyperlipidemia

Test Procedure (Standard Bruce Protocol)

• The patient walks/runs on a treadmill with staged increases in speed and incline
• Each stage lasts 3 minutes, for a total of 7 stages (maximum incline 18%, maximum speed 6.0 mph)
• ECG is recorded continuously throughout
• Blood pressure, symptoms, and perceived exertion are also measured

Criteria for Terminating the Test

• Target heart rate reached (85% of predicted max HR)
• ST-segment depression greater than 2mm or elevation greater than 1mm
• Systolic blood pressure drop greater than 10mmHg
• Serious ventricular arrhythmia
• Patient reports chest pain or dizziness

“False Positive” Risk in Endurance Athletes

Many athletes show ECG variants without any underlying pathology.

Athlete’s Heart Syndrome

• Slightly thickened left ventricular wall (physiological hypertrophy)
• Slow heart rate (resting HR below 50 bpm)
• Arrhythmias (atrial or ventricular premature contractions)
• Early repolarization of the ST segment (most common in young men)

Principles for Distinguishing Benign Variants

• The variant is benign if it resolves with recovery after exercise
• Cardiac structure is normal (normal echocardiogram)
• No symptoms are present

Recommendations for Pre-Race ECG Screening

Under Age 35

• Routine screening is not necessary
• Those with a family history are advised to have one baseline test

Ages 35-50

• A resting ECG every 2-3 years
• An exercise ECG is recommended before a first marathon or the Wuling climb

Age 50 and Above

• A resting ECG once a year
• An exercise ECG every 2 years
• Those with hypertension or diabetes should also have an echocardiogram

What to Do If Warning Signs Appear

Chest Tightness or Pain During a Race

• Stop competing immediately
• Find the medical station
• Do not push through to finish – it could be fatal

Chest Tightness During Regular Training

• See a cardiologist
• Get an exercise ECG plus an echocardiogram
• A 24-hour Holter monitor if necessary

Abnormal ECG Without Symptoms

• Do not try to interpret it yourself
• Have a sports medicine physician conduct a comprehensive evaluation
• Some variants are common and benign in athletes

Conclusion

Changes in the ST segment serve as a “warning light” for the cardiovascular health of endurance athletes. Anyone over 35, with cardiovascular risk factors, or preparing to enter a major event is advised to complete an exercise ECG. If chest tightness, chest pain, or ST-segment depression greater than 2mm occurs during exercise, seek medical attention immediately – do not bet your life on the assumption that “it’s probably nothing.”

Related Reading

• Runner’s Cardiac Enlargement and Arrhythmia: The Long-Term Effects of Endurance Training
• Cardiac Screening for Athletes: Cost-Effectiveness of 12-Lead ECG vs. ECG + Echocardiogram
• Heart Health for Endurance Athletes: Athletic Cardiac Hypertrophy vs. Pathological Heart Disease
• Heart Health for Older Athletes: Recommended Pre-Exercise Cardiac Screening Items