Heart attack

Author : Edward J McNulty Kaiser Permanente San Francisco Medical Center

2008-11-06

Heart Attack : Background, Diagnosis and Treatment

Heart attacks are a leading killer in both developed and developing nations. They are caused when part of the heart does not receive enough blood, usually because of a blockage in a coronary artery. This knol explains what is meant by a heart attack, how they are caused, and how this condition is diagnosed.

What is a heart attack?

     A heart attack occurs when there is a blockage of blood flow to the heart muscle, causing an area of the heart muscle to die.  This most commonly occurs due to a blockage of a coronary artery (see “coronary arteries”).  If there is a complete blockage in one of the three main coronary arteries, a large heart attack may occur, during which the part of the heart muscle fed by the artery dies.   If the blockage is not complete, bits of blood clot can travel downstream and clog smaller arteries, causing a smaller heart attack.  Both large and small heart attacks are serious and potentially fatal, and both require urgent attention.  Heart attacks are also called “myocardial infarctions” and often abbreviated as “MIs.”

How common are heart attacks?

     It is estimated that one half of men in the US and one third of women will develop angina (pain from a blocked artery in the heart) or suffer a heart attack in their lifetimes (2).   Fatal complications of coronary artery disease (primarily heart attack) are the most common cause of death in the United States, killing approximately half a million people per year.  There are approximately 8 million heart attacks per year in the United States (3).  While heart attacks have long been a leading killer in most developed countries, they have rapidly grown as a problem in developing nations and are now the leading cause of death in lower income countries (4).

Types of Heart Attacks

     There are two main types of heart attacks.  The large heart attacks that result when one of the main coronary arteries become blocked usually cause distinct changes on an electrocardiogram (“see Diagnosis of Heart Attack – ECG”) and are often referred to as “ST Elevation myocardial infarctions.”  (see Figure 1) Smaller heart attacks caused by bits of blood clot traveling downstream into smaller arteries usually cause different findings on an electrocardiogram and are called “non-ST Elevation myocardial infarctions.”

Figure 1.  ECG of an “ST segment elevation myocardial infarction”

What causes heart attacks?

the role of plaque

     The most common disease that affects coronary arteries and leads to heart attacks is “atherosclerosis” (literally meaning hardening of arteries due to plaque formation).  In atherosclerosis, plaque (consisting of cholesterol and other materials) is deposited within the wall of the arteries.  Not only does this plaque cause the arteries to become hardened, but the insides become narrowed.  Once plaque has grown large enough, blockage to the flow of blood may occur.  This can obstruct the flow of blood if the plaque becomes large enough (see Figure 2a), resulting in what is commonly referred to as a “blockage” or “clogged artery,” similar to a kitchen pipe becoming clogged with debris that prevents water from flowing through it.  If there is obstruction to flow through a coronary artery, there may not be enough blood for the heart muscle beyond the obstruction.   As noted above, this becomes a heart attack when the obstruction is complete and the heart muscle fed by the artery dies.

Figure 2.

   

     Over the past few decades a greater understanding of the role of plaque in causing heart attacks has emerged.   Sometimes the plaque can rupture into the inside of the artery, resulting in a blood clot forming on the plaque that completely obstructs the flow of blood and causes a heart attack (see Figure 2b).  While plaques associated with severe blockages can rupture and lead to heart attacks, there are often many less severe plaques scattered throughout the arteries, often not associated with severe narrowing, that may also rupture  and lead to obstruction of blood and also cause heart attacks (see Figure 2c).

Traditional Risk Factors
(See Table 1)

Table 1.  Traditional Risk Factors for Heart Attack

Age

Male gender

Tobacco Use

High Blood Pressure

Diabetes

High LDL (“bad”) cholesterol

Low HDL (“good”) cholesterol

Family History of early heart attack

     "Risk factors" are things that make it more likely that someone will develop a certain condition.  Over the past half a century, there have been tremendous efforts in identifying the risk factors for heart attacks.  Most patients with heart attacks have high blood pressure, abnormal cholesterol, diabetes, or a history of tobacco use (5,6).  These risk factors – along with older age, male gender, and a family history of premature coronary artery disease – comprise the “traditional” risk factors for having a heart attack and are discussed below.

Gender
     Heart attacks are more common in men, but become more common in women following menopause.  One explanation for this has been that the female hormone “estrogen” provides some protection from coronary disease.  However, taking estrogen after menopause does not appear to protect women from heart attacks and may in fact increase the risk (7).

Age
     Since plaque in coronary arteries forms over time, heart attacks are more common later in life.

High blood pressure
     The higher the systolic blood pressure (the higher of the two blood pressure readings) and the higher the diastolic blood pressure (the lower of the two readings) the greater the risk of having a heart attack.  Guidelines for treatment usually define elevated systolic blood pressure as above 140 mm Hg, and elevated diastolic blood pressure as above 90 mm Hg.  Studies including hundreds of thousands of people have shown that lower blood pressure (systolic and diastolic) indicates a reduced risk of suffering a heart attack (8,9).  The risk of dying from a heart attack is reduced by roughly half for every 20 mm Hg decrease in the systolic blood pressure or 10 mm Hg decrease in diastolic blood pressure (10).

Tobacco use 
     Tobacco use is considered the strongest potentially “modifiable” risk factor for having a heart attack (a modifiable risk factor is one that can be changed, unlike age or gender).  Cigarette smoking increases ones risk of dying from complications of coronary artery disease, primarily heart attack, by two to three times (11).  Furthermore, an estimated 35,000 nonsmokers die in the United States from complications of coronary artery disease each year as a result of exposure to “second hand” tobacco smoke (12).

Link to CDC tobacco factsheet:
http://www.cdc.gov/tobacco/data_statistics/fact_sheets/health_effects/tobacco_related_mortality/

Cholesterol

     The higher one’s total cholesterol level is, the greater the risk of having a heart attack (13).  When total cholesterol is greater than 200 mg/dl it is considered elevated.  There are many types of cholesterol, and modern guidelines rely upon looking at the various components of cholesterol. The higher one’s low density lipoprotein (LDL) cholesterol (often referred to as “bad” cholesterol) the greater the chance of developing plaque in the coronary arteries and dying as a result of a heart attack.  On the other hand, high density lipoprotein (HDL) cholesterol (often referred to as “good” cholesterol”) seems to protect individuals from heart attacks.  HDL levels tend to be higher in women than in men.  A low HDL cholesterol level (less than 40 mg/dl) therefore is not a good thing and is considered a risk factor for heart attack in current guidelines.  An HDL cholesterol of greater than 60 mg/dl is considered a “negative” risk factor; that is, having a high HDL level protects one from having a heart attack. Triglycerides are another type of cholesterol that appear to increase the risk of heart attack, especially in women.  Normal triglyceride levels are less than 150 mg/dl.  Whether other types of cholesterol, as well as ratios of various components, are more useful to measure is a subject of much current research and debate.
   
Diabetes

     Diabetes is considered a “coronary artery disease equivalent,” meaning that adults with diabetes have similar risks of dying from complications of coronary artery disease, primarily heart attacks, as do individuals with established coronary artery disease.

Family History of Premature Coronary Artery Disease

     Heart attacks are common, especially as ones get older, so the majority of individuals have a blood relative who has had a heart attack.  Early or premature coronary artery disease (including heart attack) refers to women having complications of coronary artery disease (a heart attack, angina, or a coronary procedure such as an angioplasty or bypass surgery) before the age of 65 or a man before the age of 55.  A “family history of premature coronary artery disease” means that one has a first degree relative (mother, father, brother or sister) with premature coronary artery disease, and this increases one’s risk for developing a heart attack (14).

Traditional Risk Scores

     The risk factors above are considered “traditional” risk factors.  Much of the work in identifying these factors came from following the population of Framingham, Massachusetts for many years (14).  A limitation of this work was that most of the population studied was Caucasian.  Subsequent studies have confirmed the role of these same risk factors in more diverse populations (15,16).  There are many “risk scores” that can be used to calculate an individual’s risk of having a heart attack or dying from complications of coronary artery disease.  One commonly used calculator is the Framingham Risk Calculator.

Link to Framingham risk calculator:   http://hp2010.nhlbihin.net/atpiii/calculator.asp?usertype=pub
Other Risk Factors for Heart Attack

     While there is abundant evidence linking the above risk factors to coronary artery disease, many (up to 20%) of individuals can develop complications of coronary artery disease, including heart attacks, without any of the identified traditional risk factors.  Therefore there has been much effort at identifying other risk factors for coronary artery disease and heart attacks.

Peripheral Artery Disease

     As with diabetes, individuals with peripheral artery disease (severe plaque in arteries in the neck or extremities) or abdominal aortic aneurysms have similar risks of developing complications from coronary artery disease as do individuals with established coronary artery disease, and therefore are treated as if they have coronary artery disease.

Physical inactivity/lack of exercise 

     Physical activity appears to reduce the risk of heart attack and death from coronary artery disease.  Conversely, physical inactivity and poor conditioning increases the risk of these complications (16).  Among its beneficial effects, exercise reduces weight (and the risk of developing diabetes) and blood pressure – and improves cholesterol.
Obesity 
     Obesity increases blood pressure, lowers “good” (HDL) cholesterol, raises “bad” (LDL) cholesterol and triglycerides, and increases the risks of diabetes.  In other words, obesity causes many of the other known risk factors for coronary artery disease and heart attack.  Obesity also appears to increase the risk of suffering complications of coronary artery disease in and of itself, independent of these other risk factors (17).  Furthermore, the location of adipose tissue (fat) appears to be important, with having abdominal obesity (fat in the truck or “belly”) being worse than having fat more spread out through the body.  Obesity is usually defined as having a body mass index or BMI of greater than 30.
Link to BMI calculator:
http://www.nhlbisupport.com/bmi/

The metabolic syndrome

     The metabolic syndrome refers to individuals who have three of the following: abdominal obesity, elevated fasting blood sugar (a precursor to diabetes), high blood pressure, low good (HDL) cholesterol, and elevated triglycerides.  Whether this syndrome confers a risk for coronary artery disease and heart attack beyond the additive risks of the individual risk factors is a subject of controversy and current research.

Diet

     Coronary artery disease and heart attacks are more common in countries with diets higher in animal fat and processed sugar (so called “Industrial” or “Western Diets”) (18).  Even among western countries there is considerable variation in the rates of coronary artery disease and heart attack from country to country.  In general, countries with a very high intake of animal fat have correspondingly high rates of coronary artery disease and heart attacks.  Additional evidence for the role of diet in developing coronary artery disease comes from studying individuals migrating from areas where coronary artery disease is uncommon to areas where it is very common.  For example, studies of Japanese immigrants revealed dramatic increases in the rates of coronary artery disease once individuals moved to western countries and adopted western diets.  Finally, as countries develop and adopt western diets, the incidence of coronary artery disease and heart attacks increases.

     While the evidence linking diet to coronary artery disease and heart attacks is compelling, there is also much about this relationship that is poorly understood.  Some fats appear to be harmful (saturated and “trans” fatty acids) while others seem to be protective (polyunsaturated and monounsaturated fats).  Studies have shown lower rates of coronary artery disease and heart attack in countries with diets rich in omega-3 fatty acids, part of the so called “Mediterranean Diet” (19).

Alcohol use

     Moderate alcohol intake (one drink per day for women and one to two drinks per day for men) appears to be protective against death from coronary artery disease and heart attack (16).  While some studies have suggested that substances in red wine could account for this protective effect, it appears that any alcohol intake is protective.  It should be noted that this finding refers to individuals who have established moderate patterns of alcohol intake, not individuals who begin drinking to prevent the development of coronary artery disease. At least part of the beneficial effect of alcohol intake is due to increasing levels of good cholesterol.

Stress and Depression

     Anxiety, depression, and having “type A” personality have all been shown to increase the risk of developing complications of coronary artery disease and heart attack (20).

Drugs

     Certain medications and illicit drugs can also cause heart attacks, including cocaine and methamphetamine.

What are the symptoms of a heart attack?

     The typical symptom of a heart attack is chest discomfort,  usually described as a sensation of pain or pressure over the middle or left side of the chest.  However many people having heart attacks have different symptoms such as neck pain, arm or shoulder pain (usually but not always on the left side), or pain in the upper abdomen (above the umbilicus or "belly button").  Other less "typical" symptoms that patients having heart attacks experience include nausea or a feeling of fullness in the upper abdomen.  In addition to these symptoms, people having heart attacks also have other "associated" symptoms, such as sweating and a feeling of breathlessness.  Typical symptoms occur in only about a half of individuals with heart attack, and occur more often in men than women.  Some people experience little in the way of symptoms despite having major heart attacks, especially those with diabetes. 
     Often people having heart attacks feel tired with less energy in the days leading up to heart attacks.  Sometimes patients with angina develop worsening symptoms in the days leading up to a heart attack.  If you or someone you know thinks that they may be having a heart attack, it is vital that they seek medical attention right way (in other words, call 911).  The sooner heart attack victims get treated, the more lives are saved and the more heart muscle is saved.

Complications of heart attacks

     The most dreaded complication of heart attacks is death, and up to one half of patients experiencing heart attacks die before they can receive medical attention.  Those surviving until they reach the hospital do better, but many still die.  There are two main ways that heart attacks can result in death, either through causing fatal irregular heart beats or by causing so much of the heart to die that the patient goes into “shock” and dies.  Earlier recognition and treatment can reduce these complications and improve survival.  Even if an individual survives the heart attack, the heart can be left weakened and “congestive heart failure” can result.

How is a Heart Attack Diagnosed?

1.  Electrocardiogram (“ECG”)

     If a heart attack is suspected, an electrocardiogram (ECG) is performed.  This test entails placing electrodes (simple adhesive strips) on the patient to measure the electrical activity of the heart, and can be useful in detecting signs of an ongoing or imminent heart attack as well as a prior heart attack (see figure 1).  It is quick, easy to perform and relatively inexpensive and is therefore useful as an initial tool in the diagnosis of suspected heart attack.  However, individuals with a normal ECG can still have a heart attack.

2.  Blood tests

     Certain substances are released into the blood in patients having a heart attack and can be detected by blood tests.  These tests, especially measurement of the creatine phosphokinase (“CPK”) and troponin, are useful in determining if an individual is having a heart attack.

3.  Coronary Angiography (“Cardiac Catheterization”)

     Currently, the most accurate method to detect blockages in the coronary arteries is invasive angiography.  This is an invasive test, because it requires temporarily placing objects into the body and is performed in a facility called a catheterization laboratory.   It is usually recommended in patients with a suspected major heart attack (an “ST Segment Elevation MI) and in many patients with smaller heart attacks.  It is performed by placing a hollow tube (called a sheath) into an artery in the groin or wrist and then threading small “catheters” (hollow, plastic tubes 2-3 mm in diameter) through the arteries to the coronary arteries.  This is performed using X-ray guidance, so the procedure does require exposure to radiation.  Once the catheters are placed into the beginnings of the coronary arteries, a substance called “contrast” is injected into the arteries while X-ray movies are taken.  The contrast contains iodine so that X-rays will not penetrate it; therefore arteries with contrast inside appear dark on the X-ray picture.  In this way a picture of the inside of the coronary arteries is obtained, usually from multiple positions, and blockages detected. (See figure 3). 

Figure 3.  Angiography

Blockages can sometimes be treated at the same time with angioplasty and stents (see figure 4).


Figure 4.  Angiogram showing complete blockage in right coronary artery, before (top) and after (bottom) treatment with a stent.  Arrow in top panel points to blood clot at blockage in the artery.

Treatment of heart attacks:  “Time is muscle”

     It cannot be overemphasized how important prompt treatment is in reducing the risk of death in heart attacks.  Even if it is not possible to perform an angiogram right away, clot dissolving medicines can be administered which are also effective in treating heart attacks.  The more time that elapses when there is a complete blockage in an artery in the heart, the less likely it becomes that the heart muscle can be saved and the more likely fatal complications become.

     Initial treatments for heart attack include aspirin, oxygen, nitroglycerin and sometimes morphine.  This should be done by experienced medical personnel and in a setting where the patient can be monitored.  Other medicines that are given early in heart attacks to certain patients are beta blockers and other blood thinning medicines including aspirin. 

     For larger heart attacks, or so called "ST segment elevation heart attacks," patients are taken as quickly as possible to a cardiac catheterization laboratory for a "coronary angiogram."  If a completely blocked or seriously blocked artery is found, it can usually be opened with a balloon and a stent.  Sometimes the blockages are in locations that require open heart bypass surgery.  If patients cannot be taken quickly for a cardiac catheterization, then powerful clot dissolving medicines called "thrombolytics" are given.

     For smaller heart attacks, patients are also treated with medications including beta blockers and blood thinning medications including aspirin.  Depending on how serious the heart attack is, patients may have a stress test or be sent for a coronary angiogram where blockages can be treated with balloons and stents.

     During the recovery from a heart attack, patients are usually treated with cholesterol lowering medications (especially a type called "statins") and also given medications including aspirin (or other similar blood thinning medicines), beta blockers, and ACE inhibitors.  Also important is adopting a healthy diet and lifestyle.

Other Links

National Heart Lung and Blood Institute

American Heart Association

References
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2. Jones DM, Larson MG, Beiser A Levy D , Lifetime Risk of Developing Coronary Heart Disease Lancet 1999; 353:89-92.

3.  American Heart Association/American Stroke Association Heart Disease and Stroke Statistics, 2008.

4.  World Heath Organization Global Burden of Disease Statistics 2002 (http://www.who.int/healthinfo/bodestimates/en/index.html).
     
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6.  Khot UN, Khot MB, Bajzer CT, Sapp SK et al Prevalence of Conventional Risk Factors in Patients With Coronary Heart Disease JAMA 2003; 290:891.

7.  NIH Women’s Health Initiative (www.nhlbi.nih.gov/whi/)

8.  S. MacMahon, R. Peto and J. Cutler et al., Blood pressure, stroke and coronary heart disease: Part I. Prolonged differences in blood pressure: Prospective observational studies corrected for the regression dilution bias, Lancet 335 (1990), pp. 765–774.

9.  W.B. Kannel, M.J. Schwartz and P.M. McNamara, Blood pressure and the risk of coronary heart disease: The Framingham study, Crit Rev Dis Chest 56 (1969), p. 43.

10.  Prospective Studies Collaberation Age-specific relevance of usual blood pressure to vascular mortality: a meta-analysis of individual data for one million adults in 61 prospective studies The Lancet 2002 360:1903-1913.

11.  Centers for Disease Control and Prevention, Tobacco-related Mortality Fact Sheet.  Sept 2006.  www.cdc.gov/tobacco/factsheets/Tobacco_Related_Mortality_factsheet.htm

12.  Annual Smoking-Attributable Mortality, Years of Potential Life Lost, and Productivity Losses – United States, 1997-2001, Morbidity and Mortality Weekly Report 2005;54:625-628.

13.  Third Report of the Expert Panel on Detection, Evaluation and Treatment of High Blood Cholesterol in Adults (ATP III Final Report) National Heart Lung and Blood Institute.
www.nhlbi.nih.gov/guidelines/cholesterol/atp3_rpt.htm

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 15.  Ramachandran SV, Sullivan LM, Wilson PWF, Sempos CT, Sundstrom J et al Relative Importance of Borderline and Elevated Levels of Coronary Heart Disease Risk Factors Annals Int Med 2005;142:393-402.

16.  Yusuf S, Hawken S, Ounpuu, Dans T, Avezum A et al  Effect of Potentially Modifiable Risk Factors Associated with Myocardial Infarction in 52 countries (the INTERHEART Study): a case control study.  Lancet 2004 364:937-52.

17.  Yan et al Midlife BMI and Hosp and Mortality in Older Age JAMA 2006; 295:190-198.

18.  Yusuf S, Reddy S, Ounpuu and S Anand, Global burden of cardiovascular diseases: part I: general considerations, the epidemiologic transition, risk factors, and impact of urbanization, Circulation 104 (2001): 2746–2753.

19.  Anderson CAM and Appel LJ Dietary Modification and CVD Prevention: A Matter of Fat JAMA 2006; 295(6): 693 – 695


20.  Rosengren A, Hawken S and Ôunpuu S et al., Association of psychosocial risk factors with risk of acute myocardial infarction in 11 119 cases and 13 648 controls from 52 countries (the INTERHEART study): case-control study, Lancet 364 (2004), pp. 953–962