Article

Heart attack (Myocardial infarction): Causes, Symptoms, Treatment and Prevention
February 04, 20190840

What is a heart attack?

A heart attack is the common name for the medical term “myocardial infarction”. A myocardial infarction (MI) is defined as the death of heart muscle cells due to an insufficient supply of oxygen from the blood flowing through the coronary arteries.1

 

Figure 1: Anatomy of the human heart.

Coronary arteries, which supply the heart muscle with blood, are highlighted red.

(iStockphoto/JFalcetti)

 

What causes a heart attack?

The basic cause of a heart attack is a mismatch between the amount of oxygen the heart muscle needs and the oxygen supplied by the blood.1

There are three ways this mismatch can develop:

  • increased oxygen demand by the heart
  • decreased oxygen available in the blood
  • decreased blood flow to the heart (ischemia)

There are a variety of conditions that can lead to each of these states. For example:

Increased oxygen demand by the heart may be caused by1,2:

  • high blood pressure
  • extreme exercise/rapid heart rate
  • growth of heart muscle

Decreased oxygen levels in the blood may be caused by1,2:

  • severe anemia
  • lung infections or diseases

Decreased blood flow to the heart muscle (ischemia) may be caused by1,2:

  • narrowing of coronary arteries
  • formation of a blood clot in a coronary artery
  • coronary artery spasm
  • a blood clot from another part of the body lodging in a coronary artery
  • low blood pressure
  • low heart rate

 

Types of heart attacks

There are two official systems that doctors use to categorize types of heart attacks.

One system is based on changes in electric signaling in the heart.

The other is based on a mixture of clinical manifestations, medical history, and the immediate cause of the heart attack.

 

Types of Heart Attacks based on changes in electric signaling

One of the earliest and most important diagnostic tests doctors use to determine if someone is having a heart attack is an electrocardiogram (ECG).3-5

An ECG uses sensors on the skin to detect the electric currents that flow through the heart with each beat. Doctors can use this to build a rough picture of how a heart is beating.6

A peak in a specific part of an ECG (the ST-segment) tells doctors that the heart is having serious problems beating.3-5

Doctors categorize heart attacks that show ST-segment elevation in their ECG as “STEMI” (ST-segment elevation myocardial infarction) and those without as “NSTEMI” (Non-ST-segment elevation myocardial infarction).3-5

 

Figure 2: Electorgardiogram Tracings

Left: normal tracing. Right: ST-segement elevation.

(Normal PR-segment ECG and ST-segment elevation with tombstone pattern by Bron766

are licensed under CC 3.0)

 

STEMI indicates a severe heart attack. NSTEMI suggests a milder heart attack. 

Types of Heart Attacks based on symptoms, medical history and immediate cause

There are five official categories of heart attack based on the cause, symptoms and previous medical history of the individual1:

  1. M1 MI is a heart attack caused by the formation of a blood clot in a coronary artery as a result of coronary artery disease.
  2. M2 MI is a heart attack caused by increased oxygen demand by the heart or decreased oxygen supply in the blood without a change in blood flow.
  3. An M3 MI is the diagnosis for death following what doctors believe to have been a heart attack. An M3 diagnosis is given if doctors were unable to perform all the necessary tests to make an official diagnosis before death.
  4. M4 and M5 MIs are heart attacks caused by blood clots which formed after heart surgery to treat coronary heart disease.

Types of heart attacks and coronary heart disease

While they are officially considered different types of heart attacks, the root cause of three of the five types of heart attacks (M1, M4 and M5) is, by definition, coronary heart disease.

Since M1, M4 and M5 account for about three-quarters of heart attacks, coronary heart disease may be considered the number one cause of heart attacks.2

 

Coronary heart disease

Coronary heart disease is a build-up of fat and white blood cells in the wall of coronary arteries. Doctors refer to this mixture of lipid and immune cells as a “plaque”.

Pathophysiology of plaque formation

The current belief is that the interplay between white blood cells and abnormal fat levels in the blood cause the formation of plaque in the walls of arteries.7-9

The driving force behind plaque formation is abnormally high levels of a specific type of fat in the blood – low-density lipoprotein (LDL).10

LDL from the blood is always entering the walls of arteries and then moving back into the bloodstream. When LDL levels get too high, however, some of the LDL stays in the artery wall.7,10

If LDL stays in the artery wall, it can come into contact with enzymes that turn it into a different type of fat, called oxidized low-density lipoprotein (OxLDL).8,10

The cells in the wall of the artery recognize OxLDL as abnormal, and they send out distress signals to the immune system.8

The cells that respond to this signal are white blood cells called macrophages.8

The macrophages crawl into the wall of the artery. There, they engulf the OxLDL to keep it away from the cells of the artery wall.8

If the macrophages get too full of OxLDL, they become what are called “foam cells”. Foam cells are trapped in the wall of the artery and cannot get back out into the blood.8

Foam cells send out signals to other macrophages in the blood to come help them.8

If there is more OxLDL in the wall of the artery, the new macrophages will become foam cells. These new foam cells can then call more macrophages.2

If this process continues for a long time, so many foam cells and so much OxLDL can build-up that it forms a visible bulge in the wall of your artery.7 This is a plaque.

 

Figure 3: Schematic showing how plaque build-up can narrow coronary arteries

(iStockphoto/newannyart)

 

How does coronary heart disease cause heart attacks?

Coronary heart disease can cause a heart attack in multiple ways.

  1. Plaques bulging into the artery decrease the diameter of the blood vessel. If the plaques become very large, they can decrease blood flow through the artery enough to directly cause a heart attack.1
  2. Plaques can sometimes break through the surface of the artery wall. When this happens, OxLDL and foam cells come into contact with the blood.

Blood platelets recognize something abnormal is in the bloodstream. They think there is a wound and work quickly to form a blood clot.

This blood clot makes the diameter of the artery even smaller and, in some cases, can plug the entire artery. This causes a heart attack.1

  1. Narrowing of the artery by plaques makes it more likely that a blood clot that formed somewhere else in the body will be too big to fit through the artery. If a blood clot gets stuck where the artery is narrower, it can cause a heart attack.1
  2. Coronary artery disease can indirectly increase the risk of a heart attack by increasing the likelihood of needing heart

If doctors are worried that a plaque is getting too big and may cause a heart attack, they may decide to do heart surgery.

Unfortunately, heart surgeries may result in the formation of a blood clot in the artery that was operated on. These blood clots can cause heart attacks.1

 

How common are heart attacks?

Heart attacks are one of the most common medical conditions in the world. They are also the leading cause of death worldwide.11

In the United States alone, it is estimated that around 1.1 million people have a heart attack each year. That is one person every 44 seconds.12

Of these, around 380,000 people die. That is approximately 1 in every 6 deaths in the United States.12

The World Health Organization11 estimated that, in 2015, 7.4 million people worldwide died of a heart attack. That is around 12 percent of all deaths worldwide in an entire year.

 

What are the clinical manifestations of a heart attack?

The most common symptoms of a heart attack are11:

  • pain or discomfort in the center of the chest (angina pectoris)
  • pain or discomfort in the back, arms, elbows, jaw, neck or left shoulder

Other warning signs of a heart attack may include11:

  • nausea
  • vomiting
  • sweating
  • abnormal heart beat13
  • problems breathing
  • light-headedness or fainting
  • pale skin
  • unconsciousness
  • cardiac arrest/sudden death

Heart attacks in men are more likely to present as the typical chest pain.

Women are more likely to have less common symptoms of a heart attack.

If you are a woman, you are more likely to experience a heart attack as11,14:

  • pain in your back, neck or jaw
  • shortness of breath
  • nausea or vomiting

If you have any of these warning signs – mild, severe, or in any combination – please call emergency medical services immediately.

Do not drive yourself to the hospital.

Calling an ambulance improves your chances of survival because13:

  • The medical professional on the phone can instruct you on first-aid measures before the ambulance arrives
  • EMTs can diagnose a heart attack before you get to the hospital
  • EMTs can monitor your condition on the way to the hospital
  • EMTs can begin important treatments before you arrive at the hospital
  • EMTs can re-start your heart if you go into cardiac arrest on the way to the hospital
  • Ambulance staff can call ahead to the hospital with your exact diagnosis and treatment This gives the hospital time to prepare the equipment, medications and tests you might need so that you do not have to wait after arrival.

 

How is a heart attack diagnosed?

An official diagnosis of a heart attack requires blood tests showing high levels of a heart muscle protein (troponin) in your blood. Levels over the 99th percentile of the upper normal limit qualify your symptoms as a heart attack.1

Since this blood test takes time to complete, doctors usually make a preliminary diagnosis based on3-5,13,15:

  • your medical history
  • ECG readings
  • stress test results
  • heart imaging tests

If your symptoms are consistent with a heart attack, your EMT or doctor will immediately start an ECG.5

If your ECG shows an elevation the ST-segment, you will be diagnosed with STEMI. The diagnosis will be confirmed via your blood test and/or findings during imaging or surgical treatment.5

If your ECG shows no ST-segment elevation, your doctor will diagnose you with NSTEMI.5

Your NSTEMI diagnosis will be confirmed by blood tests positive for high levels of troponin.

Other tests your doctor might order to help verify your STEMI or NSTEMI diagnosis include:

  • stress test5
  • echocardiogram (ultrasound)1,5
  • myocardial perfusion scintigraphy (MPS)1
  • single photon emission computed tomography (SPECT)1
  • magnetic resonance imagining (MRI)1
  • specific blood or lung tests if an M2 myocardial infarction is suspected2,16

If your blood tests come back negative for troponin, your diagnosis will be down-graded to unstable angina pectoris.

Unstable angina pectoris means that the decrease in blood flow to your heart was not severe enough to cause more than mild damage your heart muscle. You did not have a heart attack.1-5,13,15,16

 


Figure 4: Simplified overview of diagnostic procedures used to identify a heart attack3-5,13,15

 

How is a heart attack treated?

Initial/Diagnostic Treatment

Treatment for a heart attack usually begins before an official diagnosis.

If you have signs of a heart attack, the first medical professional you talk to will likely immediately instruct you to chew between 162 and 325 milligrams of uncoated aspirin.5,13,15

If you have been previously prescribed nitroglycerin, they may also recommend you take a single dose to see if this relieves your symptoms.15

Whoever is treating you first (your doctor, nurse or EMT) will start an ECG as soon as possible. They will closely monitor the reading from your heart beat and determine if you classify as having a STEMI or NSTEMI.3-5,13,15

Treatment of STEMI

Percutaneous coronary intervention

If you are diagnosed with STEMI, your doctor’s number one goal will be to perform a percutaneous coronary intervention (PCI) as quickly as possible. This procedure allows doctors to find and remove the blockage in your coronary artery, restoring blood flow.5,13

In this procedure, your doctor makes a small opening into a vein in your wrist or groin. She then guides a catheter through your vein to your heart and into your coronary arteries. Once there, she releases a contrast dye, that will allow her to better see the narrowed or blocked artery.17

After the doctor has found the spot(s) where blood flow has been restricted, she removes the catheter, leaving a small guide wire in your blood vessels.17

A new catheter is then attached to the guidewire and threaded to the blockage. This new catheter has a balloon attached to the top. This balloon is then expanded directly in the blockage, pushing the walls of the artery open.17

If there is a blood clot in the artery, doctors may first use suction to remove it before inflating the balloon.17,18

In some cases, the doctor may choose to place a stent directly after inflating the balloon.5,13,18

A stent is a plastic or metal mesh tube that can hold the walls of the artery open and help prevent another heart attack in the future.13,18

 

Figure 5: PCI with stent placement

(iStockphoto/anamad)

 

Fibrinolytic Agents

If it is going to take more than 2 hours to get you in for a PCI, doctors will treat you with medications that break up blood clots. These medications are called fibrinolytic agents.5,13

There are four kinds of fibrinolytic agents commonly used to treat heart attacks5,13:

  • streptokinase
  • alteplase
  • reteplase
  • tenecteplase

All four are given via IV. Their doses depend on your size and the other medications you are receiving.5,13

If these medications treat your symptoms and return your ECG to normal, you will not need a PCI. If they do not, you will still need a PCI.5

Antiplatelet/Antithrombin agents

Fibrinolytic agents and PCI are usually combined with medications that prevent new clots from forming, either antiplatelet or antithrombin agents.

You may be given13:

  • aspirin
  • clopidogrel
  • unrefracted heparin
  • enoxaprin
  • bivaliruden
  • fondaparinux

Which medications you are given, and their exact doses, depends on your size and the other medications and treatments you are receiving.13

Open heart surgery

If your symptoms are not responding to treatments or you have risk factors for specific complications, doctors may choose to do an open-heart surgery to restore blood flow to your heart muscle.13

Treatment of NSTEMI

If you are diagnosed with NSTEMI, your treatment depends on your doctor’s assessment of your risk for your heart attack worsening.

This assessment is based on the following factors5:

  • age
  • your risk of having coronary artery disease
  • the number of angina episodes you’ve had in the last 24 hours
  • whether your symptoms persist despite taking aspirin
  • changes in the ST-segment of your ECG other than elevation
  • elevated troponin in your blood

If you are at low risk, you will first be given a stress test. If this comes back negative, you will be allowed to go home.5

If your stress test comes back positive or you are at medium risk for a more severe heart attack, your doctor will likely use other imaging techniques to get a better picture of your heart (echocardiogram, MRI, etc.).5

If imaging techniques show heart damage or you are at high risk for a more severe heart attack, you will be treated with fibrinolytic medications, antiplatelet medications, PCI and/or open-heart surgery.4,5

Symptom Management

Whether you are diagnosed with STEMI or NSTEMI, you will be given medications to manage your symptoms. The most common treatments are summarized in Table 1.

 

Table 1: Common treatments for symptoms of a heart attack13,15

 

Survival-enhancing medications

Several classes of medications may be started soon after your heart attack and will likely be continued long-term. These medications have been shown to improve your chances of long-term survival following a heart attack. These medications include15:

  • Aspirin: helps prevent new blood clots from forming.
  • commonly prescribed: 75-100 mg/day5
  • Statins: help stop damage to the area of the heart that was without blood during the heart attack after blood flow is restored.
  • commonly prescribed: atorvastatin (80 mg/day)15
  • β-blockers: help keep blood pressure low, preventing a follow-up heart attack.
  • commonly prescribed: metoprolol (25-50 mg 2x-4x per day)15
  • ACE inhibitors: help keep coronary arteries open and keep blood pressure low, preventing a follow-up heart attack.
  • commonly prescribed: lisinopril (10 mg/day)15

 

Figure 6: Simplified overview of heart attack treatment strategies3-5,13,15

 

Can heart attacks be prevented?

Yes. The majority of heart attacks can be prevented.11

Most heart attacks can be prevented by dietary and lifestyle changes that decrease the risk of developing coronary heart disease.11

These include11:

  • decreasing salt intake11
  • increasing whole grains, beans, nuts, vegetables, fruit, lettuces, fiber, olives, fish and poultry19
  • increasing physical activity11
  • maintaining or reaching a healthy body weight11

Supplementing lifestyle changes with medications may decrease the risk of developing coronary heart disease even further. Useful medications may include11:

  • β-blocker or ACE inhibitors to lower blood pressure
  • blood sugar regulating medications for individuals with diabetes
  • statins to lower LDL levels in the blood

 

Heart attack FAQs

  • Is blood pressure high during a heart attack?

Your blood pressure may be high during a heart attack, but it doesn’t have to be. If there is severe damage to your heart, you may actually have low blood pressure.15

  • Is back pain a sign of a heart attack?

Back pain or discomfort may be a sign of a heart attack, especially in women.11,14

If you are experiencing unusual back pain, especially if it is combined with any of the other signs of a heart attack listed above, please call emergency medical services immediately.

  • Is heart failure the same as a myocardial infarction?

No.

Heart failure is when the heart cannot pump enough blood to your body.20

A myocardial infarction is when part of your heart is damaged because too little blood is reaching the heart muscle.1

If a myocardial infarction is severe, it may cause heart failure.3-5,13

If heart failure is severe, it may cause a myocardial infarction.1

 

References

  1. Thygesen K, Alpert JS, Jaffe AS, et al. Third universal definition of myocardial infarction. European heart journal. 2012;33(20):2551-2567.
  2. Saaby L, Poulsen TS, Hosbond S, et al. Classification of myocardial infarction: frequency and features of type 2 myocardial infarction. The American journal of medicine. 2013;126(9):789-797.
  3. American College of Emergency P, Society for Cardiovascular A, Interventions, et al. 2013 ACCF/AHA guideline for the management of ST-elevation myocardial infarction: a report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. Journal of the American College of Cardiology. 2013;61(4):e78-140.
  4. Anderson JL, Adams CD, Antman EM, et al. 2011 ACCF/AHA Focused Update Incorporated Into the ACC/AHA 2007 Guidelines for the Management of Patients With Unstable Angina/Non-ST-Elevation Myocardial infarction: a report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. Circulation. 2011;123(18):e426-579.
  5. Daga LC, Kaul U, Mansoor A. Approach to STEMI and NSTEMI. The Journal of the Association of Physicians of India. 2011;59 Suppl:19-25.
  6. National Institutes of Health. Electrocardiogram. Last updated December 9, 2016. https://www.nhlbi.nih.gov/health/health-topics/topics/ekg. Accessed July 20, 2017.
  7. Moore KJ, Tabas I. Macrophages in the pathogenesis of atherosclerosis. Cell. 2011;145(3):341-355.
  8. Moore KJ, Sheedy FJ, Fisher EA. Macrophages in atherosclerosis: a dynamic balance. Nature reviews Immunology. 2013;13(10):709-721.
  9. Tall AR, Yvan-Charvet L. Cholesterol, inflammation and innate immunity. Nature reviews Immunology. 2015;15(2):104-116.
  10. Williams KJ, Tabas I. The response-to-retention hypothesis of early atherogenesis. Arteriosclerosis, thrombosis, and vascular biology. 1995;15(5):551-561.
  11. World Health Organization. Cardiovascular Diseases Fact Sheet. Last updated May 2017. http://www.who.int/mediacentre/factsheets/fs317/en/. Accessed July 17, 2017.
  12. Go AS, Mozaffarian D, Roger VL, et al. Heart disease and stroke statistics–2014 update: a report from the American Heart Association. Circulation. 2014;129(3):e28-e292.
  13. Task Force on the management of STseamiotESoC, Steg PG, James SK, et al. ESC Guidelines for the management of acute myocardial infarction in patients presenting with ST-segment elevation. European heart journal. 2012;33(20):2569-2619.
  14. Mehta LS, Beckie TM, DeVon HA, et al. Acute Myocardial infarction in Women: A Scientific Statement From the American Heart Association. Circulation. 2016;133(9):916-947.
  15. O’Gara PT, Kushner FG, Ascheim DD, et al. 2013 ACCF/AHA guideline for the management of ST-elevation myocardial infarction: a report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. Circulation. 2013;127(4):e362-425.
  16. Lanza GA, Careri G, Crea F. Mechanisms of coronary artery spasm. Circulation. 2011;124(16):1774-1782.
  17. National Institutes of Health. Percutaneous coronary intervention. Last updated December 9, 2016. https://www.nhlbi.nih.gov/health/health-topics/topics/angioplasty/. Accessed July 21, 2017.
  18. Dudek D, Mielecki W, Burzotta F, et al. Thrombus aspiration followed by direct stenting: a novel strategy of primary percutaneous coronary intervention in ST-segment elevation myocardial infarction. Results of the Polish-Italian-Hungarian RAndomized ThrombEctomy Trial (PIHRATE Trial). American heart journal. 2010;160(5):966-972.
  19. Dalen JE, Devries S. Diets to prevent coronary heart disease 1957-2013: what have we learned? The American journal of medicine. 2014;127(5):364-369.
  20. National Institutes of Health. Heart Failure. Last updated June 22, 2015. https://www.nhlbi.nih.gov/health/health-topics/topics/hf. Accessed July 21, 2017.

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