ECG for myocardial infarction.

ECG in myocardial infarction has high diagnostic value. Despite this, its informativeness is not 100%.

In emergency and terminal conditions, standard lead II is usually used for assessment, which allows for better differentiation of a number of quantitative indicators (for example, differentiation of small-wave ventricular fibrillation from asystole).

Diagnostically significant changes in the electrocardiogram in acute coronary syndrome may appear much later than the first clinical manifestations of anginal status. For timely detection of diagnostically significant changes, an ECG should be taken in myocardial infarction as early as possible and repeated recordings should be made, especially if the patient has renewed anginal attacks. Registration should be made in 12 leads without fail. If necessary, additional leads should be used (V3R and V4R, along the posterior axillary and scapular lines (V7-V9), in the IV intercostal space, etc.).

In some cases, a comparison with an electrocardiogram recorded before the onset of an existing angina attack can help with diagnosis.

ST segment elevations may be observed not only in myocardial infarction, but also in early repolarization syndrome, complete left bundle branch block, extensive cicatricial changes in the myocardium, chronic left ventricular aneurysm, pericarditis, and other conditions. Therefore, the diagnosis of different variants of acute coronary syndrome should be based on a combination of signs and correlated with the clinical picture of the disease.

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ST segment and T wave morphology are normal

Since the main criteria for the approach to choosing treatment tactics for acute coronary syndromes are changes in the ST segment, one should have a good understanding of the morphology of the ST segment and T wave in normal and pathological conditions.

The ST segment is the section of the electrocardiogram between the end of the QRS complex and the beginning of the T wave. It corresponds to the period of the cardiac cycle when both ventricles are completely covered by excitation.

In the limb leads, the ST segment is located on the isoline (the isoline is the interval between the end of the T wave and the beginning of the P wave of the next cardiac cycle) with small fluctuations within ±0.5 mm. Rarely, in standard lead III, the decrease in the ST segment may exceed 0.5 mm in healthy people, especially if the subsequent T wave is of low amplitude or absent. In chest leads VI-V3, an ST elevation of no more than 3.5 mm is permissible, and the ST segment has a "downward arc" shape. In healthy people, such an ST segment elevation is usually combined with a deep S wave and a high positive T wave. In chest leads V4-V5-V6, a slight ST depression of no more than 0.5 mm is permissible.

Five variants of ST segment displacement below the isoline are described: “horizontal”, “obliquely descending”, “obliquely ascending”, “trough-shaped”, and “upward-arched” ST segment depression.

In typical cases, myocardial ischemia is manifested on the electrocardiogram by ST segment depression. In ischemic heart disease, ST segment depression is most often characterized as "horizontal", "slanting" or "trough-shaped". There is a well-founded opinion that it is the horizontal displacement of the ST segment that is most pathognomonic for ischemic heart disease. As a rule, the degree of ST segment depression usually corresponds to the severity of coronary insufficiency and the severity of ischemia. The greater it is, the more serious the myocardial damage. ST segment depression> 1 mm indicates myocardial ischemia, and more than 2 mm - myocardial damage or necrosis. However, this criterion is not absolutely reliable. The depth of ST segment depression in any leads depends not only on the degree of coronary insufficiency, but also on the size of the R wave, and can also vary on the respiratory rate and heart rate. ST depression of more than 1 mm at the point and in 2 leads of the electrocardiogram or more is diagnostically significant. Downsloping ST depression is less typical for patients with coronary heart disease. It is also often observed in ventricular hypertrophy, bundle branch block, in patients taking digoxin, etc.

For ST segment assessment, not only the fact of ST segment displacement is important, but also its duration in time. In patients with uncomplicated angina, ST segment displacement is transient and is observed only during an attack of angina. Registration of ST segment depression for a longer period of time requires exclusion of subendocardial myocardial infarction.

ECG in myocardial infarction shows that acute damage or myocardial infarction can lead not only to ST depression, but also to displacement of the ST segment upward from the isoline. The ST segment arc in most cases has a convex shape in the direction of displacement. Such changes in the ST segment are observed in individual ECG leads, which reflects the focal nature of the process. Dynamic changes in the ECG are characteristic of acute damage and myocardial infarction.

The T wave corresponds to the period of ventricular repolarization (i.e. the processes of excitation termination in the ventricles). In this regard, the shape and amplitude of the normal T wave are quite variable. The normal T wave:

• should be positive in leads I, II, AVF;
• the amplitude in lead I should exceed the amplitude in lead III;
• the amplitude in the leads reinforced from the limbs is 3-6 mm;
• duration 0.1-0.25 s;
• may be negative in lead VI;
• amplitude V4 > V3 > V2 > VI;
• T waves should be concordant with the QRS complex, that is, directed in the same direction as the R wave

Normally, the ST segment smoothly transitions into the T wave, due to which the end of the ST segment and the beginning of the T wave are practically not differentiated. One of the first changes in the ST segment during myocardial ischemia is the flattening of its terminal part, as a result of which the border between the ST segment and the beginning of the wave becomes clearer.

T-wave changes are less specific and less sensitive than ST segment deviation for diagnosing coronary perfusion failure. T-wave inversion may be present in the absence of ischemia as a normal variant or due to other cardiac or extracardiac causes. Conversely, T-wave inversion is sometimes absent in the presence of ischemia.

Therefore, the analysis of the ST segment and T wave morphology is carried out in combination with the assessment of all ECG elements, as well as the clinical picture of the disease. In various pathological conditions, the ST segment can move both downwards and upwards from the isoline.

ECG in myocardial infarction, ischemia, injury and necrosis

Electrocardiography can diagnose myocardial infarction in approximately 90-95% of cases, as well as determine its location, size and duration. This is possible due to disturbances in the functional currents in the myocardium during infarction (changes in the potentials of the electrical field of the heart), since the necrotically altered myocardium is electrically passive.

ECG in myocardial infarction distinguishes three zones: ischemia, damage and necrosis. In the myocardium, around the necrosis zone, there is a transmural damage zone, which, in turn, is surrounded by a transmural ischemia zone.

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ECG in myocardial ischemia

The ischemic zone is manifested on the electrocardiogram by a change in the T wave (the QRS complex and ST segment have a normal appearance). The T wave in ischemia is usually equilateral and symmetrical, both its knees are equal in size, the apex is pointed and equally distant from the beginning and end of T. The width of the wave is usually increased due to slow repolarization in the ischemic zone. Depending on the location of the ischemic area in relation to the electrocardiographic leads, the T wave can be:

1. negative symmetrical (with transmural ischemia under the differential electrode or with subepicardial ischemia under the active electrode);
2. high positive symmetrical pointed “coronary” (with subendocardial ischemia under the active electrode or with transmural ischemia on the wall opposite the electrode);
3. reduced, smoothed, biphasic (when the active electrode is located on the periphery of the ischemic zone).

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ECG in myocardial damage

Electrocardiographically, myocardial damage is manifested by ST segment displacements. Depending on the location of the damaged area in relation to the active electrode and its localization, various ST segment changes can be observed. Thus, in case of transmural damage, an ST segment elevation above the isoline with an arc facing upward is observed under the electrode. In case of transmural damage located on the wall opposite the electrode, a decrease in the ST segment below the isoline with an arc facing downward is observed. In case of subepicardial damage, under the electrode, the ST segment is located above the isoline with an arc facing upward, and in case of subendocardial damage, under the electrode, it is below the isoline with an arc facing downward.

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ECG for myocardial necrosis

Myocardial necrosis on the electrocardiogram is manifested by changes in the QRS complex, the shape of which will depend on the location of the electrode to the necrosis zone and its size. Thus, in transmural myocardial infarction, QS waves with a width of 0.04 s or more are noted under the electrode. In the area opposite to the necrosis, reciprocal changes are recorded in the form of an increased amplitude of the R waves. In non-transmural infarction, QR or Qr waves are observed on the electrocardiogram. The amplitude and width of the Q wave, as a rule, reflect the depth of the lesion.

ECG for myocardial infarction identifies myocardial infarctions of the following duration:

1. Myocardial infarction up to 3 days old (acute, fresh). Characterized by an elevation of the ST segment above the isoline in the form of a monophasic curve, when the ST segment merges with the positive T wave (in the presence or absence of a pathological Q wave).
2. Myocardial infarction up to 2-3 weeks old. Characterized by the rise of the ST segment above the isoline, the presence of a negative symmetrical T wave and a pathological Q wave.
3. Myocardial infarction more than 3 weeks old. Characterized by the location of the ST segment on the isoline, the presence of a deep negative symmetrical T wave and a pathological Q wave.
4. Cicatricial changes after myocardial infarction. Characterized by the location of the ST segment on the isoline, the presence of a positive, smoothed or slightly negative T wave and a pathological Q wave.

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ECG in myocardial infarction with ST segment elevation

A characteristic sign of myocardial infarction with ST segment elevation is an arcuate ST segment elevation in the form of a monophasic curve, so that the descending knee of the R wave does not reach the isoelectric line. The magnitude of the ST elevation is more than 0.2 mV in leads V2-V3 or more than 0.1 mV in other leads. This elevation should be observed in two or more consecutive leads. The monophasic curve persists for several hours. Then the electrocardiographic picture changes according to the stage of development of the process.

Several hours or days after the onset of the disease, pathological Q waves appear on the electrocardiogram, the amplitude of R waves decreases, or a QS-form of the ventricular complex occurs, which is due to the formation of myocardial necrosis. These changes allow us to diagnose a large-focal or Q-forming myocardial infarction.

Approximately by the beginning of the second day, a negative coronary T wave appears, and the ST segment begins to gradually descend to the isoline. At the end of the 3rd-5th day, the depth of the negative wave can decrease, on the 8th-12th day, the second inversion of the T wave occurs - it deepens again.

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ECG in non-ST-segment elevation myocardial infarction

In acute coronary syndrome without ST segment elevation, the electrocardiogram may show:

• absence of electrocardiographic changes;
• ST segment depression (diagnostic significant displacement of more than 1 mm in two or more adjacent leads);
• T wave inversion (more than 1 mm in R wave-dominant leads).

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