Electrocardiogram (ECG)
Electrocardiogram (ECG)
The electrocardiogram, ECG or EKG, directly measures microvoltages in the heart muscle (myocardium) occurring over specific periods of time in a cardiac, i.e., a heartbeat, otherwise known as a cardiac impulse. Electrocardiography is the noninvasive, virtually risk-free procedure of analyzing the electrical activity (electrical conduction and rhythm) of the heart muscle through electrodes positioned on the chest. The results are recorded on the electrocardiogram. The ECG describes a series of waves that are associated with the electrical impulses that occur during each beat of the heart. From 2000 to 2005, according to the U.S. Centers for Disease Control and Prevention (CDC), around 23 million ECGs are performed in the United States each year.
In the late 1700s medical researchers learned that muscles produce tiny electric impulses now known as action potentials. Italian biophysicist Carlo Matteucci (1811–1868) identified action potentials in a pigeon’s heart in 1843 and, in 1856, German scientists Rudolf Albert von Kölliker (1817–1905) and Heinrich Müller (1820–1864) recorded these electric currents from a frog’s heart. Reasoning that these recordings could reveal irregularities and, hence, heart disease, researchers attempted to develop accurate measuring devices. French physiologist Augustus Waller (1856–1922) found that cardiac currents could be recorded by placing surface electrodes on the body. In 1887, Waller developed a capillary electrometer—tubes of mercury that rose and fell with the changes in heart muscle current—which, unfortunately, was imprecise and difficult to use.
Therefore, Dutch physiologist Willem Einthoven (1860–1927) set out to design an improved apparatus. In 1903, he described the result as a string galvanometer that consisted of thin, silver-coated quartz stretched between the limbs of a patient. As the heart’s electric impulse flowed through, the wire was deflected and motion was magnified and projected onto moving photographic film. The extreme sensitivity of the device, which weighed 600 pounds (272.4 kilograms), allowed it to detect the tiny cardiac currents very accurately. Einthoven called his machine the electrocardiograph and the recorded electrical impulses an electrocardiogram. He devised the standard positioning of the electrodes and described the regular heart waves and the triangle used to interpret electrocardiograms. Through clinical studies, Einthoven identified a number of heart problems with his galvanometer. Einthoven won the Nobel Prize in 1924 for inventing the ECG. In 1942, Emanuel Goldberger added three augmented limb leads to Einthoven’s three limb leads and the six chest leads making the 12-lead electrocardiogram that is used today, and English physician Sir Thomas Lewis (1881–1945) established the electrocardiogram as a standard clinical tool.
With refinements in instrumentation and technique, electrocardiography became one of the most useful diagnostic tools in medicine. This highly accurate, easy to interpret, and relatively inexpensive device permits diagnosis of heart conditions without needle or incision and even portable devices are now available: the HeartMirror, weighing only 3.3 pounds (1.5 kilograms) operates on four AA NiCad (nickel cadmium) batteries and will record 12 selectable leads. Housed in a carrying case, it is ideal for a physician’s
office. The HeartVision portable emergency device weighs only 13.4 ounces (380 grams), uses four AA batteries, and fits into a shirt pocket. Four built-in emergency electrodes eliminate contact leads—the device is simply placed on a patient’s chest and displays the reading on a liquid crystal display (LCD). It also stores the reading, which can be printed out later on an analog ECG recorder or personal computer.
With each heartbeat, electrical currents called action potentials, measured in millivolts (mV), travel at predictable velocities through a conducting system in the heart. The potentials originate in a sinoatrial (SA) node, which lies in the entrance chamber of the heart, called the right atrium. These currents also diffuse through tissues surrounding the heart whereby they reach the skin. There, external electrodes, which are placed at specific positions on the skin, pick them up. They are, in turn, sent through leads to an electrocardiograph. A pen records the transduced electrical events onto special paper. The paper is ruled into mV against time and it provides the reader with a so-called rhythm strip. This is a non-invasive method to used evaluate the electrical counterparts of the myocardial activity in any series of heartbeats. Careful observation of the records for any deviations in the expected times, shapes, and voltages of the impulses in the cycles gives the observer information that is of significant diagnostic value, especially for human medicine. The normal rhythm is called a sinus rhythm if the potentials begin in the sinoatrial (SA) node.
A cardiac cycle has a phase of activity called systole followed by a resting phase called diastole. In systole, the muscle cell membranes, each called a
sarcolemma, allow charged sodium particles to enter the cells while charged potassium particles exit. These processes of membrane transfer in systole are defined as polarization. Electrical signals are generated and this is the phase of excitability. The currents travel immediately to all cardiac cells through the mediation of end-to-end high-conduction connectors termed intercalated disks. The potentials last for 200 to 300 milliseconds. In the subsequent diastolic phase, repolarization occurs. This is a period of oxidative restoration of energy sources needed to drive the processes. Sodium is actively pumped out of the fiber while potassium diffuses in. Calcium, which is needed to energize the force of the heart, is transported back to canals called endoplasmic reticula in the cell cytoplasm.
The action potentials travel from the superior part of the heart called the base to the inferior part called the apex. In the human four-chambered heart, a pacemaker, the SA node, is the first cardiac area to be excited because sodium and potassium interchange and energize both right and left atria. The impulses then pass downward to an atrioventricular (AV) node in the lower right atrium where their velocity is slowed, whereupon they are transmitted to a conducting system called the bundle of His. The bundle contains Purkinje fibers that transmit the impulses to the outer aspects of the right and left ventricular myocardium. In turn, they travel into the entire ventricular muscles by a slow process of diffusion. Repolarization of the myocardial cells takes place in a reverse direction to that of depolarization, but does not utilize the bundle of His.
The place where electrodes are positioned on the skin is important. In what are called standard leads, one electrode is fastened to the right arm, a second on the left arm, and a third on the left leg. They are labeled Lead I (left arm to right arm), II (right arm to left leg), and III (left arm to left leg). These three leads form angles of an equilateral triangle called the Einthoven triangle (Figure 1) in a sense, the galvanometer is looking at the leads from three different points of view. The standard leads are in pairs called bipolar and the galvanometer measures them algebraically, not from zero to a finite value. The ECG record is called frontal, which is a record of events downward from base to apex.
The ECG displays a second set of leads called precordial. This means that they are positioned anterior to the heart at specific places on the skin of the chest. They measure electrical events, not in a frontal plane like the standard leads do, but tangentially, from anterior (ventral) to posterior (dorsal) or vice versa across the chest wall. They are numbered from their right to left positions as V1 through V6 (Figure 2) this allows them to sense impulses directly beneath the particular electrode put into the circuit. Events in these horizontal planes add significantly to diagnosis.
The ECG also shows a third set of leads, which are three in number. These are called vectorial and are essential in obtaining vectorcardiograms because the transmission of action potentials in the heart is a directional or vector process. The direction of travel of the action potentials is found by vectorial analysis as it is in physics. It takes two measurements of a completed record that are at right angles to one another to determine the resultant direction of all the potentials occurring at a given time. The resultant, computed as an arrow with a given length and direction, is considered to be the electrical axis of the heart. In the normal young adult, it is predictably about minus 60 degrees below the horizontal isoelectric base line. The three vectorial leads are each 30 degrees away from the standard leads, appearing like spokes on a wheel. They explain why twelve leads appear in an ECG strip. In the recordings they are designated a VR, a VL and a VF. The lower case “a” means augmented, V is voltage and R, L and F are for the right arm, left arm and left foot.
The normal sinus ECG
A few selected examples of ECGs are displayed herein. In the normal ECG, as taken from standard lead II, there are three upward or positive deflections,
KEY TERMS
Action potential— A transient change in the electrical potential across a membrane that results in the generation of a nerve impulse.
Depolarization— A tendency of a cell membrane when stimulated to allow charged (ionic) chemical particles to enter or leave the cell. This favors the neutralization of excess positive or negative particles within the cell.
Diastole— The phase of rest in a cardiac cycle, allowing reconstitution of energy needed for the phase of systole which follows.
Electrocardiogram (ECG)— A moving pen or oscilloscope record of the heart beats. Essentially, the measurement of microvolt changes by a galvanometer over time.
Precordial leads— These are six leads, labeled V1 through V6, which pass from electrodes on the ventral chest wall to the electrocardiograph. They are called unipolar in that the active electrode is placed on the chest while the second electrode is placed on an extremity, but only one transmits the action potentials originating in the heart.
Repolorization— The reverse passage of ions across the cell membranes following their depolarization. This reestablishes the resting differences, i.e., polarity, on either side of a cell membrane.
Standards (limb) leads— These are conductors connecting the electrocardiograph with the right arm, left arm, and left leg. They form a hypothetical Einthoven triangle. The leads are called bipolar in that each lead involves two electrodes placed RA and LA, RA and LL, LA and LL.
Vector— A quantity or term that can be expressed in terms of both magnitude (a number) and a direction.
P, R, and T and two downward negative deflections, Q and S. The P wave indicates atrial depolarization. The QRS complex shows ventricular activity. The S-T segment, as well as the T wave, indicate ventricular repolarization. There are atrial repolarization waves but they are too low in voltage to be visible (Figure 3).
The time line on the X axis is real time. Therecording paper is read on this line as 0.04 seconds for each small vertical subdivision if the paper is running at 0.98 in (25 mm) per second. At the end of each group of five of these, which corresponds to 0.2 seconds, the vertical line is darker on the ruled paper. If the pulse rate is found to be 75 per minute, the duration of a cardiac cycle is 60/75 or 0.8 seconds. Variations in expected normal times for any part of a cycle indicate specific cardiac abnormalities. This is used to diagnose arrhythmias which have a basis in time deviation.
On the Y axis, every 0.4 in (10 mm) corresponds to 1 mV of activity in the heart. Although time on the X-axis is real, the mV on the Y-axis cannot always be taken literally. Voltages may partly lose significance in that a fatty person can to some extent insulate cardiac currents from reaching the skin.
Respiratory sinus arrhythmia
The young adult male, while resting, breathes about 12 times per minute. Each cycle takes five seconds, two for inspiration, and three for expiration. The ECG shows these differences graphically in every respiratory cycle and they are easily measurable between successive P waves. This is the only arrhythmia that is considered to be normal.
Ventricular tachycardia
The effect of the form of the wave on the ECG, as distinguished from the effect of the direction and force is illustrated in this disorder. Prominent signs include an extraordinary height of the waves and, also, the rapidity of the heart beat. Both the X-axis and Y-axis must be examined.
Resources
BOOKS
Ganong, William F. Review of Medical Physiology. 22th ed. New York: McGraw-Hill Medical, 2005.
Germann, William J. Principles of Human Physiology. San Francisco, CA: Pearson Benjamin Cummings, 2005.
Guyton, Arthur C, and John E. Hall. Textbook of Medical Physiology 11th ed. Philadelphia, PA: Elseveier Saunders, 2006.
Hill, Richard W. Animal Physiology. Sunderland, MA: Siauer Associates, 2004.
Mader, Sylvia S. Understanding Human Anatomy and Physiology. Boston, MA: McGraw-Hill Higher Education, 2005.
Randall, David J. Eckert Animal Physiology: Mechanisms and Adaptations. New York: W.H. Freeman & Company, 2002.
Van De Graaff, Kent M., and R. Ward Rhees, eds. Human Anatomy and Physiology: Based on Schaum’s Outline of Theory and Problems of Human Anatomy and Physiology. New York: McGraw-Hill, 2001.
Harold M. Kaplan