What happens during ST elevation on an electrocardiogram (ECG) in terms of contraction, depolarization, and repolarization of the heart chambers?

ST Elevation on ECG: Understanding the Electrophysiological Process

ST elevation on an ECG represents an injury current flowing across the boundary between ischemic and non-ischemic zones of the myocardium, indicating abnormal ventricular repolarization that can signify acute myocardial injury. 1

Electrophysiological Basis of ST Elevation

• ST elevation occurs during the plateau phase of the ventricular action potential, which corresponds to early repolarization of the ventricles 1
• The ST segment normally begins at the J point (the junction between the QRS complex and ST segment) and represents the period when the ventricles have been completely depolarized 1
• During normal cardiac function, the ST segment should be isoelectric (at the same level as the TP or PR segment) 1

Depolarization vs. Repolarization

• Depolarization of the ventricles is represented by the QRS complex on the ECG, which occurs before the ST segment 1
• The ST segment represents the early phase of ventricular repolarization, when all ventricular cells should be at the same electrical potential (plateau phase) 1
• When injury or ischemia is present, the damaged cells cannot maintain normal electrical potential during this plateau phase, creating a voltage gradient between injured and healthy tissue 1

Pathophysiology of ST Elevation

• ST elevation is caused by an "injury current" that flows between damaged and healthy myocardium 1
• In acute myocardial infarction, the injury current is due to the flow of electrical current across the boundary between ischemic and non-ischemic zones 1
• This current creates a vector that points toward the area of injury, causing ST elevation in leads facing the injured area 1

Chamber Involvement

• ST elevation in leads V1-V4 typically indicates anterior wall ischemia/infarction involving the left ventricle, caused by occlusion of the left anterior descending coronary artery 1
• ST elevation in leads II, III, and aVF indicates inferior wall infarction, typically involving the right coronary artery or left circumflex coronary artery 1
• The specific pattern of ST elevation can help identify which coronary artery is occluded and the location of the occlusion within that artery 1

Types of ST Elevation

ST elevation can be classified into three main categories:

1. Normal variant (early repolarization): Characterized by J-point elevation with rapidly upsloping or normal ST segment 1
2. Injury currents from acute ischemia: Typically presents with more horizontal ST segments 1
3. Injury currents from pericarditis: Usually diffuse ST elevation in multiple leads 1

ST Elevation Morphology and Clinical Significance

• The shape of ST elevation has prognostic significance - convex ST elevation is associated with a 2.7-fold increased likelihood of severe left ventricular dysfunction compared to other patterns 2
• ST elevation with a horizontal or downsloping pattern is more specific for myocardial ischemia than upsloping ST elevation 1
• The magnitude, time of appearance, duration, and number of leads with ST elevation correlate with the severity and extent of myocardial damage 1

Reciprocal Changes

• ST elevation often occurs with reciprocal ST depression in leads with axes opposite to those showing elevation 1
• For example, anterior wall infarction with ST elevation in V1-V4 may show reciprocal ST depression in leads II, III, and aVF 1
• These reciprocal changes can help confirm the diagnosis of acute myocardial infarction and localize the affected area 3

Clinical Implications

• ST elevation ≥0.1 mV (1 mm) in at least two contiguous leads is a key diagnostic criterion for STEMI 1
• The threshold for abnormal J-point elevation varies by lead, gender, and age:
  • For men ≥40 years: 0.2 mV in V2-V3, 0.1 mV in other leads 1
  • For men <40 years: 0.25 mV in V2-V3, 0.1 mV in other leads 1
  • For women: 0.15 mV in V2-V3, 0.1 mV in other leads 1
• Continuous ST-segment monitoring can identify transient ischemia and has prognostic value for predicting complications and mortality 1

Common Pitfalls in ST Elevation Interpretation

• Not all ST elevation indicates myocardial infarction - normal variants, pericarditis, and other conditions can cause ST elevation 4
• Left ventricular hypertrophy can cause repolarization abnormalities that mimic ischemic ST changes 5
• ST elevation in leads with Q waves may represent a ventricular aneurysm rather than acute ischemia 6
• The evolution of ECG changes in STEMI follows a progression from hyperacute T waves to ST elevation to Q wave development - recognizing this evolution is important for timing the ischemic event 7

Diagnostic Algorithm for ST Elevation

1. Determine if ST elevation meets threshold criteria for the specific leads, considering age and gender 1
2. Assess ST segment morphology (concave, convex, or horizontal) 2
3. Look for reciprocal changes in opposite leads 1
4. Consider clinical context and presence of symptoms 1
5. For borderline cases in patients with left ventricular hypertrophy, calculate the ratio of ST segment to R-S wave magnitude (≥25% suggests acute MI) 5

By understanding the electrophysiological basis of ST elevation, clinicians can better interpret this important ECG finding and make appropriate clinical decisions regarding patient management.