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. 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 they are picked up by external electrodes which are placed at specific positions on the skin. 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 heart beats. 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 Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group.
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 Figure 3. Illustration by Hans & Cassidy. Courtesy of Gale Group.
(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 Figure 4. ECG trace recordings. Illustration by Hans & Cassidy. Courtesy of Gale Group. 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. 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.

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, 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. The recording 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.

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.

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 X and Y axes must be examined.