Myocardial infarction is the death of tissue resulting from inadequate tissue perfusion. Myocardial infarction is also used to name the set of signs and symptoms of crushing chest pain, diaphoresis, malignant ventricular arrhythmia, heart failure and/or cardiogenic shock. MI‟s account for 750,000 hospital admissions and around 500,000 deaths every year and lead to more potential years of life loss than any other illness.
Myocardial infarctions are primarily caused by atherosclerotic lesions, disruption of atherosclerotic plagues, and thrombus formations. Risk factors that lead to MI‟s include hypertension, obesity, smoking, high levels of LDL, low levels of HDL, family history of atherosclerosis, diabetes, and possibly increased levels of triglycerides. Some risk factors can be modified, such as obesity, HDL and LDL levels, and hypertension. Other risk factors are non-modifiable, such as family history. Other etiologies of MI‟s make up a small percentage of the causes. Some of the less common causes of Myocardial infarction are trauma, connective tissue disease, long-term infectious diseases (such as Kawasaki disease), metabolic disease, congenital anomalies of the coronary arteries, and drug use (such as cocaine and amphetamines).
When a coronary artery is blocked, it causes ischemia and leads to an MI. Depending on what coronary artery is blocked or restricted, one can determine what area of the heart will be affected or vice versa. If an occlusion or restriction is in the left anterior descending (LAD) artery, the anterior left ventricle, most of the septum, the apex, and the lateral and posterior left ventricle will be affected. The LAD also supplies blood flow to the anterior fascicles of the left bundle branch and the right bundle branch.
The posterior left branch of the right coronary artery supplies the atrioventricular node in most of the population. An occlusion or restriction of the right coronary artery would affect the right ventricle.
The use of an ECG tracing can be helpful in identifying the location and severity of a cardiac event. The ST segment of the ECG tracing can tell a lot about the condition of the heart. ST segment elevation represents areas of the heart that have suffered acute injury. ST segment depression and T wave inversion shows areas that are suffering from ischemia.
Using the leads of the ECG tracing, one can narrow down the area of the cardiac event. The leads of the ECG tracing represent areas of the heart. Leads II, III, and AVF represent the inferior portion of the heart. Leads V1 and V2 show the septal area. Leads V3 and V4 portray the anterior aspect of the heart. The lateral part of the heart is shown in leads I, AVL, V5, and V6. Also, rV4 can show an event in the right ventricle.
Some factors can make ECG interpretations difficult. Examples of these factors are left bundle branch block (LBBB), paced rhythms, and left ventricular hypertrophy. However, there are key findings that can help determine if an MI or cardiac event is happening in these instances. In a LBBB, if there is ST elevation greater than or equal to 1mm in conjunction with a QTS complex or ST depression greater than or equal to 1mm in V1, V2, or V3, there is indication of an acute myocardial infarction. In the case of a right paced rhythm, ST segment elevation in a predominantly positive QRS complex or ST depression in leads V1, V2, V3 suggest an acute myocardial infarction.
ECG tracings do not always show MI or ischemia right away. A careful exam and thorough questioning can help determine if the patient is having a cardiac event. A myocardial infarction presents with many signs and symptoms. A typical complaint is one of chest discomfort that can be described as pressure, heaviness, fullness, or squeezing. This chest discomfort is less commonly described as knifelike, sharp or stabbing. The most common location of this discomfort is substernal or in the left chest with radiating pain to the arm, neck, or jaw. The patient may also complain of weakness, „sense of impending doom‟, nausea and vomiting, present as restless and confused, and may appear pale and diaphoretic. The patient‟s vital signs can show tachycardia and hypertension. However in a right ventricle MI, the patient may suffer from hypotension. If MI affects the sinus node function, the patient can suffer from hypotension. Rales or wheezing may be present due to pulmonary edema.
In many cases, the patient will have no symptoms of having a myocardial infarction other than an abnormal ECG tracing. As many as 25% of the elderly patients will have a silent MI. Some patients only complain of indigestion. Other reasons patients will exhibit no signs or symptoms of an MI are diabetes, high threshold of pain or stoicism, impaired cerebral perfusion, or obtundation.
Many illnesses can have similar signs and symptoms. An aortic dissection can have similar pain as an MI. With an aortic dissection, the patient may have pulse or blood pressure differences in the arms or legs, but will not have the same ECG changes as in an MI.
Other differential diagnosis can include pulmonary embolism, pneumothorax, and costochondritis. Cholecystitis, choleliththiasis, peptic ulcers, reflux and hiatal hernia can also have similar signs and symptoms of an MI. A thorough exam and history, along with an ECG can help determine if the patient is having an MI or some other illness.
Definitive care of a patient suffering from an acute myocardial infarction is prompt recanalization of the occluded coronary vessel. However, there are many treatments one can do in the prehospital setting. Most deaths from an MI occur due to ventricular fibrillation. Early defibrillation can save the lives of approximately 39% of the patients who die from an MI due to fibrillation. Prehospital treatments consist of placing the patient on oxygen, establishing IV access, giving the patient aspirin (180-325mg), and acquiring a 12 lead ECG. Other prehospital treatments include pain management with morphine (2-5mg every 10-15 mins), sublingual nitroglycerin (0.04mg prn) can also be helpful with pain management, suppression of ventricular arrhythmias with lidocaine (0.5-1.5 mg/kg, then dripped at 1-3 mg/min) and atropine (0.5mg every 3-5 mins) to help with bradycardia and hypotension. IV fluids can also be helpful in managing hypotension. Some EMS systems may even elect to give thrombolytics in the prehospital environment. The most important thing to keep in mind is that these patients need early thrombolytics or recanalization that is given in the hospital setting. Rapid transport to an appropriate facility is crucial to saving the heart tissue of the patient.
An EMS crew is called to the house of a 52 y/o male complaining of chest pain. Upon arrival they find the man sitting on his porch clutching his chest. The man looks very pale
and diaphoretic and appears restless. Upon questioning, the man states that he was just doing some light chores when he started having chest pain. He describes the pain as “an elephant sitting on my chest”. He rates his pain as a 7 out of 10. The patient says he takes a high blood
pressure pill, has no allergies, no medical history other than HTN, and does not smoke. The patient states that his father had 3 heart attacks. The patient is placed on 15ltrs of O2 by NRB mask, IV access is established and a 12 lead ECG is acquired. The ECG shows marked ST elevation in leads V4 and V5. The patient‟s vital signs are: pulse of 100, BP of 102/60, respirations of 22, lung sounds are clear. The patient is given a baby aspirin and given 0.04mg SL nitro. The patient states his pain is now 6/10 after the nitro. The patient‟s BP has
dropped to 88/40. The patient is given an IV bolus of 200cc NS. The patient is also given 2mg of morphine that brings the patients pain down to 4/10. After the fluid bolus, the BP is 94/48 and another nitro is given. The patient states his pain is now 1/10. His blood pressure is now 84/42, his lung sounds are clear, and another fluid bolus of 200 cc NS is given. After the nd2 bolus, the patient‟s BP is 90/46. The patient is transported rapidly to the local hospital.
Once in the hospital, the patient will be evaluated for thrombolytic therapy or recanalization.
Goldman, Lee, and Bennet, Claude MD. Cecil Textbook of Medicine. Philadelphia, PA:
W.B. Saunders Company, 2000
Lanza, Micheal, “Right Ventricular Myocardial Infarction: When the Power Fails” Nursing Center Library (Internet site at www.nursingcenter.com). Retrieved 23 March 2003.
Stahmer, Sarah, “Myocardial Infarction.” eMedicine (Internet site at www.emedicne.com). Retrieved 23 March 2003.
thTintinalli, Judith, Kelen, Gabor, and Stapcynski, J. Emergency Medicine 5 Edition, A
Comprehensive Study Guide. New York, NY: 2000
Pikes Peak Community College
March 27, 2003
I. Introduction and Statistics II. Causes and Risk Factors III. ECG Usage
IV. Signs, Symptoms and Physical Findings
V. Differential Diagnosis VI. Treatment
VII. Case History