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PART 4: ORGAN SYSTEMS
Chapter 1: Cardiac/Circulatory
Cardiac: Acute Myocardial Infarction
Lt Col Robert Allen, USAF, MCIntroduction: Acute myocardial infarction (AMI) is the leading cause of death in the US and in most of the
western world. AMI is usually thought of as a disease of older people, but almost half of the cases of AMI
in the US occur in people under the age of 65. AMI can and does occur in people in their 20s and 30s.
Early aggressive management of AMI can significantly improve mortality and morbidity. Approximately 25%
of patients with AMI die within one hour of symptom onset, usually due to a malignant cardiac arrhythmia.
Another 30-40% of patients with AMI die immediately or within several days. While some of the diagnostic
and therapeutic procedures noted in the chapter are currently not possible in the SOF environment, advances
in miniaturization of EKG machines, blood chemistry machines and pharmacology allow remarkably advanced
diagnosis and treatment even in austere environments.
Cardiac Risk Factors: Prior AMI or history of coronary artery disease, smoking/tobacco use, hypertension,
AMI before age 55 in parents or siblings, diabetes, high total cholesterol, low HDL cholesterol, obesity and
history of peripheral vascular disease.
Subjective: Symptoms (See Symptom: Chest Pain)
Chest pain associated with AMI is generally dull, diffuse, and often described as a pressure sensation. Chest
pain that lasts for a few seconds is not usually cardiac-related. Angina usually lasts less than 5 minutes, while
AMI chest pain lasts more than 5 minutes. Classical angina pain is brought on by exertion and is relieved
by rest. Chest pain from an AMI can start during exertion or at rest, and can radiate to the left arm, jaw or
epigastric area. AMI pain is frequently accompanied by diaphoresis, shortness of breath, nausea and feelings
of dread. The presence of three or more cardiac risk factors increases the likelihood of AMI.
Objective: Signs
Using Basic Tools: Tachycardia or bradycardia, hypertension or hypotension; diaphoresis in association with
chest pain; inspiratory rales and S-3 gallop (left-sided cardiac failure); hepatojugular reflux, jugular venous
distension and peripheral edema (right-sided cardiac failure).
Using Advanced Tools: Electrocardiogram (EKG): ST segment elevation is the hallmark of myocardial
infarction, while ST depression and T-wave inversion are signs of myocardial ischemia. Use patterns of ST
segment elevation to identify the location of the infarction. Elevation in leads I, AVL, and V-1 through V-3
indicate an anterior MI. Elevation in leads II, III and AVF usually indicate an inferior MI. ST elevation in V-2
through V-6 indicates an anteriolateral MI. In up to 40% of AMIs the initial EKG does not show acute ST
segment changes. A ‘normal’ EKG does not rule out AMI! The EKG may not reflect ST changes early in the
AMI. Repeat the EKG as the chest pain changes over the first few hours to ensure ST changes are captured.
(Figure 4-1: EKG ST Segment Changes)
Assessment:
Differential Diagnosis: Presumptively diagnose AMI if there is ST segment elevation of 1 mV(mm) or
greater in contiguous leads. Symptoms consistent with AMI accompanied by a new-onset left bundle branch
block (LBBB) are also considered presumptive evidence of AMI.
ST segments can be falsely elevated in several conditions, including myocarditis, left ventricular hypertrophy,
ventricular aneurysms, early repolarization, hypothyroidism, and hyperkalemia.
Other causes of chest pain are reviewed in the Symptom: Chest Pain section, including thoracic GI
causes (esophagitis, hiatal hernia), abdominal GI causes (pancreatitis, cholecyctitis), muskuloskeletal causes
(costochondritis), vascular causes (pulmonary embolus) and others.