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Experimental beating heart transplants depend upon cardiac muscle's ability to contract on its own. This new technique circulates blood and allows the heart to beat while in transit. Contrast this to typical heart transplants, where a picnic cooler filled with ice is used to preserve and transport the heart. The ability of cardiac muscle to beat on its own is dependent on the electrical impulses that are generated in the sinoatrial node. The heart has a built-in system that is responsible for generating impulses that are sequentially transmitted through the heart. This system, made up of cardiac pacemaker cells, is called the intrinsic conduction system, colored yellow in the drawing. This video will describe the components of the intrinsic conduction system; define electrocardiogram and what it measures; correlate the electrical events in the heart with the waves of an ECG tracing; define segments and intervals; and enable you to predict the effect of various scenarios on the R to R interval. The structures of the intrinsic conduction system include the sinoatrial node, the atrioventricular node, the atrioventricular bundle, the left and right bundle branches, and the subendocardial conducting network (also known as Purkinje fibers). The conduction of impulses through the heart generates electrical currents that spread and can be detected on the body's surface. A graphic recording of these electrical changes is called an electrocardiogram (abbreviated ECG or EKG). The ECG is a recording of voltage on the vertical axis against time on the horizontal axis. Shown here is a normal ECG with normal sinus rhythm. The waves or deflections on the tracing correlate to electrical changes in the heart, either depolarization or repolarization. The excitation of the heart begins with an impulse generated in the sinoatrial (or SA) node, located in the right atrium. The SA node, also known as the pacemaker, initiates and sets the rate of depolarization. The deflection of the P wave on the ECG represents the beginning of depolarization of the atria. The P wave as a whole is caused by atrial depolarization. From the SA node, impulses spread through the cardiac muscle of the atria via gap junctions and stimulate the atria to contract. So under normal conditions, the atria should contract shortly after the P wave. Note that an ECG does not detect contraction; it is inferred to occur following depolarization. The impulses travel from the SA node to the atrioventricular (or AV) node via the internodal pathway. The AV node is located in the lower interatrial septum in the right atrium. At the atrioventricular node, the impulse is slightly delayed, allowing the atria to completely contract. After the delay, the impulses travel from the AV node to the AV bundle located in the interventricular septum, then through the left and right bundle branches also located in the interventricular septum. Although the atria and ventricles are adjacent to each other, they are not connected by gap junctions. Consequently, the AV bundle is the only electrical connection between them. Let's check your understanding. Which of the following structures primarily controls the rate of heartbeats: the SA node, AV node, AV bundle, or bundle branches? >> The SA node controls the intrinsic heartbeat rate in its role as the pacemaker of the heart. The impulses travel from the bundle branches to the subendocardial conducting network, also known as Purkinje fibers, which carry the impulses to the apex of the heart and through the walls of the ventricles. The QRS complex represents the depolarization of the ventricles. There is no visible wave representing atrial repolarization because it is small in amplitude and is hidden by the QRS complex. Usually the ventricles will contract shortly after the S deflection. The ventricles are uniformly depolarized during the S-T segment. Ventricular repolarization begins at the apex and is represented by the T wave. The ventricular repolarization progresses superiorly. Let's check your understanding. Which of the following is not matched correctly? P wave and atrial depolarization; QRS complex and ventricular depolarization; or T wave and atrial repolarization? >> The T wave corresponds to ventricular repolarization. Repolarization of the atria is hidden by the large QRS complex. Abnormalities of the waves and changes in the timing of the deflections are useful in detecting cardiac abnormalities and problems with the conduction system of the heart. For example, an enlarged R wave could indicate that the ventricles are enlarged. In addition to measuring the height and duration of a wave, we also analyze segments and intervals. A segment is a region between two waves. For example, the S-T segment starts at the end of the S deflection and ends at the beginning of the T wave. An interval is a region that includes a segment and one or more waves. Two important intervals that are measured include the P to R and the Q-T intervals. The P to R interval starts at the beginning of atrial depolarization and goes to the start of ventricular depolarization. The Q-T interval is the period from the start of ventricular depolarization through ventricular repolarization. Let's check your understanding. Given the description "the end of the T wave to the start of the P wave," would this be a segment or an interval? >> It is a segment, because it is a region between two waves. There are no waves between the T wave and the P wave. Heart rate is usually expressed in beats per minute, or bpm. The R to R interval provides a convenient way to calculate heart rate, since the distance between R peaks corresponds to one heartbeat. Your instructor will explain how to convert the R to R interval into a heart rate. In this lab, you will record electrical activity of the heart of a subject lying down, sitting up, and after exercise. You will use electrocardiograms to determine heart rate. Think about how you expect heart rate to change with these scenarios.