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ST‐Segment Analysis in Ambulatory ECG (AECG or Holter) Monitoring in Patients with Coronary Artery Disease: Clinical Significance and Analytic Techniques

Since the invention of the ambulatory ECG (AECG or Holter) monitor by Norman Holter in 1961, the methodologies and applications of continuous recording of the ECG have evolved tremendously. The pioneering work of Bruce DelMar led to the first commercially available AECG in 1962 and the methodologies have become refined to the degree that the devices now only weight a few ounces and use solid-state memory circuits to record up to 7 days of continuous ECGs. The original AECGs were primarily used to detect rhythm disturbances, but early pioneering studies investigated the presence and significance of ST-segment depression.1 With better fidelity of the low-frequency signal of the ST segment, monitoring for the presence of myocardial ischemia has now become routine. Ambulatory or continuous monitoring of the ECG provides a unique insight into the presence and severity of myocardial ischemia in patients with coronary artery disease (CAD). AECG monitoring is generally performed in two clinical settings to assess ischemic risk: as outpatient evaluation of stable CAD patients and as inpatient evaluation of patients with an acute coronary syndrome (ACS) who need more prolonged recordings (1–7 days). AECG monitoring of stable CAD patients allows for assessment of ischemic jeopardy in their usual environment when the patient is exposed to the physical and emotional stresses of routine daily life. Such insights concerning ischemia are fundamentally different than assessments of ischemia in a supervised laboratory setting, where exercise stress testing is usually performed. Although there is a relationship between indices of ischemia during AECG monitoring (e.g., number and duration of ischemic episodes) and indices during exercise stress testing (e.g., exercise duration to 1.0 mm ST-