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Electrocardiographic Differential Diagnosis of ST Segment Depression

Amita Sudhir¹ and William J. Brady²

¹ Department of Emergency Medicine, University of Virginia School of Medicine, Charlottesville, VA, USA

² Departments of Emergency Medicine and Medicine, University of Virginia School of Medicine, Charlottesville, VA, USA

ST segment depression is defined as an ST segment that is depressed at least 1 mm or more below the baseline. Although ST segment depression is most commonly clinically associated with ischemia, there are several other possible causes of ST segment depression. Table 26.1 brings out the differential diagnosis of ST segment depression.

Acute Coronary Syndromes

Acute coronary syndrome (ACS) can present with ST segment depression in one of many forms. The three basic types include ST segment depression related to unstable angina or non‐ST elevation myocardial infarction (NSTEMI), due to posterior wall acute myocardial infarction (AMI), and resulting from reciprocal changes in ST elevation myocardial infarction (STEMI). These three types of ST segment depression include the following:

Unstable angina or NSTEMI with diffuse ST depression (Figure 26.1).
Posterior myocardial infarction when viewed from the anterior perspective of the 12‐lead electrocardiogram (ECG) (Figures 26.2 and 26.3).
Reciprocal ST segment depression in the setting of a coexisting STEMI (Figures 26.2 and 26.4).

Left Bundle Branch Block

Left bundle branch block (LBBB) pattern is noted when the QRS complex duration is greater than 0.12 seconds with a mainly negatively oriented QRS complex in leads V1 and V2 and mainly positively oriented QRS complexes in leads I, aVL, V5, and V6. In leads where the primary portion of the QRS complex is positively oriented, the ST segment is depressed below the baseline. Therefore, the ECG leads with large monophasic R waves demonstrate ST segment depression. The ST segment is depressed in a downsloping shape that fuses with the inverted T wave (Figure 26.5). Ventricular paced patterns will demonstrate similar findings (Figure 26.6).