Electrocardiogram ST Segment Elevation

ST segment elevation on an electrocardiogram can signal a life-threatening heart attack requiring immediate treatment, but it can also appear in several other conditions that are less urgent. Understanding the difference is crucial for proper medical care.

Table of contents

• What is the ST Segment?
• Causes of ST Segment Elevation
• ST Segment Elevation in Heart Attacks
• Other Medical Conditions Causing ST Elevation
• How ST Elevation is Detected
• Treatment Approaches

What is the ST Segment?

The ST segment is a specific part of the electrocardiogram (ECG), which is a test that records the electrical activity of your heart. The ECG shows the heart’s activity as a wave pattern on paper or a screen, with different parts of the wave labeled with letters^[1].

The ST segment appears on the ECG between the end of the QRS complex (which represents the electrical signal that causes the lower chambers of your heart to contract) and the beginning of the T wave (which represents the heart muscle recovering after contraction). In a healthy heart, the ST segment is normally flat and level with the baseline, appearing as an isoelectric or electrically neutral section^[1][2].

The ST segment corresponds to a period in the heart’s electrical cycle called the plateau phase. During this time, the heart muscle maintains contraction to push blood out of the lower chambers. Normally, there are very small voltage changes in the heart muscle cells during this phase, which is why the ST segment appears flat^[1].

The point where the QRS complex ends and the ST segment begins is called the J point. This junction point is important because doctors measure any elevation or depression of the ST segment from this point^[1].

Causes of ST Segment Elevation

ST segment elevation occurs when the ST segment rises above the normal baseline level on an electrocardiogram. An ST elevation is considered significant if the vertical distance from the baseline is at least 0.1 millivolts (usually 1 millimeter or 1 small square on ECG paper) in a limb lead, or 0.2 millivolts (2 millimeters or 2 small squares) in a precordial lead^[6].

The most important cause of ST segment elevation is myocardial ischemia or infarction, which means reduced blood flow to the heart muscle or death of heart tissue^[2]. However, there are many other conditions that can cause ST elevation. These include^[2]:

• Acute heart attack (myocardial infarction)
• Coronary artery spasm (also called Prinzmetal’s angina)
• Pericarditis (inflammation of the sac surrounding the heart)
• Benign early repolarization (a normal variation, especially in young, healthy people)
• Left bundle branch block
• Left ventricular hypertrophy (thickening of the heart’s main pumping chamber)
• Ventricular aneurysm (a bulge in the heart wall)
• Brugada syndrome (an inherited heart rhythm disorder)
• Ventricular paced rhythm
• Raised intracranial pressure (increased pressure in the skull)
• Takotsubo cardiomyopathy (stress-induced heart muscle weakness)

ST Segment Elevation in Heart Attacks

An ST-segment elevation myocardial infarction (STEMI) is a severe type of heart attack that happens when one or more of the arteries supplying blood to the heart become completely blocked. This blockage causes transmural myocardial ischemia, meaning the full thickness of the heart muscle is affected, which leads to injury or death of heart tissue^[3].

STEMI represents about 30% of all cases of acute coronary syndrome, which is the term for conditions that involve a sudden decrease in blood flow to the heart. Every year in the United States, there are approximately 550,000 new heart attacks and 200,000 repeat occurrences. About 38% of patients who come to the hospital with heart attack symptoms have a STEMI^[3][11].

The blockage in STEMI typically occurs when an atherosclerotic plaque (a fatty, waxy buildup inside the artery) ruptures. This rupture leads to the formation of a blood clot, or thrombus, that quickly closes the artery and interrupts blood flow to the heart muscle^[3][4].

Risk factors for STEMI include high blood pressure, high levels of unhealthy fats in the blood, smoking, diabetes, and a family history of coronary artery disease. These factors contribute to the development of atherosclerosis and increase the likelihood of plaque instability^[3][11].

When ST elevation is present during a heart attack, it happens because of what is called an injury current. This is electrical current flowing between areas of heart muscle that are receiving blood (non-ischemic zones) and areas that are not (ischemic zones). This affects the plateau phase of the heart’s electrical cycle and causes the ST segment to elevate^[1].

In STEMI, there is usually reciprocal ST depression, which means that the leads electrically opposite to the area of ST elevation show ST segment depression. For example, if there is ST elevation in the high lateral leads, there is typically reciprocal ST depression in the opposite lead^[1][2].

The pattern of ST elevation on the ECG can indicate which area of the heart is affected. STEMI patterns include septal, anterior, lateral, inferior, right ventricular, and posterior infarctions, each showing characteristic changes in specific ECG leads^[2].

Other Medical Conditions Causing ST Elevation

Coronary Artery Spasm

Coronary artery spasm, also known as Prinzmetal’s angina, causes a pattern of ST elevation that looks very similar to acute STEMI on an ECG. The ST elevation occurs during episodes of chest pain along with reciprocal ST depression. However, unlike acute STEMI, the ECG changes are temporary, reversible with medications called vasodilators, and usually not associated with heart muscle death. It may be impossible to tell these two conditions apart based on the ECG alone^[2].

Pericarditis

Acute pericarditis, which is inflammation of the sac surrounding the heart, causes widespread concave (saddle-shaped) ST segment elevation with depression of the PR segment in multiple leads. This typically involves leads I, II, III, aVF, aVL, and V2-6. There is reciprocal ST depression and PR elevation in leads aVR and V1. A specific finding called Spodick’s sign, which is a downward sloping segment of the ECG, is specific for acute pericarditis^[2][6].

Benign Early Repolarization

Benign early repolarization is a normal variation commonly seen in young, healthy patients. It causes mild ST elevation with tall T-waves mainly in the precordial leads (those placed across the chest). There is often notching of the J-point, creating what is called a “fish-hook” pattern. The ST changes may be more prominent at slower heart rates and disappear when the heart rate is fast^[2].

Left Bundle Branch Block

In left bundle branch block, which is a condition affecting the heart’s electrical conduction system, the ST segments and T waves show “appropriate discordance.” This means they are directed opposite to the main direction of the QRS complex. This produces ST elevation and upright T waves in leads with a negative QRS complex (dominant S wave), while producing ST depression and T wave inversion in leads with a positive QRS complex (dominant R wave)^[2].

Left Ventricular Hypertrophy

Left ventricular hypertrophy, which is thickening of the heart’s main pumping chamber, causes a similar pattern of abnormalities as left bundle branch block. It shows ST elevation in leads with deep S-waves (usually V1-3) and ST depression with T-wave inversion in leads with tall R waves^[2].

Ventricular Aneurysm

A ventricular aneurysm is a bulge in the heart wall that can develop after a previous heart attack. The ECG pattern shows residual ST elevation and deep Q waves. It is associated with extensive heart muscle damage and abnormal movement of the left ventricular wall during contraction^[2].

Brugada Syndrome

Brugada syndrome is an inherited disease of the heart muscle’s sodium channels that leads to sudden, dangerous heart rhythms and sudden cardiac death in young patients. The characteristic sign on the resting ECG is ST elevation in leads V1-2^[2].

How ST Elevation is Detected

The primary tool for identifying ST segment elevation is the 12-lead electrocardiogram (ECG). This test uses several sensors (usually 10) attached to the skin that detect the heart’s electrical activity. That activity is then displayed as a wave pattern^[1][4].

Because STEMI is a life-threatening emergency, rapid recognition is crucial to optimizing patient outcomes. Patients who present to the emergency department with chest pain should have an electrocardiogram done within 10 minutes of arrival. Some emergency medical services will transmit ECGs to the hospital from the field to promote rapid recognition and treatment^[5][14].

The ECG is considered a dynamic diagnostic tool because it captures the heart’s electrical activity at a specific moment in time. An initial absence of ST-segment elevation does not completely rule out a heart attack, as the changes can develop over time^[4].

About 80% to 90% of ischemic episodes detected by ECG are clinically silent, meaning the patient may not feel symptoms. Continuous ST-segment monitoring can reveal changes in the ST segment over time, providing a more accurate picture of the heart’s condition than a single 10-second snapshot^[15].

The diagnosis of STEMI is confirmed by characteristic findings on electrocardiography, including ST-segment elevation in specific leads. Blood tests, such as elevated cardiac troponins (proteins released when heart muscle is damaged), further support the diagnosis^[3].

Studies have shown that ECG interpretation is not perfect. One study found that 51% of patients identified in the pre-hospital setting as having ST elevation meeting criteria for treatment did not have myocardial infarction as their final diagnosis. Another study showed that correct differentiation of benign early repolarization from STEMI by emergency physicians occurred in 81% of cases, while cardiologists were correct 90% of the time. Experience plays an important role in accurate interpretation^[18].

Treatment Approaches

The treatment for ST segment elevation depends on whether it is caused by a STEMI or another condition. For STEMI, immediate treatment is critical because every minute of delay increases the amount of heart muscle death. The phrase “time is muscle” emphasizes the importance of early recognition and treatment^[5][14].

Emergency Treatment for STEMI

STEMI requires emergency assessment and treatment. If symptoms started within the past 12 hours, patients are usually offered a procedure called percutaneous coronary intervention (PCI), also known as coronary angioplasty. This is a procedure to widen blocked coronary arteries^[10].

During PCI, a thin tube with a balloon at the end (called a catheter) is inserted into a blood vessel in the groin or arm and passed through blood vessels up to the heart. Once the catheter reaches the narrowed section of the coronary artery, the balloon is inflated to open it. A flexible metal mesh called a stent is usually inserted into the artery to help keep it open^[10].

If PCI cannot be performed quickly, patients may be given medicines to break down blood clots, known as thrombolytics or fibrinolytics. These are usually given by injection. Patients may still need coronary angiography and PCI once their condition is stable or if thrombolysis does not work^[10].

If PCI is not suitable, patients may be treated with two types of antiplatelet medicines (usually aspirin and one other medicine). These medicines make blood flow through veins more easily, reducing the likelihood of blood clot formation. Both medicines usually need to be taken for up to 12 months^[10].

In some cases, an alternative operation called coronary artery bypass graft (CABG) may be considered. This involves taking a blood vessel from another part of the body and attaching it to the coronary artery above and below the narrowed area or blockage to improve blood flow and oxygen supply to the heart^[10].

Early management is critical to limit heart muscle damage. Additional therapies, including antiplatelets and anticoagulants, are often used to prevent further blood clot formation^[3].

Treatment for Other Causes

For ST elevation caused by conditions other than STEMI, treatment focuses on the underlying cause. For example, coronary artery spasm is treated with vasodilators, while pericarditis may be treated with anti-inflammatory medications. Each condition requires specific management based on its cause and severity.

• Heart
• Coronary arteries
• Myocardium (heart muscle)
• Ventricles (lower heart chambers)