Chapter 25
Limitations of the Conventional ECG: Utility of Other Techniques

Introduction

Throughout this book, we have discussed the value of the conventional surface ECG for the diagnosis of heart diseases in different situations. We have emphasized the need for a comprehensive approach to diagnosis, to always perform the interpretation of the surface ECG in light of the patient’s clinical setting. This approach is essential and sufficient in many cases, but in other circumstances, as demonstrated, it may not provide the necessary information for the correct diagnosis with the highest sensitivity, specificity, and overall predictive value. Furthermore, it is often not sufficient for risk stratification and for arriving at therapeutic decisions. For that, the use of other diagnostic techniques may be necessary. We will now make some comments about the importance of the clinical context for ECG interpretation, and later we will briefly describe the utility of other electrocardiologic techniques.

Interpretation of the surface ECG in light of the patient’s clinical setting

As discussed throughout the text, the following examples illustrate the need to use information related to the patient’s clinical setting for the interpretation of the ECG:

The importance of premature ventricular impulses is strongly related to the patient’s clinical characteristics (see Chapter 16).
The correlation of the voltage of ECG complexes with left ventricular enlargement (LVE) is related to the presence of anatomic LVE according to Bayés’ theorem and other characteristics (see Chapter 10).
The various patterns of bundle branch block have various prognostic and diagnostic implications according the clinical setting (Chapters 11 and 20).
For the differential diagnosis of a tachycardia with a wide QRS, the presence of underlying structural heart disease suggests the ventricular origin (see Chapter 16).
The prognosis for a ventricular tachycardia is related more to the patient’s clinical setting than the ECG morphology (see Chapter 16).
With precordial chest pain, a normal ECG (or an ECG with minimal repolarization changes) does not exclude an ischemic origin (see Chapters 13 and 20).
The importance of most repolarization changes (e.g. ST shifts/T wave inversion) should be evaluated in relation to the clinical setting. In the absence of underlying heart disease, they may be non‐specific or unimportant. However, it is necessary to first rule out their relation to ischemic heart disease or other diseases (see Chapter 13).

The abnormal ECG in the absence of heart disease and patients with normal ECG and advance heart disease (Bayés de Luna et al. 2020a)

Patient with advanced heart disease and normal ECG

The ECG can be normal even just before the patient experiences sudden death. This can occur in the following circumstances:

Inherited heart disease. This can occur in some cases of hypertrophic cardiomyopathy, arrhythmogenic right ventricular dysplasia, and non‐compaction cardiomyopathy. It is important to bear in mind that the morphology with r′ in V1 can be normal, and also the presence of negative T in V1–V2 is frequently normal if it is asymmetric, but is usually abnormal if it is symmetric (Nunes de Alencar Neto et al. 2018).
The Brugada Syndrome. In some occasions, the r′ in V1 may be considered normal. It is convenient is some cases to perform a flecainide test.
A accurate measurement of the QT interval must be made, especially in the case of a family history of short‐long QT syndrome.
Ischemic heart disease. An ECG may be normal just before sudden death from ventricular fibrillation or AV block appears in a patient with acute ischemic heart disease. Sometimes, the only ECG finding in a patient with ST‐elevation ACS is the appearance of a tall and symmetric pointed T wave, especially in the right precordial leads (V1–V2).
Other clinical changes that may accompany severe arrhythmias and sudden death that present a normal ECG are: a) pulmonary embolism; b) rupture of an aortic aneurysm; and c) many patients with heart disease may have a normal ECG or with small changes. This is less frequent in patients with valvular or congenital heart disease.

Patients without heart disease and with abnormal ECG
1. Due to incorrect ECG recording, consequence of bad placement of electrodes or inappropriate filters.
2. Presence of Q waves that could be pathological, but are not, such as the abnormal Q wave in lead III that disappears with deep inspiration.
3. Athletes can present altered repolarization without evident pathology.
4. The alteration of repolarization that can be seen in electrolytes.
5. The presence of the high voltage of R can be seen without LV enlargement in athletes and other normal patients, especially with vertical heart.
6. The presence of r′ can be seen in patients with heart disease, but also in normal people, especially if the electrode of V1 is located high.
7. Presence of atrial or ventricular arrhythmias that appear sporadically.

Additional value of other techniques

We will briefly discuss different techniques that may help us when the information derived from the clinical setting and conventional ECG are not sufficient to arrive at correct diagnosis, projecting the prognosis and/or developing a therapeutic approach in each case.

We will assume that the reader is familiar with the most important features of each technique. However, we will give now a brief discussion on the most widely used.

Unified interpretation of the ECG: computerized interpretation and the use of ECG classification systems

We made some comments about computerized ECG interpretation systems in Chapters 3 and 10. More information on that is beyond the scope of this book.

With regard to the ECG classification systems, the most widely known are the Minnesota Code (MC) (Figure 25.1) and the Nova code (Nova). Both are valuable and useful for clinical trials and epidemiological studies. For more information about computers and classification systems, please consult Macfarlane (2010).

Vectorcardiography (Figures 25.2 and 25.3)

Vectorcardiography (VCG) is a technique that records the cardiac electrical activity as closed loops: the atrial depolarization (P loop), ventricular depolarization (QRS loop), and ventricular repolarization (T loop) loops. VCG curves originate from X, Y, and Z leads (Figure 25.2A), which are three orthogonal leads that are perpendicular to each other. The X lead is right–left (similar to lead 1), Y is supero‐inferior (similar to aVF), and Z is postero‐anterior (similar to V2). These leads are generally recorded with the Frank system by means of different electrodes located on various points of the body. Inscription of the curves is done with a preamplifier system that magnifies the voltage of the currents coming from the heart through the lead systems, and a cathodic X‐ray tube by which the vectorcardiographic curve is seen.

VCG machines interrupt the current every 5 ms, 2.5 ms, or 1 ms using an oscillator; the VCG continuous curves are thus divided into tears or comma‐shaped fragments, the head of which represents the direction of the electrical current (Figure 25.2B). Figure 25.2C shows the ECG morphology that corresponds to the VCG of Figure 25.2B. We draw the loops as continuous lines when we use the VCG curves to aid in the comprehension of the ECG.

Schematic illustration of an ECG interpretation using the Minnesota Code: 1.1. 7; 2.1, frontal plane axis -30°, 5.1, repolarization disorder; negative T wave; 8.8, sinus bradycardia. — Figure 25.1 An ECG interpretation using the Minnesota Code: 1.1. 7 (sign of necrosis); 2.1, frontal plane axis −30°, 5.1, repolarization disorder; negative T wave; 8.8, sinus bradycardia. In this case, our interpretation would be: sinus bradycardia, old anteroseptal myocardial infarction with inferior extension, anterolateral subepicardial ischemia, and mild inferolateral subepicardial injury.

Schematic illustration of (A) Orthogonal leads with the corresponding vectorcardiographic (VCG) loop and its projection on the frontal, horizontal, and right sagittal planes. (B) Normal VCG corresponding to ECG of (C). — Figure 25.2 (A) Orthogonal leads with the corresponding vectorcardiographic (VCG) loop and its projection on the frontal, horizontal, and right sagittal planes. (B) Normal VCG corresponding to ECG of (C). At a sensitivity of 4, the P and T loops are poorly seen and the entire QRS loop is clearly visible (upper part of B). In the middle and lower panels of (B), the P and T loops with the respective onset and end of the QRS loop with amplified sensitivity may be seen.

Characteristics of different loops

P loop: The normal morphology, rotation, and orientation of the P loop are shown in Figure 25.3. This loop starts at point E and finishes at point O of the figure.
QRS loop: This loop begins at point O and terminates at point J, where the T loop begins. The normal morphology, orientation and rotation of this loop are shown in Figure 25.3. The intermediate part is inscribed more rapidly. Three zones can be distinguished: the Q loop (initial vectors), R loop (intermediate vectors), and S loop (terminal vectors). The QRS loop has a centrifugal or efferent part, and a centripetal or afferent part.
T loop: This loop extends from point J to point E. The centrifugal branch is inscribed more slowly. The loop is usually twice as long as it is wide. Figure 25.3 shows the normal morphology, orientation, and rotation of the T loop on three planes.

Usefulness of vectorcardiography

Vectorcardiography is useful both as a clinical and teaching tool, especially for training in electrocardiography. Electrocardiography should be integrated with VCG, as described in this text; one should be able to deduce ECG morphology from the VCG curve, and vice versa.

Table 25.1 shows the classical usefulness of VCG (Benchimol et al. 1972). However, currently the clinical utility of vectorcardiography has progressively decreased, and as we have explained in Chapter 3 with the correlation of the morphology of the loops with the hemifields and ECG curves, most of the advantages of VCG loops may be found in the surface ECG. In recent years, the possible usefulness of spatial QRS–T angle for risk stratification and some other utilities of VCG have been described. However, the most important contribution of VCG nowadays is its use, as we have done in this book, for teaching purposes (see Chapter 3).

Schematic illustration of the normal morphologies, rotation, and orientation of the P, QRS, and T loops in three planes: frontal (A), horizontal (B), and sagittal (C). — Figure 25.3 Normal morphologies, rotation, and orientation of the P, QRS, and T loops in three planes: frontal (A), horizontal (B), and sagittal (C). The QRS loop in the frontal plane may have clockwise rotation with an upward initiation (sometimes it forms a figure‐of‐eight) or, less often, counterclockwise rotation with an inferior onset. The latter is seen especially in obese subjects. EO = P loop; OJ = QRS loop; JE = T loop. The onset of the ST vector begins at point E, and the termination, at point J.

Table 25.1 Classical clinical usefulness of vectorcardiography (see text)

To better study atrial activity, as the P loop may be amplified
To confirm the presence of pre‐excitation when it is not clear on the surface ECG
For better evaluation of intraventricular conduction disorders and repolarization alterations
For improved diagnosis of certain types of necrosis and the association of necrosis and left bundle branch block
As a useful non‐invasive tool (continuous VCG recording) for detecting effective reperfusion after thrombolytic treatment

Exercise testing (Figures 25.4 and 25.5)

Exercise is considered isotonic or dynamic when several muscle groups alternately contract and relax, as in running, and isometric or static, when few muscle groups contract for more prolonged periods against a fixed force, as in weightlifting. Isotonic exercise such as that on a bicycle or treadmill is the most appropriate form of exercise to assess cardiac functional capacity.

Exercise testing represents more than the establishment of ECG changes during exercise. Hemodynamic or metabolic (O₂ consumption, quantity of exercise, changes in blood pressure, and heart rate, etc.) and clinica1 changes (presence of anginal pain, dyspnea, etc.) should also be evaluated.

Schematic illustration of the six examples of exercise ECG–thallium scintigraphy correlation. — Figure 25.4 Six examples of exercise ECG–thallium scintigraphy correlation (S = exercise image, R = redistribution image). (A) Positive exercise testing in an asymptomatic patient with negative thallium and normal coronary angiography. (B) Patient with exercise angina, with a negative exercise test and stress images showing a mild defect in the lower septum, with complete redistribution. (C) A patient with an inferolateral infarction with a positive exercise test and exercise thallium image showing an inferolateral defect without redistribution. (D) Patient with angina, with a positive exercise test from V2 to V6 and 1, 11, VL, and thallium images showing an inferolateral defect with complete redistribution. (E) Patient with anterior myocardial infarction, without significant change in exercise testing. The thallium images show the existence of an anteroseptal defect without redistribution and marked dilatation of the left ventricle. (F) Patient with inferolateral myocardial infarction, with a positive exercise test in the lateral leads. Thallium images show an inferoposterolateral defect with only lateral redistribution (positive for inferoposterior necrosis and lateral ischemia).

Methodology

The bicycle and treadmill are equally useful for exercise testing. There is no ideal protocol, but the Bruce protocol is the most widely used (Table 25.2). The following general principles should be taken into account: (i) The intensity of exercise should be increased gradually, not suddenly. Increments are generally made at a minimal interval of 3 minutes. (ii) Patients should be monitored for symptoms (precordial pain, etc.), ECG changes, and hemodynamic changes (blood pressure and heart rate) during the exercise and for at least 6–8 minutes after the test. (iii) Exercise should not be stopped abruptly. Exercise capacity is described using the product of heart rate and blood pressure, which is the so‐called double product.

A submaximal exercise test (85–90% of theoretical maximum heart rate for the patient’s age and sex) is adequate for clinical purposes and is much easier to perform for patients with ischemic heart disease. Metabolic equivalents (METS) (multiples of basal metabolic requirements) are used to express the work performed at different stages of the exercise test. In patients with ischemic heart disease, a workload of 8 METS is usually sufficient to evaluate angina. Healthy sedentary individuals do not usually exceed 10–11 METS, while athletes usually achieve more than 16 METS.

The exercise testing should be interrupted when: (i) significant symptoms or arrhythmias appear, (ii) significantly abnormal ST segment changes are detected, or (iii) the target heart rate is reached.

Image described in caption. — Figure 25.5 Diagnosis of ischemic heart disease by correlating clinical data (upper left) with exercise test results. Counterclockwise from the upper left, presence or absence of chest pain on the treadmill (lower left) (B); positive exercise ECG test with ST depression (ECG‐ST) (lower right) (C); or positive thallium imaging (upper right) (D). Shaded curves indicate the mean (± standard deviation) for 96 patients. Different shaded curves represent positive (+) or negative (−) results; non‐diagnostic ECG–ST results are shown by a question mark. For clinical data (upper left), the age and sex are shown on the vertical axis versus the probability of ischemic heart disease on the horizontal axis. Separate curves are shown according to the number of risk factors (0, 1–2, or 3–5) for asymptomatic patients (men and women). Symptomatic patients are classified as having no angina (NACP), atypical chest pain (ATCP), or typical angina pectoris (TAP). The post‐test probability of ischemic heart disease according to each test becomes the pretest probability of the next test in sequence, moving counterclockwise. The lines represent the range of probabilities of ischemic heart disease (IHD) for two patients. Patient 1 is a 45‐year‐old man with no typical symptoms but three risk factors, and patient 2 is 45‐year‐old man with chest pain typical of angina pectoris.

(Reproduced with permission Patterson *et al*. 1984).

Table 25.2 Bruce protocol for exercise (treadmill) ECG test

(Based on Benchimol et al. 1972).

Stage	Speed (mph)	Grade (%)	Duration (min)	METS (units)	Total time (min)
1	1.7	10	3	4	3
2	2.5	12	3	6–7	6
3	3.4	14	3	8–9	9
4	4.2	16	3	15–16	12
5	5.0	18	3	21	15
6	5.5	20	3	—	—
7	6.0	22	3	—	—

METS: metabolic equivalents.

Usefulness

Exercise testing is very useful in patients with ischemic heart disease to arrive at the diagnosis, to evaluate functional capacity, and to monitor the response to treatment. It can also be useful in other heart diseases and in the evaluation and assessment of cardiac arrhythmias. The most important indications and contraindications of exercise testing are listed in Tables 25.3 and 25.4.

The most important indication is to determine the presence of ischemia (positive test) in patients with dubious precordial pain and in post‐infarction patients to stratify prognosis. The combination of the appearance of anginal pain or other clinical or hemodynamic signs (Table 25.5) and electrocardiographic ST depression confirms the diagnosis. If it is only electrocardiographically positive, the diagnosis can only be suggested, although other tests (another exercise test with isotopic methods and on occasions, coronary angiography) are needed to clarify the problem and to exclude false positive results. The degree of abnormality of the exercise test is very important. If it is clearly positive (early and/or important ST depression, and/or precordial pain or hypotension, etc.), coronary angiography is recommended. If it is equivocal for ischemia (minor ST depression at the final stage of the Bruce protocol) without severe clinical findings (angina, hypotension), we recommended other techniques (isotopic or CV), magnetic resonance studies of perfusion, to assure the diagnosis of ischemia. If these tests are positive for ischemia, coronary angiography or multislice scanner is recommended.

Table 25.3 Main indications for exercise testing

Diagnostic indications	Doubtful precordial pain. Early detection of ischemic heart disease
Functional	Arrhythmias and exercise Prognosis in patients with ischemic heart disease (post‐myocardial infarction patients) Severity of ischemic heart disease Functional capacity of patients with heart disease Therapeutic effectiveness Behavior, with exercise, of known arrhythmias Level of physical training in asymptomatic patients and athletes
Other indications	Rehabilitation Research

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Limitations of the Conventional ECG