EKG Interpretation and the Cardiac Cycle

In order to understand EKG interpretation, it is important to first understand the cardiac cycle. This is the cycle of how the heart works to pump blood through it. There are electrical signals and resulting physical events. This article discusses the physical events – that is, how blood flows through the pumping heart.

The purpose of EKG interpretation is to identify abnormal events in the functioning of the heart. The EKG measures electrical signals which can be matched to physical events of muscular contraction.

Realize that the circulatory system is a closed system. This means that an increase in pressure in one segment results in decreased pressure in another segment. And the volume of blood is constant.

Blood from the body travels through the veins ending at either the superior vena cava or the inferior vena cava, both of which empty into the right atrium. In a cycle called diastole, the right atrium relaxes and blood flows into it from the vena cavas. As the right atrium fills with blood, it enlarges, causing the tricuspid valve to open. Thus, blood flows into the right ventricle. The ventricle fills about 70% with blood. Then the atrium enters its cycle of systole. This event is called atrial kick, and enforces additional blood into the ventricle. As the atrium relaxes in its period of diastole, then ventricular systole begins. As the right ventricle contracts, blood is forced into the pulmonary artery which leads to the lungs. There, carbon dioxide and oxygen are exchanged in the blood.

The newly oxygenated blood returns to the heart through the pulmonary vein. During the period of atrial diastole the left atrium fills with blood. As the pressure grows inside the bicuspid valve opens, allowing blood to flow into the left ventricle. In the next cycle of atrial systole, additional blood is forced into the left ventricle. During this cycle of ventricular systole, blood is forced through the aortic semi lunar valve into the aorta. From here it begins its journey to the rest of the body.

The heart has two atria and two ventricles. The tricuspid atrioventricular valve separates the right atrium and the right ventricle. The bicuspid atrioventricular valve separates the left atrium and the left ventricle. This valve is also called the mitral valve. The semi lunar pulmonic valve is located between the right ventricle and the pulmonary artery. The semi lunar aortic valve is located between the left ventricle and the aorta.

The tissue between the ventricles is called the interventricular septum. There are three layers of tissues that make up the heart. The innermost layer is called the endocardium. It is a thin smooth layer of tissue that lines the hearts inner chambers. The middle layer of the heart is called the myocardium. This layer is a thick muscular layer that make ups the middle portion and majority of the heart.

An infarct is an area of necrosis – or tissue death – resulting from a loss of blood supply.

Interpreting a 12-Lead EKG is a matter of using the heart’s electrical signals to determine the status of the heart – specifically if there are areas where an infarction has occurred in the myocardium that would prevent it from properly functioning as a blood pump.