ECG for myocardial infarction
Last reviewed: 20.11.2021
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ECG with myocardial infarction has a high diagnostic value. Despite this, its informativeness is not absolute.
In urgent and terminal states, II standard leads are usually used for evaluation, which allows better differentiation of a number of quantitative indicators (for example, differentiation of small-scale ventricular fibrillation from asystole).
Diagnostically significant changes in the electrocardiogram in acute coronary syndrome may occur much later than the first clinical manifestations of anginal status. For timely detection of diagnostically significant changes, ECG should be removed with myocardial infarction as early as possible and repeated recording, especially if the patient resumes anginal attacks. Registration must be made in 12 leads. If necessary, additional leads (V3R and V4R, on the back axillary and scapular lines (V7-V9), in the IV intercostal space, etc.) should be used.
In some cases, the comparison with an electrocardiogram registered before the onset of an existing anginal attack can help in diagnosis.
Elevations of the ST segment can be observed not only in myocardial infarction, but also in the syndrome of early repolarization, complete blockade of the left leg of the bundle of the Hisnia, 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 symptoms and correlate with the clinical picture of the disease.
The morphology of the ST segment and the T wave is normal
Since the main criteria of the approach to the choice of therapeutic tactics of acute coronary syndromes are changes in the ST segment, it is necessary to represent the morphology of the ST segment and the T wave in norm and pathology well.
The segment ST is the segment 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 with excitation.
In limb leads, the ST segment is located on the contour (isolines - 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. Occasionally, in the III standard lead, a decrease in the ST segment may exceed 0.5 mm in healthy people, especially if the subsequent low-amplitude T wave is absent. In thoracic leads VI-V3, the elevation ST is allowed to be no more than 3.5 mm, while the segment ST has the form "arc down". In healthy people, such an elevation of the ST segment, as a rule, is combined with a deep S-tooth and a high positive T wave. In the thoracic leads of V4-V5-V6, a slight depression of ST of not more than 0.5 mm is permissible.
Five variants of ST segment displacements below the isoline are described: "horizontal", "skewed", "oblique", "trough" and ST segment depression "arch up".
In typical cases, myocardial ischemia manifests itself on an electrocardiogram with ST-segment depression. With ischemic heart disease, ST segment depression is more often characterized as "horizontal", "skewed" or "trough". There is a well-founded opinion that it is the horizontal displacement of the ST segment that is most pathognomonic for coronary heart disease. Typically, the degree of ST-segment depression usually corresponds to the severity of coronary insufficiency and the severity of ischemia. The more it is, the more serious the defeat of the myocardium. Depression of ST> 1 mm indicates myocardial ischemia, and more than 2 mm - about myocardial damage or necrosis. Nevertheless, this criterion is not absolutely reliable. The depth of depression of the ST segment 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 from the respiratory rate and the heart rate. Diagnostically significant depression of ST is more than 1 mm at the point and in 2 leads of the electrocardiogram and more. Skewed ST depression is less common in patients with ischemic heart disease. It is also often observed with hypertrophy of the ventricles, blockade of the bundle branch, in patients taking digoxin and the like.
To assess the ST segment, it is important not only the fact of the ST segment displacement, but also its duration in time. In patients with uncomplicated angina, the displacement of the ST segment is transient and is observed only during an attack of angina pectoris. Registration of ST segment depression for a longer time requires the exclusion of subendocardial myocardial infarction.
ECG in myocardial infarction suggests that acute damage or myocardial infarction can lead not only to ST depression, but also to the displacement of the ST segment up from the isoline. The arc of the segment ST in this case in most cases has the form of convexity in the direction of displacement. Such changes in the ST segment are observed in separate leads of the ECG, which reflects the foci of the process. For acute damage and myocardial infarction, dynamic ECG changes are characteristic.
The tine T corresponds to the period of repolarization of the ventricles (i.e., processes of cessation of excitation in the ventricles). In this regard, the shape and amplitude of the normal T wave are highly variable. Tine T in norm:
- must be positive in leads I, II, AVF;
- amplitude in the I lead should exceed the amplitude in the III lead;
- The amplitude in the leads that are amplified from the extremities is 3-6 mm;
- duration 0.1-0.25 s;
- may be negative in lead VI;
- the amplitude V4> V3> V2> VI;
- the T teeth must be concordant to the QRS complex, that is, pointing in the same direction as the tooth R
Normally, the ST segment smoothly passes into the T-wave, and therefore the end of the ST segment at the beginning of the T wave is practically not differentiated. One of the first changes in the ST segment in myocardial ischemia is the flattening of its end part, as a result of which the boundary between the segment ST and the beginning of the tooth becomes clearer.
The changes in the T wave are less specific and less sensitive than the ST segment deviation for the diagnosis of coronary perfusion deficiency. Inversion of the T wave can also be observed in the absence of ischemia as a variant of the norm, or due to other cardiac or non-cardiac causes. Conversely, T wave inversion is sometimes absent in the presence of ischemia.
Therefore, the analysis of the morphology of the ST segment and the T wave is carried out in conjunction with the evaluation of all elements of the ECG, as well as the clinical picture of the disease. For various pathological conditions, the ST segment can be mixed downward or upward from the isoline.
ECG with myocardial infarction, ischemia, damage and necrosis
With the help of electrocardiography, it is possible to diagnose myocardial infarction approximately in 90-95% of cases, and also to determine its localization, size and prescription. This is possible in connection with violations of functional currents in the myocardium in case of an infarction (changes in the potentials of the electric field of the heart), since the necrotically altered myocardium is electrically passive.
ECG with myocardial infarction distinguishes three zones: ischemia, damage and necrosis. In the myocardium around the necrosis zone there is a zone of transmural damage, which, in turn, is surrounded by a zone of transmural ischemia.
[8], [9], [10], [11], [12], [13], [14]
ECG with myocardial ischemia
The ischemia zone is manifested on the electrocardiogram by changing the T wave (the QRS complex and the ST segment have the usual form). The prong T in ischemia is usually equilateral and symmetrical, both its knees are equal in magnitude, the apex is pointed and is equally removed from the beginning and end of T. The width of the tooth is usually increased because of delayed repolarization in the ischemic zone. Depending on the location of the ischemia site in relation to electrocardiographic leads, the tooth T can be:
- negative symmetrical (with transmural ischemia under the trim electrode or with subepicardial ischemia under the active electrode);
- high positive symmetrical acute coronary (with subendocardial ischemia under the active electrode or with transmural ischemia on the opposite electrode wall);
- reduced, smoothed, two-phase (when the active electrode is located on the periphery of the ischemic zone).
[15], [16], [17], [18], [19], [20], [21]
ECG with myocardium damage
Electrocardiographically, myocardial lesions manifest themselves in the displacement of the ST segment. Depending on the location of the damage zone with respect to the active electrode and its location, various changes in the ST segment can be observed. So for transmural damage under the electrode, the ST segment is seen to rise above the isoline by an arc facing upward with a bulge. With transmural damage located on the opposite electrode wall, the ST segment decreases below the isoline with an arch facing downwards. With subepicardial damage under the electrode, the ST segment is located above the isoline with an arch facing upwards, with subendocardial damage under the electrode - below the isoline with an arch facing downward.
[22], [23], [24], [25], [26], [27], [28]
ECG with 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, with transmural myocardial infarction, the QS teeth with a width of 0.04 s or more are marked under the electrode. In the area opposite to necrosis, reciprocal changes are recorded in the form of an increased amplitude of the R wave. In non-transural myocardial infraction, QR or Qr denticles are observed on the electrocardiogram. The amplitude and width of the Q wave, as a rule, reflect the depth of the lesion.
ECG with myocardial infarction distinguishes myocardial infarctions of the following prescription:
- Myocardial infarction up to 3 days (acute, fresh). It is characterized by the rise of the segment, ST above the isoline in the form of a monophasic curve, when the segment ST merges with the positive T wave (with or without pathological Q wave).
- Myocardial infarction up to 2-3 weeks old. Characterized by the rise of the segment ST above the isoline, the presence of a negative symmetrical wave T and pathological Q wave.
- Myocardial infarction lasting more than 3 weeks. Characterized by the location of the segment ST on the contour, the presence of a deep negative symmetrical wave T and a pathological Q wave.
- Cicatricial changes after myocardial infarction. It is 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.
[29], [30], [31], [32], [33], [34]
ECG with myocardial infarction with ST segment elevation
A characteristic sign of myocardial infarction with ST segment elevation is the arcuate rise of the ST segment in the form of a monophasic curve, so that the descending elbow of the R wave does not reach the isoelectric line. The magnitude of the ST rise in this case is more than 0.2 mV in leads V2-V3 or more than 0.1 mV in other leads. This rise should be observed in two or more consecutive leads. Monophasic curve persists for several hours. Then the electrocardiographic picture changes according to the stage of development of the process.
A few hours or days after the onset of the disease, an electrocardiogram appears abnormal Q wave, the amplitude of the R wave decreases, or the QS form of the ventricular complex occurs, which is caused by the formation of myocardial necrosis. This change allows you to diagnose a large-focal or Q-forming myocardial infarction.
Approximately to the beginning of the second day, a negative coronary tooth T appears, and the segment ST begins to gradually descend to the isoline. At the end of 3-5 days the depth of the negative tooth can decrease, on the 8th-12th day the second inversion of the T-wave comes - it deepens again.
ECG with myocardial infarction without ST segment elevation
In acute coronary syndrome without ST segment elevation, an electrocardiogram can be:
- absence of electrocardiographic changes;
- depression of the ST segment (a diagnostically significant displacement of more than 1 mm in two or more adjacent leads);
- inversion of the T wave (more than 1 mm in the leads with the predominant R wave).