In some situations, conventional electrocardiography is the technique with the best diagnostic accuracy. In other words, it enables us to define criteria that make it possible to determine if a certain disease process is present or absent better than any other diagnostic method (Fisch 1989; Schlant 1991).

As a whole, conventional electrocardiography is considered a reference, or “gold standard,” method for the diagnosis of atrial and ventricular conduction abnormalities, ventricular pre‐excitation, most cardiac arrhythmias, and acute myocardial infarction. In some situations, other electrocardiological techniques may be needed for a reliable diagnosis. For example, in some tachycardias with a wide QRS, it may be difficult to distinguish between aberrancy and ventricular rhythm with a conventional surface ECG alone. The same limitation applies when locating an accessory pathway.

In other circumstances, such as atrial and ventricular enlargement, alterations due to chronic ischemic heart disease (ECG findings of ischemia, injury, or necrosis), or other repolarization disorders, conventional electrocardiography may yield valuable information for diagnosis. However, it has less potential than other imaging techniques (for example, echocardiography for detecting atrial and ventricular enlargement, etc.).

It is important to know the true value of ECG diagnostic criteria in diseases in which electrocardiography is not the “gold standard” diagnostic tool (i.e. diagnostic criteria of atrial and ventricular enlargement, necrosis in chronic phases, tachycardia with wide QRS, etc.). The specificity and sensitivity of electrocardiological techniques in the presence or absence of disease should be known. Thus, the concepts of sensitivity, specificity, and predictive value are indispensable for such evaluations.

The validity of the diagnostic criteria of a method or technique is determined by comparing the results obtained with the method being evaluated (in this case, surface ECG or other electrocardiological technique) with the results of a “gold standard” diagnostic test, a reference test (Ransohoff and Feinstein 1979; Willems et al. 1985). The reference test may be another electrocardiological technique (such as endocavitary techniques in the case of wide QRS) or a different type of test (echocardiography, hemodynamic evaluations, clinical findings such as the presentation of disease, etc.). It is only in the last few years that such concepts have been applied to ECG diagnosis criteria; prior to that, electrocardiographic diagnoses were often more intuitive than rational.

The information derived from both the diagnostic test under evaluation (presence or absence of a determined ECG criterion, or positivity or negativity result of an electrocardiological technique) and the reference test considered “gold standard” for diagnosis can be represented in a table with two rows (results of the test under evaluation) and two columns (distribution of the disease according to the reference test). This is referred to as a 2 × 2 table (Table 8.1) in which four subgroups of individuals can be defined:

1. true positives (TP): those patients with the disease/abnormality properly identified by the diagnostic method under evaluation;
   
2. false negatives (FN): those patients with the disease but not identified as such by the diagnostic method, thus classified as healthy;
   
3. false positives (FP): those individuals without the disease but identified as having the disease by the diagnostic method;
   
4. true negatives (TN): those individuals without the disease properly identified as healthy by the diagnostic method.

Using this type of table and these four subgroups, the sensitivity, specificity, and predictive values of the diagnostic test can be estimated (Diamond and Forrester 1979; Wulff 1980; Altman and Bland 1994a, 1994b).

100 Patients with valvular heart disease
		LAE assessed by echocardiography		Total
		Yes	No
100 patients with valvular heart disease	P± in II, III, VF	2	0	2
	without P± in II, III, VF	88	10	98
Total		90	10	100

^a An example to demonstrate whether the presence of an electrocardiographic criterion (in this case, a ±P wave in II, III, and aVF in patients with valvular heart disease) does or does not predict the presence of left atrial enlargement (LAE) as detected by echocardiography.

FN: false negative; FP: false positive; NPV: negative predictive value; PPV: positive predictive value; SE: sensitivity; SP: specificity; TN: true negative; TP: true positive.

Sensitivity

The sensitivity of an ECG criterion is defined as the percentage of individuals with a given abnormality who present that ECG criterion (i.e. the percentage of persons with left ventricular enlargement (LVE) who present an R ≥ 35 mm in V5). If all individuals with this abnormality (LVE) present the determined criterion, the sensitivity is 100%, in which case no false negative cases exist. Sensitivity is also defined as the probability that the result of a diagnostic test will be positive when the disease is present. Sensitivity is calculated using the following formula:

where TP is the number of the subjects with the disease or abnormality (in this case, LVE) who present the criterion; FN is the number of the subjects with the disease or abnormality who do not present the criterion and are classified as healthy.

Specificity

The specificity of a test (in this case, a diagnostic criterion obtained by surface ECG or another electrocardiological technique) is defined as the percentage of individuals without the abnormality/disease who do not present the criterion. Therefore, the smaller the number of healthy individuals who present the criterion, the more specific it is. For example, in evaluating the criterion R wave ≥35 mm in V5 for the diagnosis of LVE, the specificity is 90% if the criterion is found in only 10 out of 100 healthy individuals. If no healthy individual presents the criterion, the specificity is 100%, no false positive cases exist. Specificity is also defined as the probability that the result of a diagnostic test will be negative when the disease is not present. Specificity is calculated using the following formula:

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