Can ST Segment Depression Occur in Lead I?
Yes, ST segment depression can and does occur in lead I of an EKG, most commonly as a reciprocal change in the setting of inferior wall myocardial infarction, particularly when the right coronary artery is occluded. 1, 2
Mechanisms and Clinical Contexts
Reciprocal Changes in Inferior MI
ST depression in lead I occurs primarily as a reciprocal change when acute ischemia causes ST elevation in the inferior leads (II, III, aVF):
Right coronary artery (RCA) occlusion is the most common cause, with the ST-segment spatial vector directed inferiorly and to the right, producing reciprocal ST depression in leads I and aVL (which have positive poles oriented to the left and superiorly). 1, 3
Significant ST depression in lead I indicates RCA-associated inferior wall MI with 70% sensitivity and 63% specificity. 2
When the RCA is occluded, ST elevation is typically greater in lead III than lead II, with associated reciprocal depression in leads I and aVL. 1, 3
Left Circumflex Occlusion Pattern
In contrast, when the left circumflex coronary artery (LCx) is occluded:
The ST-segment spatial vector is more likely directed to the left, resulting in ST segments that may be isoelectric or even elevated in leads I and aVL rather than depressed. 1, 3
The absence of significant ST depression in lead I suggests proximal LCx obstruction with 71% sensitivity and 65% specificity. 2
Prognostic Significance
ST depression in lateral leads including lead I carries important prognostic implications:
ST depression in two or more lateral leads (I, aVL, V5, V6) in non-ST elevation MI predicts higher in-hospital mortality (14.3% vs 2.6%), with an adjusted odds ratio of 3.5 for death. 4
Patients with lateral ST depression have more frequent left main or three-vessel coronary disease (60% vs 22%) and lower left ventricular ejection fraction. 4
Important Clinical Pitfalls
Avoid Misinterpretation
The American Heart Association recommends avoiding labeling leads as "anterior," "inferior," or "lateral" and instead using original nomenclature (I, II, III, aVR, aVL, aVF, V1-V6), as this prevents the misconception that ST changes in a lead always represent ischemia in that anatomic region. 1
Non-Ischemic Causes
ST depression in lead I can also occur in non-ischemic conditions including:
Stress Testing Considerations
During stress testing in patients with prior inferior MI, ST depression in lead I (high lateral leads) often represents reciprocal changes for inferior ST elevation rather than true ischemia, with an inverse correlation (r = -0.8) between ST shifts in inferior and high lateral leads. 6