Basic Electrocardiographic Techniques

The 12-Lead ECG

Although there is variability depending on the workplace, most ECGs in use today are three-channel recorders with computer memory. Such multichannel systems, which record electrical events in several leads concurrently, offer advantages over the antiquated single-channel recorder systems as follows: capturing transient events on multiple leads simultaneously; banking the data in computer memory for storage, comparison, and transmission; and allowing presentation of data on a single sheet of paper. The electrocardiographic tracing is printed in a standardized manner on standardized paper by the electrocardiograph, which has default settings regarding the speed at which the paper moves through the machine, in addition to the amplitude of the deflections to be made on the tracing ( Fig. 14.1 A ) . Electrocardiographic paper is divided into a grid with a series of horizontal and vertical lines; the thin lines are 1 mm apart, and the thick lines are separated by 5 mm. At the standard paper speed of 25 mm/sec, each vertical thin line thus represents 0.04 second (or 40 msec), and the thick vertical lines correspond to 0.20 second (or 200 msec). Recordings from each of the 12 leads are typically displayed for 2.5 seconds by default setting; the leads appearing horizontally adjacent to each other are separated by a small vertical hash mark to represent lead change.

The standard ECG includes 12 leads derived from 10 electrodes placed on the patient; each is color-coded and represented by a two-character abbreviation ( Table 14.1 ; see also Fig. 14.1 B ). Distinction is made between the terms lead and electrode with the former referring to the electrical view or perspective and the latter the actual wire attached to the patient. Placement of limb leads includes on the left and right arms (LA and RA, respectively) and on the left and right legs (LL and RL, respectively). Caution is advised as to the correct anatomic placement of the leads; incorrect lead placement produces electrocardiographic findings which can mimic illness with resultant unnecessary clinical interventions and further diagnostic testing; please refer to the section on Electrode Misplacement and Misconnection later in this chapter for a more complete discussion of this important issue.

Standard 12 Leads

The standard 12-lead ECG depicts cardiac electrical activity from 12 points of view, or leads, that can be grouped according to planar orientation. Six leads (I, II, III, aVR, aVL, and aVF) are oriented in the frontal, or coronal, plane, and are derived from the four limb electrodes. The six precordial leads (V ₁ , V ₂ , V ₃ , V ₄ , V ₅ , and V ₆ ) are oriented in the horizontal, or transverse, plane, with each representing cardiac electrical activity from that perspective. Leads I, II, and III are termed limb leads and are bipolar in that they record the potential difference between two electrodes ( Fig. 14.2 ). The fourth electrode located on the right leg serves as an electrical ground. The positive poles of these bipolar leads lie to the left and inferior, a position approximating the major vector forces of the normal heart. This early convention was established so that the tracing would feature primarily upright complexes. In contrast, augmented leads aVR, aVL, and aVF are unipolar leads with the positive electrode located at the respective extremities. These augmented leads serve to fill the “electrical gaps” between leads I, II, and III. Lead aVR stands alone with a polarity and resultant orientation opposite that of the other limb and augmented leads because of the fact that its positive electrode is located in the opposite direction (superior and to the right) of the major vector force of the normal heart (inferior and to the left); thus its complexes usually appear “opposite” those in most or all of the other leads.

Merging of the vector axes of the limb and augmented leads around a central axis yields a hexaxial system representing cardiac electrical activity in the frontal plane ( Fig. 14.3 ). The six precordial leads, oriented in the horizontal plane, represent six unipolar electrodes with vector positivity oriented toward the chest surface and the central terminal of the hexaxial system serving as a negative pole. In contrast to the frontal-plane leads, the angles between each of the precordial leads in the horizontal plane are not equal. They can vary depending on electrode placement and body habitus.

Electrode Placement

The four limb electrodes are conventionally placed on the extremities as follows: RA on the right wrist, LA on the left wrist, RL on the right ankle, and LL on the left ankle. Electrodes may be affixed more proximally on the limbs if necessary (e.g., amputation, severe injuries), ideally with a notation made on the ECG. Others note that the electrodes may be placed on any part of the arms or legs provided that they are distal to the shoulders or inguinal/gluteal folds, respectively.

Mason-Likar electrode placement is commonly used by hospital staff and paramedics; this approach does not alter precordial electrode placement but instead moves the limb electrodes to the torso. Originally described in 1966, the Mason-Likar configuration differs from standard electrode placement in that the arm electrodes are relocated to the infraclavicular fossae (medial to the borders of the deltoid muscles and 2 cm below the clavicles), and the leg electrodes are positioned along the anterior axillary lines (halfway between the costal margins and the iliac crests). Actual torso positioning may differ in practice because of individual variation or an attempt to simulate limb electrode placement. A rightward frontal-plane access shift has been described when torso electrode placement is used for the limb electrodes instead of standard positioning on the extremities. Mason-Likar positioning has also been associated with diminution of inferior Q waves, thus making detection of inferior MI more difficult. An alternative electrode configuration is the Lund system, in which the arm electrodes are placed laterally on the left and right arms at the level of the axillary folds, and the leg electrodes are positioned laterally on the greater femoral trochanters. The Lund system has been found to more directly approximate the electrocardiographic recordings obtained with conventional positioning than the Mason-Likar configuration does.

The precordial electrodes should be placed as follows: V ₁ —right sternal border, fourth intercostal space; V ₂ —left sternal border, fourth intercostal space; V ₃ —midway between V ₂ and V ₄ ; V ₄ —left midclavicular line, fifth intercostal space; V ₅ —left anterior axillary line, same horizontal level as V ₄ ; and V ₆ —left midaxillary line, same horizontal level as V ₄ and V ₅ ( Fig. 14.4 ). Note that V ₄ to V ₆ are placed at the same horizontal level, not all in the fifth intercostal space. If V ₅ and V ₆ are situated so that they follow the contour of the intercostal space rather than being on the same horizontal level, they will be superiorly displaced as the ribs curve around the side of the thorax. Minor changes in the position of the precordial leads will alter the ECG tracing, so it is important to keep the adhesive leads in place throughout the ED stay so that lead placement is identical during serial ECG comparisons.

Intercostal space number can be determined by first palpating the sternal angle (angle of Louis), which is the junction of the manubrium and body of the sternum (see Fig. 14.4 ). This transverse bony ridge is located about 5 cm caudal from the sternal notch in adults. Immediately lateral and inferior to it is the second intercostal space; two spaces farther down lies the fourth intercostal space, where V ₁ and V ₂ should be placed. Alternatively, one can count down from the medial aspect of the clavicle; beneath the clavicle lies the first rib, below which is the first intercostal space. The precordial electrodes should not be simply “eyeballed” by the technician because as little as 1 to 2 cm of electrode displacement can result in significant morphologic alteration in the precordial QRS complexes.

If the patient's body habitus or pathologic process precludes placement of a precordial electrode as just described, it is permissible to attach it within the radius of the width of one interspace of the recommended position, with appropriate notation on the tracing. If the situation demands further displacement, it is recommended that the lead be omitted, with appropriate documentation on the tracing.

Preformatted lead placement is used by some clinicians. For instance, the use of disposable, prewired electrodes in a manner similar to Mason-Likar positioning allows for more rapid placement, 20% faster compared to standard lead placement. The quality of the resultant ECG tracing is excellent, with lower rates of artifact and minimal impact on standard electrocardiographic structures. These preformatted lead placement systems are more expensive, however, as compared to standard lead placement wiring.

Pediatric Electrode Placement

In addition to the standard 12-lead tracing, leads V ₄ R and V ₃ R should also be recorded; these are mirror images of their left-sided counterparts (see section on Additional Leads later in this chapter). The chest of a tiny infant may not accommodate all the precordial electrodes; in such cases the following array is recommended: V ₃ R or V ₄ R, V ₁ , V ₃ , and V ₆ . Limb electrode placement is the same as in adults.

Information Provided by the Computer

In addition to the patient demographic data entered by the operator, the tracing will often feature computations regarding rate, intervals, and axes along the top of the paper. On some tracings, a computer-generated interpretation, or “reading,” will also be displayed at the top of the tracing. These interpretations are not infallible. A sample of nine of these programs was compared with the readings of eight cardiologists; the gold standard in this study was clinical diagnosis made independently of the interpretations of these tracings based on other objective data (e.g., echocardiography, cardiac catheterization). The performance of the programs was good, with correct interpretations in a median of 91% of cases, but the cardiologists were significantly better (median of 96% correct). The computer programs demonstrated a median sensitivity for anterior and inferior MI of only 77% and 59%, respectively. Of note, this study did not evaluate interpretation of acute ischemia and cardiac rhythm disturbance, other critical issues in electrocardiographic interpretation. Others have found both the computer programs and clinicians to be lacking in their ability to exclude cardiac disease with the ECG, with a negative predictive value for each of between 80% and 85%. When diagnosing atrial fibrillation, both general practitioners (sensitivity, 80%; specificity, 83%) and computer software (sensitivity, 83%; specificity, 99%) are flawed; when combined, diagnostic accuracy improves but is still imperfect (sensitivity, 92%; specificity, 91%).

It is worthwhile to read and consider the computer reading of the ECG, but the emergency clinician should not be beholden to it. The properly trained and experienced clinician provides more accurate electrocardiographic interpretation, as compared with the machine's computer algorithm.