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

From the Guidelines

Demand ischemia on an electrocardiogram (ECG) is typically diagnosed by looking for ST-segment depression ≥1 mm (0.1 mV) at 60 to 80 ms after the J point, particularly in patients with symptoms of angina.

Key ECG Findings

• ST-segment depression: 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.
• Upsloping ST depression: While upsloping ST depression during exercise is not usefully predictive for the presence of myocardial ischemia in general populations, markedly depressed upsloping ST-depression responses to exercise could identify underlying CAD and future adverse events in highly symptomatic patients with angina 1.
• T-wave changes: Inverted T waves (>1 mm) in leads with predominant R waves can also indicate ischemia, although this finding is less specific 1.

Clinical Context

• Symptomatic patients: A recording made during an episode of the presenting symptoms is particularly valuable, and transient ST-segment changes (greater than or equal to 0.05 mV [i.e., 0.5 mm]) that develop during a symptomatic episode at rest and that resolve when the patient becomes asymptomatic strongly suggest acute ischemia and a very high likelihood of underlying severe CAD 1.
• Comparison with prior ECG: Comparison with a previous electrocardiogram, if available, is extremely valuable, particularly in patients with co-existing cardiac pathology such as left ventricular hypertrophy or a previous myocardial infarction 1.

Limitations and Pitfalls

• False negatives: A completely normal ECG in patients presenting with suspicious symptoms does not exclude the possibility of an acute coronary syndrome, and up to 5% of patients with normal electrocardiograms who were discharged from the emergency department were ultimately found to have either an acute myocardial infarction or unstable angina 1.
• Alternative causes: The common alternative causes of ST-segment and T-wave changes, such as pericarditis, myocarditis, Prinzmetal’s angina, early repolarization, and Wolff-Parkinson-White syndrome, must be considered 1.

From the Research

Diagnosing Demand Ischemia on an ECG

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

• The ECG should be read in conjunction with the clinical assessment of the patient 2.
• ST segment elevation (and ST depression in leads V1-V3) in patients with active symptoms usually indicates acute occlusion of an epicardial artery with ongoing transmural ischemia 2.
• ST depression in leads other than V1-V3 is typically indicative of subendocardial ischemia secondary to subocclusion of the epicardial artery, distal embolization to small arteries, or supply/demand mismatch 2.
• Reversible ST-segment depression is the characteristic finding associated with exercise-induced, demand-driven ischemia in patients with significant coronary obstruction but no flow limitation at rest 3.

ECG Patterns and Ischemia

Different ECG patterns are associated with various types of ischemia:

• ST segment elevation is often seen in transmural ischemia 2, 4.
• ST depression is commonly observed in subendocardial ischemia 2, 4.
• Negative T waves can also be indicative of ischemia, particularly in non-ST elevation acute coronary syndromes (NSTE-ACS) 4.
• Normal ECGs can be seen in some cases of ischemia, highlighting the importance of clinical correlation and repeated ECGs 4.

Modern ECG Systems and Ischemia Detection

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

• These systems can improve the detection of ECG alterations typical for ischemia compared to conventional 12-lead ECGs 5.
• Smart nonvector-based devices for patients are primarily useful for detection of arrhythmias but can be useful for documentation of temporary ECG alterations, including ST-segment changes 5.