What does a flat ST segment above the isoelectric line suggest?

ST Segment Above the Isoelectric Line: Clinical Significance

A flat ST segment above the isoelectric line suggests early repolarization, a normal variant, but can also indicate myocardial injury or ischemia depending on the clinical context and degree of elevation. 1

Normal ST Segment Physiology

• The ST segment corresponds to the plateau phase of the ventricular transmembrane action potential, during which voltage changes slowly and remains at approximately the same level in all ventricular myocardial cells 1
• Under normal conditions, the ST segment should be nearly flat and at approximately the same level as the TP segment (isoelectric) due to the absence of significant voltage gradients in ventricular myocardial cells 1
• The absence of pronounced voltage gradients during the ST segment is similar to that which occurs during electric diastole (TP segment) 1

Causes of ST Segment Elevation

ST segment elevation above the isoelectric line can be attributed to several causes:

1. Normal Variant (Early Repolarization)

• Early repolarization is characterized by J-point elevation and a rapidly upsloping or normal ST segment 1
• This is a common normal variant, especially in young, healthy individuals 1
• During exercise, subjects with resting J-junction elevation (early repolarization) may develop an isoelectric J junction, which is a normal finding 1

2. Myocardial Injury

• ST elevation can represent injury currents associated with acute ischemia or ventricular dyskinesis 1
• The ST-segment changes associated with acute ischemia are due to the flow of current across the boundary between the ischemic and nonischemic zones, referred to as injury current 1, 2
• In acute myocardial infarction, this injury current creates a vector that points toward the area of injury, causing ST elevation in leads facing the injured area 2

3. Pericarditis

• ST elevation can be associated with injury currents in pericarditis 1
• This typically produces diffuse ST elevation in multiple leads 1

Normal Limits and Interpretation

• The upper normal limit for J-point amplitude varies by age, gender, and race 1
• For white men less than 40 years of age, the upper normal limit is approximately 0.3 mV (3 mm) in V2 1
• For white men 40 years and older, the upper normal limit is approximately 0.25 mV (2.5 mm) in V2 1
• For white women, the upper normal limit remains relatively unchanged with age at approximately 0.15 mV (1.5 mm) 1
• The threshold value for abnormal J-point elevation in V2 and V3 is 0.2 mV for men 40 years and older and 0.25 mV for men less than 40 years of age 1

Clinical Significance and Evaluation

• When evaluating ST elevation, it's important to consider ST-segment waveform in addition to amplitude 1
• Normal ST elevation in V2 and particularly V1 is generally sloping down steeply 1
• Normal ST elevation at 60 ms past the J point is combined with an upsloping ST segment rather than the more horizontal ST segment present in myocardial ischemia 1
• The shape of ST elevation has prognostic significance, with certain patterns associated with increased likelihood of severe left ventricular dysfunction 2
• ST elevation with a horizontal or downsloping pattern is more specific for myocardial ischemia than upsloping ST elevation 2

Common Pitfalls in ST Segment Interpretation

• Positional changes can cause ST-segment fluctuations that mimic ischemia - a telltale sign is an associated QRS change 1
• When an ST alarm sounds and a patient is found in a side-lying position, they should be returned to the supine position to accurately evaluate the ST segment 1
• If ST-segment deviation persists in the supine position, it should be considered indicative of myocardial ischemia 1
• Left ventricular hypertrophy can cause ST elevation without true ischemia, possibly related to transient electrocardiographic repolarization changes in the hypertrophied ventricle 3
• Baseline ST abnormalities are common - in one study, 63% of patients admitted for non-urgent cardiac catheterization had baseline ST-segment deviation of 1 mm 1

Measurement Considerations

• ST segment deviation is usually measured at the J point (junction of QRS and ST segment) 1
• In some settings, such as exercise testing, measurements are taken 40-80 ms after the J point 1
• At ventricular rates >130 beats/min, 60 ms after the J junction is optimal to determine ST segment displacement in patients with an upsloping ST segment 1
• When the J point is elevated at rest (early repolarization) and becomes more depressed during exercise, the magnitude of ST-segment displacement should be determined from the P-Q junction, not from the resting elevated J junction 1