The Function of an ECG in Diagnosing Heart Conditions A useful guide to the function of the heart s electrical system for patients receiving an ECG Written by Erhan Selvi July 28, 2014
Audience and Scope The purpose of this document is to describe the electrical activity of the heart and how it relates to an ECG (electrocardiogram) for patients if they receive one during a visit to the hospital. The focus will be placed on anatomical and physiological functions of the heart. Further focus will be directed towards abnormalities in the heart and how they can be diagnosed by using an ECG. Everyone is familiar with the typical heart beat pulse that is monitoring a patient in the hospital that is often dramatized in TV shows or movies. That waveform that is displayed on the machine s screen is a typical readout from an ECG machine. This document will inform patients who are going to receive an ECG test about how the ECG produces the waveform and what different waveforms in the readout mean. Receiving an ECG is very simple procedure that is both brief and painless, but is very important to ensure the wellbeing of an individual. Background Anatomy Information An ECG is a test that is used to determine the electrical activity of the heart. The information produced from an ECG can be further interpreted to determine the heart rate, heart rhythm, and contractility of the heart. If a patient experience symptoms that include, chest pain, uneven heartbeat, breathing issues, unusual fatigue, and abnormal heart sounds, a physician may recommend an ECG to diagnose the cause of these symptoms. An ECG examination may also be part of a routine checkup to ensure the heart s conduction system is working properly. Autorhythmic cells and Action Potentials The heart beats with a steady rhythm, and that is the result of specialized cells in the heart that are known as autorhythmic cells. Autorhythmic cells have the ability to contract the entire heart due to its electrical features. These cells go through a process known as an action potential. The action potential is a quick change in the electrical voltage of a heart cell. For instance, when a heart cell is at rest it has a voltage of -70 millivolts. During an action potential, the voltage of the cell will quickly become very positive (depolarization), and then immediately drop back down to a negative value (repolarization). This action potential causes the heart cells to contract. Most cells in the human body need an external agent to cause an action potential; autorhythmic cells can go through an action potential on their own. Autorhythmic cell a cell that can undergo action potentials, and thus contractions, on its own Action potential a rapid depolarization and repolarization that results in the cell contracting SA node specialized group of autorhythmic cells that undergo action potentials at the fastest rate; sets heart rate in healthy hearts; causes atrial contraction AV node specialized group of autorhythmic cells that that receives action potential signals from the SA node; spreads signal to ventricular fibers Specialized Autorhythmic Cells Multiple regions of the heart have autorhythmic cells so that different parts of the heart may contract at different times. The first and most important group of autorhythmic cells is located at the top of the right atrium and is called the SA node as seen in figure 1. These cells undergo action potentials at the fastest rate. Therefore the SA node is often called the pacemaker of the heart. Another important node of autorhythmic cells is the AV node, located at the bottom of the right atrium. When the SA node undergoes an action potential, its action potential signal is sent to the AV node which 1
signals cells in that region to contract. After the AV node, there are fibers that are spread out in the ventricles that will also relay the action potential signals that were started at the SA node. Therefore, after the AV node receives the signal from the SA node, the signal moves to the fibers which signal the ventricles to contract last. Figure 1 Anatomical Structure of the Heart Source: http://www.heart-valve-surgery.com/images/cardiac-conduction-system.jpg Gap Junctions Gap junctions play an important role in the contraction of the entire heart. The heart can contract without the help of the SA node. While there are the specialized groups of autorhythmic cells such as the SA and AV nodes, autorhythmic cells are located all throughout the heart. All cells in the heart also contain a structure called gap junctions that link neighboring cells together. This structure acts like an open doorway for voltages to move from one cell to the other. This is important because the spread of voltages from one cell will cause depolarization of the cells in that region, and this ultimately results in the spread of action potentials and contractions in the heart cells (heart cells contract after undergoing an action potential). With the ability for a heart to spread an action potential through the gap junctions in its cells with the help of the SA node setting the Ventricular fibers receive action potential signals from AV node and send the signal to ventricular cells; causes ventricular contraction Gap junction linkage between neighboring all cardiac cells that allows for the spread of voltage and thus contraction 2
pace of action potentials, the heart can successfully beat in a coordinated rhythm that delivers blood efficiently to the body. ECG Readings Electrical signals can be detected using an ECG with sensors placed on different parts of the body. The sum of all the action potentials that occur in the heart is what an ECG measures. Sensors (or leads) can be placed on several parts of the body such that when action potentials occur in its region, they will detect the change in voltages. These changes in voltage from the ECG waveform are seen in figure 2. Figure 2 Typical ECG Waveform Source: lessons4medicos.blogspot.com The level point at the beginning of the wave is known as the isoelectric point and this represents a resting voltage value where there are no major action potentials occurring. The first major event to occur is the P wave. The P wave is the depolarization of the atria of the heart. Following the P wave is the QRS complex, and this P wave atrial depolarization and contraction QRS complex ventricular depolarization and contraction T wave - ventricular repolarization and relaxation Arrhythmia heartbeat that is out of rhythm represents the ventricular depolarization. Lastly is the T wave which is the ventricular repolarization. The atrial repolarization is not seen in the ECG waveform because the overall magnitude of it is not strong enough for it to create a distinguishable wave. For the sake of simplicity, when a region of the heart depolarizes, that means the cells in that region will contract. 3
Diagnosis of Heart Abnormalities An ECG test is useful to detect when the heart does not beat in rhythm (arrhythmia). The sequence of contraction in the heart starts with both atria at the same time contracting into the ventricles, followed by both ventricles contracting blood into the body. This order is dependent upon the heart s electrical system and the order that it follows. The order of contraction is SA node, atrial cells, AV node, then ventricular cells. Any abnormalities that occur in this sequence result in arrhythmia. Figure 3 shows the ECGs of several different arrhythmias that reveal abnormalities in the heart and how it contracts. Second-degree block is seen as the first graph in figure 3. This ECG differs from the normal waveform in that not every P wave is followed by a QRS complex. Since the P wave is associated with atrial contraction and the QRS complex is associated with ventricular contraction, so there must be a problem with the conduction of action potentials from the atria to the ventricles, so the malfunctioning component of the heart is the AV node. Figure 3 ECG waveform of second-degree heart block Source: cnx.org/content/m46664/latest/ In the atrial fibrillation ECG graph in figure 4, the only noticeable wave is the QRS complex, and the rest of the graph is just peaks and valleys of weak signals. These small deflections in the wave are representative of the autorhythmic cells of the atria contracting on their own without any order. Thus, it can be concluded that atrial fibrillation is the result of the SA node not properly sending its signal and setting the pace for the rest of the heart to beat. 4
Figure 4 ECG waveform of atrial fibrillation Source: cnx.org/content/m46664/latest/ Ventricular fibrillation, seen in figure 5, is similar to atrial fibrillation, except that there are none of the distinguishable waves of a normal ECG present. The peaks and valleys are seen again, however they are slightly larger. This is due to there being more ventricular cells than atrial cells. Thus in ventricular fibrillation, all of the autorhythmic cells of the ventricles are beating on their own without any coordination and this results in an extremely ineffective pumping of the blood. The cause of ventricular fibrillation is not exactly known, but it is clear that the pathway of the electrical signal starting in the SA node is not reaching the AV node and the subsequent fibers that allow the heart to contract in order and rhythm. Figure 5 ECG waveform of ventricular fibrillation Source: cnx.org/content/m46664/latest/ Third-degree heart block is represented in figure 6 and is the result of miscommunication between the atria and ventricles contracting. The most notable feature is the slower rate of QRS complexes appearing. The reason for this is that the since the SA node and AV node are not communicating correctly, the AV node takes over the pace-making responsibilities because it contracts and the next fastest rate. However, this rate is significantly slower than the rate at which the SA node sets for the heart. This loss of signal from the SA node that doesn t reach the ventricles results in the long interval between ventricular contractions. 5
Figure 6 ECG waveform of third-degree heart block Source: cnx.org/content/m46664/latest/ Conclusion The heart s electrical system is the spark behind the action of the heart and is the reason why it can pump blood through the human body so effectively. The electrical system of the heart is made up of the SA node, AV node, ventricular fibers, and all the other autorhythmic cells located throughout the heart. The SA node sets the pace for the heartbeat because it undergoes action potentials at the fastest rate, and that action potential spreads throughout the entire heart resulting in ordered contraction of the atria first, followed by the ventricles. When the SA node fails, the AV will take up responsibility for setting the pace of the heartbeat, but it will produce action potentials at a slower rate than the SA node. If both the SA node and the AV node fail, then the remaining autorhythmic cells in the heart will contract independently without any specific pace or rhythm. Second-degree heart block - AV node malfunctioning resulting in atrial contraction that is always not immediately followed by ventricular contraction Atrial fibrillation SA node malfunctioning resulting in uncoordinated contractions of atrial cells Ventricular fibrillation uncoordinated contraction of ventricular cells most likely due to a malfunctioning SA and AV node Third-degree block SA node malfunctioning resulting in slower heart rate and uncoordinated atrial contraction Understanding how the heart works and how an ECG monitors its functions is useful to become an informed patient. Some of these abnormalities are asymptomatic, so it would be impossible to determine whether or not one has it without an ECG examination. Therefore, it is important to have regular appointments with a physician to ensure that the heart it pumping properly and that there are no issues in the wiring of the heart s electrical system. Note: Most of the information comes from prior knowledge of the author from college-level coursework so cited materials were used as general reference Works Cited "Cardiac Muscle and Electrical Activity." OpenStax CNX. OpenStax College, 19 June 2013. Web. 27 July 6
2014. "Understanding the Heart's Electrical System and EKG Results." National Heart, Lung, and Blood Institute. National Institute of Health, n.d. Web. 27 July 2014. "What Is Atrial Fibrillation?" National Heart, Lung, and Blood Institute. National Institute of Health, n.d. Web. 27 July 2014. "What Is an Electrocardiogram?" National Heart, Lung, and Blood Institute. National Institute of Health, n.d. Web. 27 July 2014. Figure 1. (cropped) Electrical System of the Heart. Digital image. HeartValveSurgery.com. HeartValveSurgery.com, 2012. Web. 27 July 2014. Figure 2. Normal ECG waveform. Digital image. Medicine Decoded. N.p., 18 June 2008. Web. 27 July 2014. Figure 3. (cropped) Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19 June 2013. Web. 27 July 2014. Figure 4. (cropped) Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19 June 2013. Web. 27 July 2014. Figure 5. (cropped) Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19 June 2013. Web. 27 July 2014. Figure 6. (cropped) Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19 June 2013. Web. 27 July 2014. 7