How to diagnose demand ischemia on an electrocardiogram (ECG)?

From the Guidelines

Demand ischemia on an electrocardiogram (ECG) is diagnosed by looking for ST-segment depression, specifically horizontal or downsloping ST depression ≥1 mm (0.1 mV) at 60 to 80 ms after the J point.

Diagnostic Criteria

The standard criteria for test positivity include:

• Horizontal or downsloping ST depression ≥1 mm (0.1 mV) at 60 to 80 ms after the J point 1
• When modest resting ST depression is present on the upright control ECG before exercise, only additional ST depression during exercise is measured for analysis
• In the presence of resting ST-segment elevation at 60 to 80 ms after the J point because of early repolarization, only ST-segment changes below the P-Q baseline should be used for analysis

Key Considerations

• Upsloping ST depression during exercise is not usefully predictive for the presence of myocardial ischemia in general populations, and is generally defined as an “equivocal” test response 1
• The degree, time of appearance, duration, and number of leads with ST-segment depression are related to the probability and severity of CAD 1
• The anatomic and functional severity of CAD can also be related to the time of appearance of ischemic ST-segment depression, with earlier appearance indicating worse prognosis and more likely presence of multivessel disease 1

Clinical Context

In patients with suspected acute coronary syndromes (ACS), the ECG pattern provides important prognostic information, with patients having ST-segment deviation (ST-segment elevation or depression) at higher risk for death than those with isolated T-wave inversion or normal ECG patterns 1

• The magnitude of the ECG abnormality also provides important prognostic information, with greater ST-segment deviation associated with higher risk of death or new MI 1

From the Research

Diagnosing Demand Ischemia on an ECG

To diagnose demand ischemia on an electrocardiogram (ECG), several key points should be considered:

• Demand ischemia, also known as supply/demand mismatch, occurs when the heart's oxygen demand exceeds its supply, often due to increased heart rate or blood pressure, without a complete blockage of a coronary artery 2.
• The ECG is a crucial tool in diagnosing ischemia, with changes in the ST segment and T waves being indicative of ischemia 2, 3.
• ST segment depression in leads other than V1-V3 is often indicative of subendocardial ischemia, which can be caused by a supply/demand mismatch 2.
• Exercise-induced ST-segment depression is a characteristic finding associated with exercise-induced, demand-driven ischemia in patients with significant coronary obstruction but no flow limitation at rest 3.
• The diagnostic capacity of the exercise ECG can be improved by considering the sum of ST-segment depression or the most depression in the three leads representing the three main areas of the myocardium (II, V2, and V5) 3.

ECG Patterns and Ischemia

Different ECG patterns can be indicative of ischemia, including:

• ST segment elevation, which usually indicates acute occlusion of an epicardial artery with ongoing transmural ischemia 2.
• ST segment depression, which can be indicative of subendocardial ischemia or a supply/demand mismatch 2, 3.
• T wave changes, which can also be indicative of ischemia 2.
• The clinical scenario and comparison to previous ECGs can aid in the diagnosis and interpretation of ECG changes 2.

Modern ECG Systems and Ischemia Detection

Modern ECG systems, including vector-based electrocardiography, can facilitate and optimize the detection of ischemic ECG alterations 4.

• These systems can improve the detection of ECG alterations typical for ischemia compared to the conventional 12-lead ECG 4.
• Smart nonvector-based devices for patients can be useful for detection of arrhythmias and documentation of temporary ECG alterations, but do not replace the 12-lead ECG for detection of ischemia 4.