The 12 lead electrocardiogram or ECG is a valuable diagnostic tool for assisting in the diagnosis of cardiac disorders or acute injury events. Knowledge of lead configuration, proper skin preparation and placement are components to obtaining an accurate picture of cardiac activity.
An understanding of cardiac anatomy, the electrophysiology of the conduction system and basic rhythm interpretation knowledge will be helpful in understanding the 12 lead ECG tracing. This tutorial will focus on proper lead placement and various waveform representations.
While the first electrical current associated with a heart beat was identified in 1842 by an Italian physicist, it was a British scientist, Augustus Waller, who published the first human ECG in 1887. And, it was not until 1893 that Dutch scientist, Willem Einthoven, introduced the term "electrocardiogram" and subsequently refined the concept of cardiac electrical conduction, naming the deflections P, Q, R, S and T waves. Those same basic waves are used today to identify and interpret the ECG rhythm strip.1
Einthoven's concept was a triangle, an imaginary area on the body, formed by the intersection of the standard bipolar limb leads with the heart at the center. The bipolar leads have a negative and positive pole that captures the direction of the electrical activity of the heart in the various lead configurations. Looking at the heart from the various electrical vectors that are produced helps determine which way the impulse is traveling.
Standard Limb Leads
The hexaxial system of looking at the heart basically cuts the center of the heart in half into two planes, a front and back, combining the view from leads I, II and III and creating three additional augmented vector pictures in AVR, AVL and AVF. These three additional vectors complete the standard six limb leads with the notion that the positive electrode lies at the end of the lead name. If a camera is placed at the end of the positive electrode, looking toward the negative pole, the electrical impulse of the heart's activity passing under would create the ECG tracing. As the heart normally depolarizes from right to left, a positive impulse moving away from the camera would create a negative deflection of the QRS wave on the ECG tracing. While a positive impulse moving toward the camera would be recorded as a positive or upward wave on the tracing.2
The Precordial System
The six unipolar precordial leads are those that lie directly on the chest wall and in a theoretical plane perpendicular to the limb leads. Thus creating a transverse plane through the center of the heart cutting it into a top and bottom half. Combining all three planes together (the front, back, top and bottom) will now give a 3D picture of the heart, and the electrical activity recorded will be a reflection of the subsequent mechanical or muscular contraction with each cardiac beat or cycle. The six precordial leads are those named V1, V2, V3, V4, V5 and V6.
All leads give a view of the heart, which when combined together, transmit a wealth of diagnostic information used to identify physiologic or pathologic processes occurring with cardiac function. For identification of arrhythmias to hypertrophies and infarcts, the 12 lead ECG is an indispensable tool for use in daily healthcare practice.
Correct electrode and lead placement is important to receive an accurate rhythm strip tracing. Laying the patient supine or in a comfortable reclined position will allow easier access to the upper body. Most importantly, proper skin care is the first step to assure reliable results. Skin should be clean, dry and free of hair, oils or substances that may interfere with placement or conduction to the electrode. Hair should be clipped and lead placement should avoid implanted devices such as pacemakers or catheters that could impede signal transmission.
Avoiding boney prominences, open wounds and very muscular or hairy areas will provide a more accurate picture. While exact placement for limb leads can vary, correct precordial lead position is essential.
What the Leads Tell Us
The sum of the total 12 lead system gives a wealth of information to the practitioner during controlled or emergent situations, essentially directing treatment and interventions for care.
An important concept to remember is that the electrical activity of the heart precedes the mechanical function of the cardiac cycle. And, each wave is associated with a particular function of the cardiac cycle. Changes in the specific waves can indicate abnormalities in heart chamber size, electrolyte values, acute myocardial infarction or STEMI patterns and electrical conduction defects as a result of ischemia, clot or hypertrophic changes related to congestive failure.
Reviewing trends in wave form pattern changes can be important in determining treatment for acute coronary syndromes where ischemia often precedes injury. Follow the guidelines from the American Heart Association for prevention of cardiac syndromes and when determining treatment for acute injury events. Use the12 lead ECG as a tool in the armamentarium of options to prevent and treat cardiac emergencies.