the Electrocardiogram
Duc H. Do
Noel G. Boyle
INTRODUCTION
The electrocardiogram (ECG) is one of the main diagnostic tools of the clinician, whether in the clinic, emergency department, or hospital. The first ECG machine was developed by William Einthoven in 1901 in Leiden, the Netherlands, for which he received the 1924 Nobel Prize in Medicine. Since then the ECG has undergone many improvements, including an increase in the number of leads recorded, electrical noise filtering, automated algorithms for ECG interpretation, and miniaturization of the necessary equipment. But it fundamentally performs the same task: recording the electrical activity of the heart.1
INDICATIONS
The ECG has many applications. A standard 12-lead ECG records a 10-second snapshot of the heart’s electrical activity. This is useful in diagnosing causes of chest pain, including myocardial infarction, causes of palpitation, including supraventricular and ventricular arrhythmias, causes of syncope, including tachyarrhythmias and bradyarrhythmias and other conduction abnormalities. Continuous ECG recording can be used to monitor a patient’s cardiac electrical activity in the hospital, a procedural suite (operating room, catheterization lab, etc.), or in the outpatient setting and to alert for the presence of ischemia, arrhythmias, and QT interval prolongation following the administration of certain medications.2 Stress ECGs utilize continuous ECG monitoring during treadmill exercise or simulated pharmacologic stress to detect signs of ischemia. Continuous ambulatory ECG monitoring can be used to record cardiac rhythm on a patient at home for periods between 24 hours and 1 month to diagnose causes of palpitations and/or syncope. ECG recording is also combined with other types of testing to correlate cardiac electrical activity and other organ functions, such as in sleep studies and electroencephalography. A new application of a body vest of 224 electrodes, called ECG imaging (ECGI), has also been in use in recent years predominantly in the evaluation and research of arrhythmias.
Use of the ECG can be broadly defined as for both diagnostic and screening purposes. Specific populations where screening ECGs have been advocated include elderly patients at risk of atrial fibrillation who may benefit from anticoagulation for stroke prevention, competitive athletes to evaluate for genetic abnormalities (ie, long QT syndrome, Brugada syndrome, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy), which may increase the risk of sudden cardiac death during exercise, and individuals in high-risk occupations such as military service and pilots to evaluate for the preceding conditions and other abnormalities such as Wolff-Parkinson-White (WPW) pattern. The positive predictive value of a test for various abnormalities of interest should be considered when recommending general screening requirements; a high false positive rate can lead to a significant amount of unnecessary medical testing. For example, normal variants and physiologic mimics of pathologic states (eg, physiologic ventricular hypertrophy from intense athletic training) can result in a high rate of false positive reading by inexperienced readers or if the clinical setting is unknown.3
ANATOMIC CONSIDERATIONS
The heart in young adults is generally more vertically oriented and gradually shifts leftward throughout life. Hence, a vertical axis of 90 to 100° is normal in young individuals (axis discussed in detail further on). Particularly in females with pendulous breasts, placement of the precordial leads in the standard locations may be difficult; it is generally recommended to place the electrodes immediately under the left breast line. Placement of the precordial leads outside of the standard locations or in patients with a deformed chest, orthotopic heart transplant, prior cardiac surgery, and prior pneumonectomy may result in changes in R wave progression (discussed further later) owing to changes in the heart position within the chest cavity.
FUNDAMENTALS OF ELECTROCARDIOGRAPHY
An ECG is recorded by attaching electrodes (flat paper stickers with adhesive) to the patient’s skin. Each electrode records electrical potentials from the heart’s depolarization and repolarization. In a standard 12-lead ECG, electrodes are placed on the patient’s right arm (RA), left arm (LA), and left leg (LL), and a ground electrode is generally placed on the right leg to reduce interference. Six precordial leads are placed across the patient’s chest. Three types of lead exist in modern-day ECG: (1) bipolar leads; (2) augmented leads; and (3) unipolar leads. Bipolar leads measure the difference in potential between two electrodes, whereas unipolar leads measure the electrical
activity directly beneath the electrode only. Augmented leads are calculated from bipolar leads.
activity directly beneath the electrode only. Augmented leads are calculated from bipolar leads.
The standard bipolar leads are leads I, II, III. These are calculated by subtracting the potentials between the (LA-RA), (LL-RA), and (LL-LA), respectively (Figure 49.1). Based on the “Einthoven triangle,” the third bipolar lead can always be calculated if the other two leads are recorded through the formula Lead I + Lead III = Lead II.
The augmented leads are then calculated from a combination of the bipolar leads using the following formulas:
 FIGURE 49.1 Einthoven triangle. Einthoven triangle is created by the three bipolar limb leads I, II, III, which are formed by three electrodes placed on the right arm (RA), left arm (LA), and left leg (LL). The augmented leads (aVR, aVL, and aVF) are calculated from leads I, II, and III. |
The bipolar limb leads and augmented leads together can be described as the “frontal plane” of the heart (Figure 49.2). The precordial leads are unipolar leads and are placed across the chest to record the horizontal plane of the heart (Figure 49.3). The most common configuration in a 12-lead ECG system is V1 to V6. In specific situations for diagnosing myocardial infarction, discussed in more detail further on, right-sided (V3R-V6R) or posterior precordial leads (V7-V9) can be placed (Table 49.1).
The 12 standard ECG leads can be further categorized by which part of the left ventricle it records potentials from, this being most useful for the purposes of diagnosing myocardial infarction (Table 49.2).
In some specific uses of ECG, such as continuous telemetry ECG monitoring in the hospital or ambulatory Holter monitoring, a more limited set of leads are typically used. A standard 5-electrode continuous ECG system in the hospital generally provides all 6 frontal leads and 1 precordial lead, generally in the V1 or V2 position. A standard 4-lead Holter monitoring system generally provides 3 limb bipolar leads.
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 FIGURE 49.3 The precordial leads (V1-V6) record the horizontal plane of the heart. |
In general, when electrical depolarization of the heart occurs in the direction of a particular lead, the deflection is positive (above the isoelectric baseline). If depolarization is away from the lead, the deflection is negative below the isoelectric baseline.
Standard ECG paper consists of large boxes and small boxes. Each large box is comprised of 5 × 5 small boxes, which are in turn 1 × 1 mm. Standard ECGs are printed at a speed of 25 mm/s (ie, 25 horizontal small boxes [5 large boxes] make up
1 second). Hence, each horizontal small box is equal to 40 ms, and each large box represents 200 ms. The vertical calibration (amplitude) is most commonly 10 mm/1 mV. Hence, each small vertical box equals 0.1 mV (Figure 49.4). In rare cases, such as in ECGs with very large QRS complexes, the calibration may be different (eg, 5 mm/mV, Figure 49.5). Amplitude interpretations need to be performed carefully with this different scale. Typically, a calibration box is recorded at the beginning of the ECG tracing to inform the reader of the paper speed (ie, mm/s) and amplitude (ie, mm/mv) calibration.
1 second). Hence, each horizontal small box is equal to 40 ms, and each large box represents 200 ms. The vertical calibration (amplitude) is most commonly 10 mm/1 mV. Hence, each small vertical box equals 0.1 mV (Figure 49.4). In rare cases, such as in ECGs with very large QRS complexes, the calibration may be different (eg, 5 mm/mV, Figure 49.5). Amplitude interpretations need to be performed carefully with this different scale. Typically, a calibration box is recorded at the beginning of the ECG tracing to inform the reader of the paper speed (ie, mm/s) and amplitude (ie, mm/mv) calibration.
TABLE 49.1 Precordial Leads | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 49.2 Lead Position Categorization (in Relation to Left Ventricle) | ||||||||||
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CLINICAL APPLICATIONS
P-QRS-T Waves
The heart normally depolarizes electrically first in the atria and then in the ventricle. This is reflected on the ECG by a lower amplitude P wave, reflecting atrial depolarization, followed by an isoelectric segment (PR segment) when electrical activity traverses the atrioventricular (AV) node and the His-Purkinje system within the heart, which does not generate sufficient depolarization to show up on the surface ECG. A taller amplitude QRS complex reflects ventricular depolarization. The Q wave component is the initial negative deflection, the R wave the first positive deflection, the S wave a negative deflection that occurs after an R wave. In cases where there is another positive deflection after the S wave, that deflection is termed R’ (read R-prime). The ST segment is the segment between the QRS and T wave that reflects ventricular repolarization (Figure 49.6). Occasionally, another low-amplitude wave is seen after the T wave, which is called the U wave. The origin of the U wave is widely debated and may represent repolarization of the Purkinje fibers.
A systematic approach should always be utilized to interpret an ECG. Although each clinician may choose a different approach, one should maintain a consistent approach so as not to miss important diagnoses. We generally approach ECG by assessing the following: rate; rhythm; axis; intervals; hypertrophy; ischemia; infarction; and other findings (eg, electrolyte or drug effects, clinical diagnosis patterns, lead misplacements, etc.)
Structured Approach to ECG Analysis Includes:
Rate
Rhythm
Axis
Intervals
Hypertrophy
Ischemia & infarction
Other findings: electrolyte or drug effects; clinical diagnosis patterns; lead misplacements
 FIGURE 49.4 A normal electrocardiogram showing normal sinus rhythm at approximately 75 beats per minute. Note the standard scales. Each small box on the paper is 1 × 1 mm. The standard paper speed is 25 mm/s (5 large horizontal boxes = 1 second; hence, each large horizontal box = 200 ms, each small horizontal box = 40 ms). The standard amplitude scale is 10 mm/mV (10 small boxes = 1 mV, each small box = 0.1 mV). The calibration for amplitude is seen by the rectangular wave at the far left. |
 FIGURE 49.5 Left ventricular hypertrophy. Note the amplitude scaling on this electrocardiogram (ECG) is 5 mm/mV, as seen by the calibration marker on the far left. This ECG shows sinus rhythm at a rate of approximately 90 bpm with left ventricular hypertrophy. Note the asymmetrically inverted T waves in the inferolateral leads, representing repolarization abnormalities attributable to left ventricular hypertrophy. |
 FIGURE 49.6 P-QRS-T complex. The P, QRS, T waves are marked in red. The PR interval is measured from the beginning of the P wave to the beginning of the QRS. The QRS interval (duration) is measured from the beginning to the end of the QRS complex. The QT interval is measured from the beginning of the QRS to the end of the T wave (where the line tangent to the downslope of the T wave crosses the isoelectric line). Normal ranges for each interval are displayed within parentheses. The ST segment is the normally isoelectric segment between the orange arrows. |
Rate
When the rhythm is regular, the ventricular rate can initially be roughly estimated by counting the number of large boxes between one QRS complex and the next (eg, if there is 1 large box between two QRS complexes, the rate is 300 beats per minute [bpm]; 2 large boxes: 150 bpm; 3 large boxes: 100 bpm; 4 large boxes: 75 bpm; 5 large boxes: 60 bpm; etc.) (Figure 49.5). When there is an irregular rhythm, the number of QRS complexes can be counted within the standard 10-second recording and multiplied by 6 (eg, if there are 10 QRS complexes across the 10-second recording strip, the heart rate is approximately 60 bpm). Rates <60 bpm are considered bradycardic, 60 to 100 bpm are normal, and >100 bpm are tachycardic. However, many normal healthy individuals have resting heart rates in the 45 to 60 range, or even lower.
While the atrial and ventricular rate is generally the same, this may not be the case in various arrhythmias. An atrial rate can be calculated separately from a ventricular rate using a similar manner, but using the P waves instead of the QRS deflections.
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