Myocardial infarction, commonly referred to as a heart attack, is a life threatening condition caused by a sudden reduction or complete interruption of blood flow to a portion of the heart muscle.
Without adequate oxygen delivery, myocardial tissue begins to suffer ischemia and can progress to irreversible cell death. Although myocardial infarction is primarily considered a cardiac event, it has significant respiratory implications.
Respiratory therapists frequently encounter patients with acute coronary syndromes, pulmonary edema, and respiratory failure related to myocardial injury.
What is a Myocardial Infarction?
A myocardial infarction occurs when blood flow through a coronary artery is severely reduced or completely blocked, depriving a segment of the myocardium of oxygen and nutrients. The coronary arteries originate from the root of the aorta just beneath the semilunar valves. Blood flows into these arteries primarily during diastole when the semilunar valves are closed.
Partial obstruction of a coronary artery reduces oxygen delivery and results in myocardial ischemia. Ischemia may cause chest pain and shortness of breath, a clinical presentation commonly referred to as angina pectoris. If the obstruction becomes complete and prolonged, the affected myocardial tissue undergoes necrosis. This irreversible injury is defined as a myocardial infarction.
Myocardial infarction is part of a broader category known as acute coronary syndrome, which includes unstable angina, non ST segment elevation myocardial infarction, and ST segment elevation myocardial infarction. These conditions reflect varying degrees of coronary artery obstruction and myocardial damage.
Pathophysiology of Myocardial Infarction
The underlying mechanism of a myocardial infarction is typically atherosclerotic plaque rupture within a coronary artery. When the plaque ruptures, platelets aggregate and a thrombus forms. This clot can partially or completely occlude the vessel lumen.
When blood flow is interrupted, myocardial cells switch from aerobic to anaerobic metabolism. Anaerobic metabolism produces less adenosine triphosphate and generates lactic acid, leading to metabolic acidosis at the cellular level. If perfusion is not restored, cellular membranes fail, and myocardial cells die.
Even when systemic perfusion appears adequate, localized reductions in coronary blood flow can result in regional hypoxia. The heart muscle is highly dependent on continuous oxygen delivery. Within minutes of severe ischemia, myocardial contractility declines. As more tissue becomes involved, cardiac output may decrease significantly, leading to hypotension and systemic consequences.
Note: The extent of damage depends on the size of the affected artery, the duration of occlusion, and the presence of collateral circulation.
Types of Myocardial Infarction
Myocardial infarctions are commonly classified based on electrocardiographic findings.
ST Segment Elevation Myocardial Infarction
An ST segment elevation myocardial infarction, or STEMI, is characterized by elevation of the ST segment on the electrocardiogram. The ST segment represents the time between ventricular depolarization and repolarization. Under normal conditions, it appears as a flat isoelectric line.
ST segment elevation indicates transmural injury, typically caused by complete occlusion of a coronary artery. STEMI is generally associated with more extensive myocardial damage and requires rapid reperfusion therapy.
Non ST Segment Elevation Myocardial Infarction
A non ST segment elevation myocardial infarction, or NSTEMI, does not show ST segment elevation on the electrocardiogram. Instead, the tracing may reveal ST depression or T wave inversion. NSTEMI usually results from partial coronary artery obstruction.
Although NSTEMI may involve less extensive myocardial injury compared to STEMI, it still represents significant myocardial damage and carries serious risk if not treated appropriately.
Clinical Presentation
Classic symptoms of myocardial infarction include chest pain or pressure that may radiate to the neck, jaw, back, or arms. Patients often describe the discomfort as tightness or heaviness. Associated symptoms may include shortness of breath, diaphoresis, nausea, fatigue, lightheadedness, palpitations, and anxiety.
Women are more likely than men to present with atypical symptoms such as fatigue, shortness of breath without chest pain, or discomfort in the back or jaw. Older adults and individuals with diabetes may also have atypical or silent presentations.
As myocardial function declines, signs of heart failure may develop. These include dyspnea, orthopnea, pulmonary edema, and hypoxemia. In severe cases, cardiogenic shock can occur, characterized by hypotension and evidence of end organ hypoperfusion.
Diagnostic Evaluation
Electrocardiogram
The electrocardiogram is a primary diagnostic tool in suspected myocardial infarction. Evaluation of the ST segment is critical. A depressed ST segment often indicates ischemia, while an elevated ST segment is associated with certain types of infarction.
Because ST segment changes reflect potentially life threatening myocardial injury, rapid identification is essential. Serial ECGs may be required to detect evolving changes.
Cardiac Biomarkers
Cardiac enzymes and proteins help confirm myocardial injury. Creatine kinase MB, also known as CK MB, is released from injured myocardial cells. Levels typically rise within 4 to 6 hours after injury and peak between 12 and 24 hours.
Troponin I is now more commonly used than CK MB. It is highly specific for cardiac muscle injury. Troponin I levels generally peak 12 to 16 hours after myocardial infarction and remain elevated longer than CK MB, allowing for extended diagnostic detection.
Note: Serial measurements of cardiac markers provide valuable information about the timing and extent of myocardial damage.
B Type Natriuretic Peptide
B type natriuretic peptide, or BNP, is released in response to increased cardiac muscle stretch. While BNP is primarily used to evaluate heart failure, it has important implications in patients with myocardial infarction who develop pulmonary edema or ventricular dysfunction.
Elevated BNP levels help differentiate cardiogenic causes of dyspnea from other conditions such as acute respiratory distress syndrome. High BNP levels suggest elevated filling pressures and cardiac origin of respiratory symptoms.
Relevance to Respiratory Therapists
Although myocardial infarction is a cardiac event, respiratory therapists play a critical role in its management and complications.
Oxygen Therapy
Patients with myocardial infarction may experience hypoxemia due to pulmonary edema, ventilation perfusion mismatch, or reduced cardiac output. Supplemental oxygen is administered to maintain adequate arterial oxygen saturation, particularly in patients with respiratory distress or low oxygen levels.
Respiratory therapists are responsible for selecting appropriate oxygen delivery devices and monitoring oxygenation status. Excessive oxygen administration should be avoided in patients with normal oxygen saturation, as unnecessary hyperoxia may increase oxidative stress.
Management of Pulmonary Edema
Left ventricular dysfunction following myocardial infarction can lead to elevated left atrial pressures and pulmonary venous congestion. This may result in cardiogenic pulmonary edema.
Respiratory therapists frequently manage these patients with noninvasive positive pressure ventilation such as continuous positive airway pressure or bilevel positive airway pressure. Positive pressure improves oxygenation, reduces preload and afterload, and decreases the work of breathing.
In severe cases, endotracheal intubation and mechanical ventilation may be required. Careful ventilator management is essential to avoid excessive intrathoracic pressure that could further compromise cardiac output.
Monitoring and Assessment
Respiratory therapists often assess breath sounds, work of breathing, oxygenation, and ventilatory status in patients with myocardial infarction. They may assist in obtaining arterial blood gas samples to evaluate oxygenation and acid base balance.
Metabolic acidosis can occur due to anaerobic metabolism and poor tissue perfusion. Identification of acid base disturbances is important in guiding therapy.
Cardiopulmonary Resuscitation
Myocardial infarction increases the risk of life threatening arrhythmias such as ventricular tachycardia and ventricular fibrillation. Respiratory therapists are integral members of resuscitation teams. They manage airway support, ventilation, and oxygenation during cardiac arrest and post resuscitation care.
Complications and Respiratory Impact
Complications of myocardial infarction include heart failure, cardiogenic shock, arrhythmias, and mechanical complications such as papillary muscle rupture. Many of these complications have significant respiratory consequences.
Heart failure can lead to pulmonary congestion and hypoxemia. Cardiogenic shock reduces systemic perfusion and may cause metabolic acidosis. Respiratory failure may develop secondary to pulmonary edema or severe hypoperfusion.
In some cases, distinguishing cardiogenic pulmonary edema from acute respiratory distress syndrome is necessary. BNP levels, clinical history, and imaging findings assist in this differentiation.
Note: Because myocardial infarction often affects older adults with comorbid pulmonary disease, respiratory compromise may be more pronounced in patients with chronic obstructive pulmonary disease or other chronic lung conditions.
Prevention and Risk Factors
Major risk factors for myocardial infarction include hypertension, hyperlipidemia, diabetes mellitus, smoking, obesity, and sedentary lifestyle. Age and male sex increase risk, though postmenopausal women also face significant risk.
Patients with two or more major risk factors have substantially increased lifetime risk of developing coronary heart disease. Recognition and management of these factors are essential for prevention.
Note: Respiratory therapists who provide patient education can reinforce smoking cessation counseling and promote cardiovascular and pulmonary health.
Myocardial Infarction Practice Questions
1. What is the definition of a myocardial infarction?
A myocardial infarction (i.e., heart attack) is a medical emergency where a blockage in the coronary arteries cuts off blood supply to the heart, causing tissue damage or death.
2. What is the chief complaint for a heart attack?
Chest pain or pressure
3. What are some other symptoms of a myocardial infarction?
Nausea, vomiting, shortness of breath, and diaphoresis.
4. What are the risk factors of a myocardial infarction?
Coronary artery disease, hypertension, high-density lipoproteins, diabetes mellitus, smoking, family history of coronary artery disease, greater than 55 years old, sedentary lifestyle, and obesity. This condition is twice as likely to occur in men as in women. It has the highest prevalence in African American men ages 45 and older and for women 55 and older.
5. How are heart attacks diagnosed?
EKG (STEMI) or elevated troponin (non-STEMI).
6. What medications are used to treat a heart attack?
ASA, NTG, and thrombolytic (Heparin)
7. What is the incidence of a myocardial infarction? The incidence decreases with higher socioeconomic status, and less than 10 percent are fatal in the United States.
8. What are the causes of a myocardial infarction?
Coronary atherosclerosis, atherosclerotic plaque where there is a large amount of fat or cholesterol in the diet, coronary vasospasm, thrombosis, increased vasomotor tone, chest trauma (laceration or contusion), severe anemia, and collagen vascular disease.
9. What diagnostic tools are used for a myocardial infarction?
ECG is used to record the electrical activity of the heart via electrodes on the skin. Blood tests are done to assess the enzymes present in the blood. Other tests include chest x-rays, arterial blood gas (ABG), echocardiograms, angiograms, and exercise stress tests.
10. What are the complications of a myocardial infarction?
Heart arrhythmias, heart failure, heart rupture, valve damage, stroke, angina, and pericarditis.
11. What happens during the early stages of a myocardial infarction?
The patient experiences chest pain.
12. What happens during the later stages of a myocardial infarction?
One or more arteries become completely blocked, causing a heart attack and damage/death of heart tissue due to lack of blood flow and oxygen to the tissue.
13. What is the prognosis of a myocardial infarction?
Survival rates have increased steadily since 1970.
14. What can be seen on the ECG of patients with a myocardial infarction?
An elevated ST segment.
15. What is the purpose of a stress test?
It determines the effectiveness of oxygen delivery to myocardial tissue.
16. What specific enzymes are released by dying muscle?
Cardiac troponin and creatinine phosphokinase.
17. What population commonly has silent myocardial infarctions?
Elderly, post-menopausal women, and diabetics.
18. What is the Levine’s sign?
Clenched right fist held over the chest to describe ischemic chest pain.
19. What is common for an EKG diagnosis of a myocardial infarction?
ST-elevation or depression, T-wave inversion, and Q-waves.
20. Why does the heart need the coronary arteries?
The myocardium is too thick for diffusion to take place efficiently.
21. What is atherosclerosis?
Atherosclerosis is a chronic condition characterized by the buildup of plaque inside arteries, leading to reduced blood flow and increased risk of cardiovascular events.
22. How long does it take for plaque to form and cause a clinically significant event?
Decades
23. What is stable angina?
Stable angina is a condition marked by predictable chest pain or discomfort typically triggered by physical exertion or stress due to reduced blood flow to the heart.
24. What is angina pectoris?
Angina pectoris is a clinical syndrome characterized by chest discomfort or pain due to transient myocardial ischemia, which occurs when the demand for oxygen by the heart muscle exceeds the supply.
25. What is unstable angina?
Unstable angina is a form of acute coronary syndrome where unexpected chest pain occurs at rest or with minimal exertion, signaling a high risk of imminent myocardial infarction.
26. What is a thrombosis?
When a clot completely blocks a vessel.
27. What happens during a heart attack?
During a heart attack, a blockage in the coronary arteries interrupts blood flow to the heart, causing heart muscle cells to be deprived of oxygen and leading to tissue damage or death.
28. What does an angioplasty use to do its job?
A balloon
29. What is an angiogram?
A method for looking at the coronary vessels under a live x-ray that uses radioactive dye and imaging to locate blockages.
30. What is a stent?
A mesh material that holds a plaque against the arterial wall, allowing blood flow through the vessel.
31. What is PCI?
Percutaneous Coronary Intervention.
32. What is a mural thrombus?
Blood that no longer pumps through the heart and clots, eventually turning into fibrous tissue.
33. When does instant death occur from a heart attack?
It occurs when a heart attack softens the affected myocardium, thus leading to a rupture of the heart wall itself.
34. What is a coronary artery bypass graft (CABG)?
A surgical procedure that improves blood flow to the heart by diverting blood around narrowed or blocked coronary arteries using grafts.
35. What is an infarction?
Necrosis (i.e., tissue death) from a lack of blood supply.
Access our quiz with sample TMC practice questions and detailed explanations to help you master the key concepts of pathology.
36. What do coronary thrombosis and vascular spasms cause?
They can cause a complete block of part of the coronary circulation and death of cardiac muscle cells beyond the blockage.
37. What does the severity of the heart attack depend on?
The site and nature of the blockage.
38. What causes endothelial damage?
Endothelial damage is often caused by risk factors such as high blood pressure, smoking, high cholesterol, diabetes, and inflammation, which can compromise the integrity of the inner arterial lining, leading to atherosclerosis and cardiovascular diseases.
39. How does aspirin help treat a heart attack
Aspirin helps treat a heart attack by inhibiting platelets, which prevents the formation of new blood clots and reduces the growth of existing ones, thereby improving blood flow to the heart muscle.
40. What is an EKG?
An electrocardiogram (EKG or ECG) is a diagnostic tool that records the electrical activity of the heart to help detect and monitor cardiac conditions.
41. What is an echocardiogram?
An echocardiogram is a noninvasive ultrasound test that uses sound waves to create images of the heart, allowing doctors to assess its structure and function.
42. What is the most common cause of a heart attack?
A blood clot that blocks one of the coronary arteries.
43. What causes a blood clot?
Blood clots are caused by factors such as endothelial injury, slow or turbulent blood flow, and hypercoagulability, which can lead to the aggregation of platelets and the activation of coagulation factors, resulting in the formation of a thrombus.
44. What is plaque made up of?
Plaque is composed of lipids, particularly cholesterol, calcium deposits, fibrin, cellular waste, and a variety of cells, including smooth muscle cells and inflammatory cells such as macrophages.
45. What happens when the plaque inside a blood vessel tears?
Platelets form blood clots at the sight of the tear, which blocks the vessel.
46. What are the normal levels of creatine phosphokinase?
10-120 mcg/liter of blood
47. What level of creatine phosphokinase indicates a heart attack?
400-800 mcg/liter
48. What causes heart tissue to die at a faster-than-normal rate?
Blood clots in the coronary arteries.
49. What is the job of the coronary arteries?
To provide the heart muscle with oxygenated blood.
50. What are the phases of a heart attack?Â
Hyperacute phase, fully enveloped phase, resolution phase, and permanent phase.
51. What is the fully enveloped phase?
Appears a few hours to days after a heart attack.
52. What is the resolution phase?
Appears weeks after a heart attack.
53. What is the stabilized chronic phase?
The final phase that shows permanent changes compared to a normal EKG.
54. What is the hyperacute phase?
Happens directly after a heart attack.
55. As a respiratory therapist, when a patient presents with signs of a heart attack, what’s the first thing you should do?
Give the patient 100% oxygen.
56. How fast do creatine phosphokinase levels rise?
Within 8-24 hours
57. What is the normal level of white blood cells?
4,000-10,000
58. What level of white blood cells may indicate a heart attack?
12,000-15,000
59. How often do they perform a creatine phosphokinase blood test?
Every hour for 24 hours.
60. Why is it important to check the creatine phosphokinase levels often?
If they keep rising, patients may still be experiencing a myocardial infarction.
61. What is the immediate treatment for a heart attack?
Intravascular line (IV), blood thinners (e.g., aspirin), heart monitor, and 100% oxygen.
62. When is an electric shock (defibrillator) indicated?
When the patient has an arrhythmia with a shockable rhythm.
63. What chemical is used to relax the blood vessels?
Nitroglycerin
64. What is coronary artery disease?
The atherosclerotic narrowing of the coronary arteries.
65. What does plaque lead to in coronary artery disease?
Ischemia
66. What are the risk factors for coronary artery disease?
Smoking, hyperlipidemia, diabetes mellitus, obesity, sedentary lifestyle, and uncontrollable stress.
67. What are the entrance criteria for metabolic syndrome?
Hypertension, abdominal obesity, and pre-diabetes.
68. What are the risk factors for an acute myocardial infarction?
Smoking, hypertension, genetics, and obesity.
69. What decreases the risk of an acute myocardial infarction?
No smoking, healthy diet, exercise, and moderate alcohol consumption.
70. How does plaque form?
Plaque forms when the inner wall of an artery is damaged by factors like high cholesterol or hypertension. This leads to a build-up of LDL cholesterol, white blood cells, and other substances, creating a hard, fatty deposit that narrows the artery and impedes blood flow.
71. What is the difference between a STEMI and an NSTEMI?
A STEMI shows ST-segment elevation on ECG due to complete coronary occlusion, whereas an NSTEMI does not show ST elevation and is usually caused by partial occlusion with elevated cardiac biomarkers.
72. Which cardiac biomarker is most specific for myocardial infarction?
Cardiac troponin I or T is the most specific and sensitive biomarker for myocardial injury.
73. How soon after symptom onset do troponin levels typically begin to rise?
Troponin levels generally begin to rise within 3 to 6 hours after myocardial injury.
74. How long can troponin remain elevated after a myocardial infarction?
Troponin may remain elevated for 7 to 14 days.
75. What is the typical time frame for CK-MB elevation after myocardial infarction?
CK-MB rises within 3 to 6 hours, peaks at about 12 to 24 hours, and returns to baseline within 48 to 72 hours.
76. Why is rapid reperfusion therapy critical in myocardial infarction?
Early reperfusion limits infarct size, preserves myocardial function, and reduces mortality.
77. What is the goal door-to-balloon time for primary PCI in STEMI patients?
The recommended goal is 90 minutes or less from hospital arrival to balloon inflation.
78. What is the goal door-to-needle time for fibrinolytic therapy?
Fibrinolytics should ideally be administered within 30 minutes of hospital arrival.
79. What ECG changes are associated with transmural myocardial infarction?
ST-segment elevation and the eventual development of pathologic Q waves.
80. What ECG finding may indicate myocardial ischemia without infarction?
ST-segment depression or T-wave inversion.
81. What is the most common arrhythmia during the first 24 hours after myocardial infarction?
Ventricular arrhythmias, particularly ventricular fibrillation.
82. Why are beta-blockers commonly used after myocardial infarction?
They reduce myocardial oxygen demand by lowering heart rate and blood pressure.
83. What is the role of ACE inhibitors following myocardial infarction?
ACE inhibitors reduce ventricular remodeling and decrease the risk of heart failure.
84. Why is morphine sometimes administered during a myocardial infarction?
It relieves pain and anxiety and reduces sympathetic stimulation.
85. What is cardiogenic shock in the context of myocardial infarction?
A state of severe pump failure resulting in inadequate tissue perfusion.
86. What hemodynamic parameter is typically reduced in cardiogenic shock?
Cardiac output
87. What is the most common location for myocardial infarction?
The left ventricle, particularly the anterior wall supplied by the left anterior descending artery.
88. Which coronary artery is most commonly involved in anterior wall myocardial infarction?
The left anterior descending (LAD) artery.
89. What complication may occur if the right coronary artery is occluded?
Inferior wall myocardial infarction and possible bradyarrhythmias.
90. Why should oxygen be administered cautiously in normoxic myocardial infarction patients?
Excess oxygen may cause coronary vasoconstriction and oxidative stress.
91. What is the mechanism behind chest pain in myocardial infarction?
Ischemia leads to anaerobic metabolism and accumulation of metabolites that stimulate pain receptors.
92. What is reperfusion injury?
Additional myocardial damage that occurs after blood flow is restored to ischemic tissue.
93. What is ventricular remodeling after myocardial infarction?
Structural changes in ventricular size and shape due to myocardial damage.
94. Why is smoking cessation critical after myocardial infarction?
Smoking increases thrombosis risk and accelerates atherosclerosis.
95. What is the significance of elevated BNP after myocardial infarction?
It may indicate the development of heart failure.
96. What is Dressler syndrome?
An autoimmune pericarditis that may occur weeks after myocardial infarction.
97. What mechanical complication can occur days after a large infarction?
Papillary muscle rupture leading to acute mitral regurgitation.
98. Why are antiplatelet agents prescribed long term after PCI?
To prevent stent thrombosis.
99. What lifestyle modifications reduce recurrence risk after myocardial infarction?
Smoking cessation, dietary changes, regular exercise, and blood pressure control.
100. What is the long-term goal of cardiac rehabilitation after myocardial infarction?
To improve functional capacity, reduce recurrence risk, and enhance quality of life.
Final Thoughts
Myocardial infarction is a serious condition resulting from impaired coronary blood flow and subsequent myocardial tissue death. While it originates in the cardiovascular system, its effects frequently extend into respiratory function through pulmonary edema, hypoxemia, and respiratory failure.
Respiratory therapists contribute to oxygen therapy management, ventilatory support, arterial blood gas analysis, and emergency resuscitation.
A thorough understanding of myocardial infarction pathophysiology, diagnostic markers, and cardiopulmonary interactions allows respiratory professionals to provide safe and effective care within the multidisciplinary team.
Written by:
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
References
- Ojha N, Dhamoon AS. Myocardial Infarction. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.