Measurement

ST segment depression may be determined by measuring the vertical distance between the patient's trace and the isoelectric line at a location 2-3 millimeters from the QRS complex

QRS complex

The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...

.

It is significant if it is more than 1 mm in V5-V6, or 1.5 mm in AVF or III.

In a cardiac stress test

Cardiac stress test

Cardiac stress test is a test used in medicine and cardiology to measure the heart's ability to respond to external stress in a controlled clinical environment....

, an ST depression of at least 1 mm after adenosine administration indicates a reversible ischaemia, while an exercise stress test requires an ST depression of at least 2 mm to significantly indicate reversible ischaemia.

Physiology

For non-transmural ischemia, the pathophysiological cause of ST depression is a slightly elevated resting potential

Resting potential

The relatively static membrane potential of quiescent cells is called the resting membrane potential , as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential....

 in myocardial cells, but with the ST segment less affected, as it represents a depolarized state. Still, the resting potential is the reference line in ECG, making it display an apparent ST depression rather than an elevation of the other segments.

Causes

It is often a sign of myocardial ischemia

Ischemia

In medicine, ischemia is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue. It may also be spelled ischaemia or ischæmia...

, of which coronary insufficiency is a major cause. Other ischemic heart diseases causing ST depression include:

• Subendocardial ischemia or even infarction
• Non Q-wave myocardial infarction
  Myocardial infarction
  Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
  
  
• Reciprocal changes in acute Q-wave myocardial infarction (e.g., ST depression in leads I & aVL with acute inferior myocardial infarction)

Depressed but upsloping ST segment generally rules out ischemia as a cause.

Also, it can be a normal variant or artifacts

Visual artifact

Visual artifacts are anomalies during visual representation of e.g. digital graphics and imagery.-Examples in digital graphics:* Image quality factors, different types of visual artifacts...

, such as:

• Pseudo-ST-depression, which is a wandering baseline due to poor skin contact of the electrode
• Physiologic J-junctional depression with sinus tachycardia
• Hyperventilation

Other, non-ischemic, causes include:

• Side effect of digoxin
  Digoxin
  Digoxin INN , also known as digitalis, is a purified cardiac glycoside and extracted from the foxglove plant, Digitalis lanata. Its corresponding aglycone is digoxigenin, and its acetyl derivative is acetyldigoxin...
  
  
• Hypokalemia
  Hypokalemia
  Hypokalemia or hypokalaemia , also hypopotassemia or hypopotassaemia , refers to the condition in which the concentration of potassium in the blood is low...
  
  
• Right or left ventricular hypertrophy
  Ventricular hypertrophy
  Ventricular hypertrophy is the enlargement of ventricles in the heart. Although left ventricular hypertrophy is more common, enlargement can also occur in the right ventricle, or both ventricles.- Physiology :...
  
  
• Intraventricular conduction abnormalities (e.g., right
  Right bundle branch block
  A right bundle branch block is a defect in the heart's electrical conduction system.During a right bundle branch block, the right ventricle is not directly activated by impulses travelling through the right bundle branch. The left ventricle however, is still normally activated by the left bundle...
  
   or left
  Left bundle branch block
  Left bundle branch block is a cardiac conduction abnormality seen on the electrocardiogram . In this condition, activation of the left ventricle is delayed, which results in the left ventricle contracting later than the right ventricle....
  
   bundle branch block
  Bundle branch block
  A bundle branch block refers to a defect of the heart's electrical conduction system.-Anatomy and physiology:The heart's electrical activity begins in the sinoatrial node , which is situated on the upper right atrium. The impulse travels next through the left and right atria and summates at the...
  
  , WPW, etc.)
• Hypothermia
  Hypothermia
  Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
  
  
• Tachycardia
  Tachycardia
  Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...
  
  
• Reciprocal ST elevation
• Mitral valve prolapse
  Mitral valve prolapse
  Mitral valve prolapse is a valvular heart disease characterized by the displacement of an abnormally thickened mitral valve leaflet into the left atrium during systole. There are various types of MVP, broadly classified as classic and nonclassic. In its nonclassic form, MVP carries a low risk of...
  
  
• CNS disease