The differential diagnosis of chest pain is vast, as chest pain may originate from the cardiovascular, gastrointestinal, respiratory, musculoskeletal, and nervous systems. Potentially lethal causes of chest pain, which must be considered in every patient with this complaint, include acute myocardial infarction (MI), aortic dissection, pneumothorax, pneumonia, pulmonary embolism, and esophageal perforation (Box 12.1). While many presentations of chest pain are not cardiac, the time‐sensitive nature as well as dire consequences of missed cardiac chest pain make early consideration and diagnosis of this disease essential. The 12‐lead electrocardiogram (ECG) plays a critical role in the evaluation of cardiac chest pain, and therefore it is important that clinicians to use the ECG early when there is a suspicion of cardiac chest pain.

Cardiac Anatomy and Basic Physiology of Depolarization

In a very basic sense, the heart is divided into two halves – the right heart and the left heart; each half of the heart is further divided into two specific chambers – atrium and ventricle. Thus, the heart is composed of four chambers, including the right atrium, right ventricle, left atrium, and left ventricle. The atria are passive receptacles for blood returning to the heart, and they, in turn, push blood into the ventricles. The ventricles are much larger and stronger than the atria because they are responsible for pumping blood to the lungs (right ventricle) and the body (left ventricle). The right heart moves deoxygenated blood from the body and pumps it to the lungs to be reoxygenated. The left heart then takes this newly oxygenated blood from the lungs and pumps it throughout the body.

When the clinician is evaluating the patient for ACS, the left ventricle is the only cardiac region that is currently considered significant and important from the perspective of the 12‐lead ECG. Thus, the left ventricle is imaged by the 12‐lead ECG. In fact, the 12‐lead ECG primarily images the walls of the left ventricle, including the anterior, lateral, inferior, and posterior segments; the right ventricle is imaged but less completely when compared to the left ventricle.

The triggers for the atrium and the ventricle to contract come in the form of electrical impulses, and these impulses are controlled by the cardiac pacemaker and conduction system (Figures 12.1 and 12.2a). The sinus node, which is located in the right atrium, is the primary pacemaker for the heart. The electrical signal from the sinus node (Figure 12.2b) is carried through the atrium to the atrioventricular (AV node; Figure 12.2c), which is located between the atria and the ventricles. The AV node then passes the signal to the His–Purkinje system, which passes it to the ventricles, initiating repolarization (Figure 12.2d). Repolarization occurs in a reverse manner, from ventricular tissue to atrial tissue (Figure 12.2e).

Coronary Anatomy and Electrocardiograhic Regional Anatomic Issues

Blood is supplied to the heart muscle, which is also called the myocardium, via blood vessels called the coronary arteries. These coronary arteries arise from the base of the aorta and run along the surface of the heart, dividing into smaller branches. Eventually, very small arteries come off of the coronary arteries and dive down into the heart tissue, providing blood directly to the myocardium. Each of the major coronary arteries follows a typical course around the heart and usually supplies a specific area of the heart muscle. The two coronary arteries that come directly off of the aorta are named the right coronary artery and the left main coronary artery (Figure 12.3

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