a. Heart Cells. Two types of cells are found in the heart -- electrical cells and mechanical cells (see figure 1-1). The heart's conduction system is made up of electrical cells. These cells have the ability to begin and transmit electrical activity in the heart. Myocardial cells (the mechanical cells) make up the bulk musculature of the heart. When an electrical stimuli reaches these cells, the cells contract. An electrical impulse stimulates the mechanical action of the heart causing the heart to pump effectively. If the electrical system of the heart does not function properly, arrhythmias (a mechanical activity) may occur.
Figure 1-1. Electrical cells and mechanical cells.
b. Electrical Activity In The Heart. The heart's electrical activity begins in the sinoatrial (SA) node and flows toward the ventricles (see figure 1-2). The SA node is the heart's pacemaker. All the areas of this conduction system initiate impulses, become irritable, and respond to an impulse. Impulses are initiated in each area of the conduction system as shown here:
|SA Node ||60-1 00 per minute |
|AV Junction ||40-60 per minute |
|Ventricle ||20-40 per minute |
Figure 1-2. Electrical conduction system of the heart.
c. The Pacemaker Site. The common pacemaker site is the SA node because it initiates electrical impulses at a faster rate than the junction or ventricle. An impulse from the AV junction can take over if the SA node should fail. If the AV junction also fails, the ventricle can take over. This is a protective backup system, a system which helps the heart maintain electrical efficiency. Sometimes the junction or the ventricle becomes irritable and starts impulses at a faster than normal rate which overrides the SA node. When this happens, the pacemaker site that is the fastest dominates and takes over control of the heart.
The autonomic nervous system (ANS) regulates internal organ activities, usually involuntarily and automatically. This system regulates sweating, alters the size of the pupils, and directs many other body adjustments. The ANS influences the heart rate and myocardial contractility (the ability of cardiac muscle cells or tissues to shorten when stimulated) by sympathetic and/or parasympathetic stimulation. Sympathetic impulses increase heart activity; parasympathetic impulses decrease heart activity. Both of these systems must be in balance for the heart to function properly. If one or the other system is stimulated abnormally or blocked, the result will be heart arrhythmias. See figure 1-3 for autonomic nervous system control of the heart.
Figure 1-3. Innervation of the heart.
1-5. GRAPHIC DISPLAY OF ELECTROCARDIOGRAM
a. Heart Electrical Forces. During the cardiac cycle (one contraction of the heart plus the relaxation period that follows), electrical changes take place in the heart. These changes can be visualized and recorded.
(1) Detection of electrical forces in the heart. Electrical forces in the heart can be detected on the body's surface. Therefore, electrodes attached to the patient's skin can detect electrical forces in the heart.
(2) Recording of electrical forces in the heart. The recording of the electrical changes during the cardiac cycle is called an electrocardiogram (ECG or EKG). The instrument used to record these changes is an electrocardiograph.
b. Electrocardiogram Graph Paper and Machines. Electrocardiogram graph paper and the speed of the EKG machines are standard and uniform. Lines on the graph paper are horizontal and vertical with four light lines between two heavy lines. The horizontal lines indicate voltage. The electrical voltage of the heart impulse is measured in millivolts and determined by the magnitude of deflection (the power of a wave).
(1) Determination of electrical impulse strength. Compare the height of a wave spike to the horizontal lines to determine the strength of the electrical impulse.
(2) Vertical lines. Vertical lines indicate the speed of the electrical current traveling within the heart. The distance in time between two heavy vertical lines is 0.20 seconds and between two light vertical lines or across one small square is 0.04 seconds.
(3) Heavy lines. Heavy lines are necessary to determine rates, rules, and normal values. Light lines are composed of five small columns between two heavy vertical lines.
(4) Squares. There are 25 smaller squares in each large square.
(5) Standard rate of EKG paper. The standard rate of EKG paper travels past the stylus at a rate of millimeters per second.
(6) Graph paper markings. The markings on the graph paper can be examined and compared to normal markings to give the reader an idea of the electrical activity of the patient's heart. See figure 1-4 for an example of standard EKG graph paper measurements.
Figure 1-4. Standard graph paper measurements.
A Graph paper. B Enlargement of one square of graph paper.
c. Electrical Impulses of Electrocardiogram Waves. Each part of the cardiac cycle produces a different electrical impulse. The electrical flow of the heart starts with the SA node (right atrium) and continues to the Purkinje fibers (ventricles). Electrodes transmit impulses to a recording pen which graphs the impulses in a series of up and down waves called deflection waves. The cardiac cycle includes all of the wave patterns produced by electrical activity beginning with the pacemaker impulses and including ventricular repolarization. An isoelectric line occurs when there is no current strong enough to produce either a positive or negative deflection. The positive and negative forces are equal with the result that a flat line is shown (usually following the "T" wave). An electrical force (from the heart) toward the positive electrode will draw the stylus in an upright wave while an electrical force toward the negative electrode will draw the stylus in a downward wave. A single cardiac cycle is expected to produce one heartbeat. Deflections above or below the isoelectric line are called waves. Each wave is labeled with a letter. The waves are called the P wave, QRS complex, and the T wave. The letters were arbitrarily selected and do not stand for any words.
(1) P wave. A small upward (positive) wave that indicates atrial polarization (the spread of an impulse from the SA node through the muscle of the two atria). The atria contract a fraction of a second after the P wave begins.
(2) QRS wave (complex). This second wave begins as a downward deflection and continues as a large, upright, triangular wave which finally ends as a downward wave at its base. This wave complex shows the spread of the electrical impulse through the ventricles.
(3) T wave. The third wave shows ventricular repolarization.
NOTE: There is no deflection to show atrial repolarization because the stronger QRS wave masks this event.
NOTE: Figure 1-5 shows EKG wave patterns produced by the electrical activity of the heart.
Figure 1-5. Electrocardiogram wave patterns produced by electrical activity in the heart.
d. Size and Time Intervals of EKG Waves. The size of the deflection waves and particular time intervals are important when you are reading an electrocardiogram. For example, the duration of a normal "P" wave is between 0.06 and 0.1 seconds, the time it takes for depolarization current to pass through the atrial musculature. An increased width of "P" wave may indicate left atrial abnormality or right atrial hypertrophy (enlargement). The deflection of a normal "P" wave is small due to the thin walled structure of the atria. A "P" wave is usually no more than 3 mm high. A taller "P" wave may indicate that atrial enlargement has occurred due to hypertension, coronary pulmonade, or congenital heart disease.
(1) P-R interval. Measured from the beginning of the P wave to the beginning of the R wave, this wave pattern represents the conduction time from the beginning of atrial excitation to the beginning of ventricular excitation. This is the time it takes for an electrical impulse to travel through the atria and atrioventricular node to the remaining conducting tissues. A medical condition that disrupts this electrical impulse will display itself as a P-R interval that is longer than 0.2 seconds due to the increased time it takes to travel the conducting tissues. The normal P-R interval is between 0.12 and 0.20 seconds.
(2) Q wave. The Q wave is defined as the first down (negative) deflection following the P wave but coming before the R wave.
(3) QRS complex. This complex is made up of three waves: the Q, the R, and the S waves. The QRS complex represents ventricular depolarization. A normal QRS measurement is less than 0.12 seconds. The QRS complex is larger than the P wave on an EKG because ventricular depolarization involves a greater muscle mass than atrial depolarization.
(4) R wave. The R wave is the first upward (positive) deflection that follows the Q wave.
(5) S-T segment. Beginning at the end of the S wave ending at the beginning of the T wave, this wave represents the time between the end of the spread of the heart's electrical impulse through the ventricles and repolarization of the ventricles. When a patient has acute myocardial infarction, the S-T segment is elevated. When the heart muscle does not receive enough oxygen, the S-T segment is depressed.
(6) T wave. Representing repolarization of the ventricular cells, the T wave is flat when the heart muscle does not receive enough oxygen; for example, in atherosclerotic heart disease. When the body's potassium level is increased, the T wave may be elevated. This wave occurs after the QRS complex.
NOTE: Refer to figure 1-6 to see these waves.
Figure 1-6. EKG wave, segment, and internal definitions.
(7) The refractory period. During this period, cell charges are depolarized and have not returned to their polarized state. A cell that is electrically "refractory" cannot receive another impulse until it is repolarized. The refractory period on an EKG includes the QRS complex and the T wave. The absolute refractory period includes the QRS and the upslope of the T wave and is NOT a dangerous period. The relative refractory period may allow depolarization of ventricles. This period occurs on the downslope of the T wave; it is dangerous if an impulse occurs at this time.
e. Electrocardiogram Uses. The EKG has a variety of uses; for example, abnormal cardiac rhythms and conduction patterns and following the course of recovery from a heart attack. Some people carry a Holter monitor to monitor heart electrical activity. This machine can be carried around by the patient while he goes about his everyday routines. The Holter monitor is especially useful in detecting rhythm disorders in the conduction system. It is also useful in correlating rhythm disorders and symptoms and then following the effectiveness of drugs in dealing with these disorders.