1-16
1-16. ELECTROCARDIOGRAM
a. The electrocardiogram (ECG or EKG) is a graphic recording of the electrical impulses produced in association with the heartbeat. Impulse formation and conduction produce weak electrical currents that spread throughout the entire body. By connecting certain points on the body to a recording instrument, these currents can be recorded as a graphic representation of the heartbeat, measured against time. Time is expressed on the special ECG graph paper by vertical and horizontal lines.
b. Normally, each heartbeat is represented as five major waves: P, Q, R, S, and T. The Q,R, and S waves all represent the same portion of the heart and are referred to as a unit: the QRS complex.
(1) The P wave represents atrial depolarization. The QRS complex represents ventricular depolarization.
(2) The QRS complex represents the impulse traveling through the ventricles, at which time there is no heart contraction.
(3) The T wave is produced by electrical recovery of the ventricles, at which time there is no heart contraction it represents ventricular repolarization.
c. The standard ECG machine utilizes 12 "leads." These leads represent paths of electrical activity and are designated as leads I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5, and V6. It is neither practical nor necessary to go into an explanation of leads in this text. To do so would require extensive explanation of electrophysiological principles. It will suffice to say that each lead senses and records the electrical impulses from different positions related to the heart's surface. Since each lead takes a different view of heart activity, it generates its own characteristic tracing. Wave abnormalities that appear in the different leads indicate damage or defects in particular portions of the heart muscle.
d. The ECG provides quite a bit of valuable information for the small amount of effort involved in obtaining an ECG recording. It is a procedure that is completely noninvasive and without risk to the patient. It is easily performed by anyone with the proper training. The ECG provides information about the heart rate, rhythm, the condition of the myocardium, the presence of ischemia or necrosis, conduction abnormalities, the presence of certain drugs, and the effects of disturbed electrolytes.
e. Because it does provide so much valuable information, it is important that the procedure be performed correctly. Correct procedure will vary depending upon the type of equipment used in your facility. Be sure to read the local standard operating procedures (SOP) and the manufacturer's instructions before attempting to
use the equipment. Another important factor in correct performance is proper placement of the electrodes. Electrodes should be secured over a fleshy area, not over a bone, as bone interferes with the electrical impulse readings. In order to obtain accurate readings the patient may need to be shaved using a prep razor if the area where the electrodes are to be placed. Location of the electrodes is standard for all designs of ECG equipment.
(1) Leg electrodes are best placed on the medial or lateral aspect of the calf to avoid contact with bone.
(2) Arm electrodes are best placed on the inner aspect of the arm or forearm, several inches above or below the antecubital space.
(3) Chest electrodes are placed as follows. Remember to place the electrodes over the intercostal spaces, not over the ribs.
(a) V1: 4th intercostal space at the right sternal border.
(b) V2: 4th intercostal space at the left sternal border.
(c) V3: Halfway between V2 and V4.
(d) V4: 5th intercostal space at the midclavicular line.
(e) V5: 5th intercostal space at the anterior axillary line.
(f) V6: 5th intercostal space at the midaxillary line.
