notes by dr Claudio Italiano
Ischemia, from the Greek, literally means the reduction of the blood supply to a
tissue or an organ and manifests itself differently, depending on the districts
involved. If it concerns the tissue of the myocardium, that is the heart, it
will give a series of clinical pictures ranging from simple angina, ie anginal
chest pain to myocardial infarction. This affection can be associated with
dangerous, severe and severe arrhythmias, up to the terminal picture of sudden
death. The reason for which the tissue due to ischemia is damaged is due to the
fact that each tissue has its own metabolism, that is a set of chemical
reactions that take place at a temperature of 37 ° C and allow to burn the fuel
and the comburent, ie glucose and oxygen, brought from the blood and precisely
from the red blood cells.
Therefore, whenever the blood fails to penetrate into the tissue, for example because an arterial vessel is damaged or stenosed, then narrowed, the tissue is referred to as being in ischemia. When the tissue is in ischemia for a long time, especially if this condition has happened in a short time, so it is an acute condition, it derives the death of the tissue itself which is called technically "necrosis". The tissues particularly sensitive to ischemia are the brain, the heart, the kidneys. When we talk about the heart, in the myocardium the ischemia can give the infarct: a part of the tissue goes to breakup and it generates what the cardiologists define "a current of lesion", that is recording an ECG there is a unbalance between the charges electrical effects of the tissue and alteration of the same vector resulting, with particular signs to the electrocardiogram ; in addition, the damaged tissue releases substances that the doctor goes to dose in the periphery: myocardial-specific enzymes, transaminases, myoglobin and troponin.
The ventricle, and in the meantime we invite you to read the first and second parts of the interpretation of the ecg (ECG and ECG 2), when it is reached by the excitation wave, is depolarized. The exploring electrode sees this phenomenon as a positive deflection wave, that is, the nib of the machine gives a touch up, since it is so established by convention. To the phase of depolarization follows the phase of repose which is also called repolarization phase, this is a slower phase, when the myocardium takes again the charge values; however, if the ventricle is ischemic, there will be an alteration of the T wave, ie this wave expressing precisely the repolarization phase, ie the repolarization vector, will be different from the norm, for example or negative or pointed symmetric branches. This will not occur on the QRS complex, unless significant damage has occurred (eg Q waves of transmural necrosis of the infarct).
The T wave becomes, say, in the myocardial ischemies with symmetrical and
pointed branches, while normally the branches are asymmetric, since the
descending phase of the wave is usually less steep.
Moreover, the T wave can have further modifications:
- Its meaning can be negative where it was positive
- Its height can be considerable, above 5 mm
- The alterations are a function of ischemia localizations.
The electrode that it explores sees an ischemic tissue in front of it and,
therefore, the vector of repolarization moves away, the deflection is negative;
moreover the lesion must be evident in several derivations (see the figure above)
In this case it is exactly the opposite because the vector of repolarization has
a positive sense, ie proceeds towards the electrode that it explores, see the
figure on the right; in this case the resulting vector is positive and goes
towards the electrode it is exploring, because the tissue in front of itself is
healthy, while the ischemic tissue is subendocardial.
Examples of ECG that document "ischemia"; the first subendocardial: the vector
of the repolarization is directed towards the electrode exploring in V2-V3-V4
where the T waves are pointed, with symmetrical branches.
Subendocardial ischemia
Subendocardial ischemia
Another example of ischemia, this time subepicardial, with a more discrete path, so to speak! But where are the negative T waves? Look for them! It is a postero-inferior ischemia, therefore evident in the posterior derivations that are projected in D3 and aVF, the electrode that looks at the heart from below, from the feet of the subject, sees a vector T that moves away from it, therefore negative. There are still high and sharp T-waves from V2 to V5
index of cardiology