In this chapter, we address the basic notions of myocardial ischemia and
myocardial infarction. Cardiac ischemia changes the electrical activity and the genesis
of the action potential and of the resting potential. It can be divided into 3 forms;
ischemia, lesion and necrosis. Modification of the QRS complex, the ST segment and T
wave is observed. Ischemia is a biochemically reversible anomaly. Moreover, it is
mainly ionic, notably potassium disturbances which underlie ST and T wave changes.
Lesion is a more severe form of cardiac ischemia but is still reversible, with interstitial
oedema and biochemical disturbances. Essentially, it is the ST segment, which is
modified, in that it becomes displaced from the isoelectric baseline. The ST segment
vector is determined in the same manner as that of the QRS complex: it allows for better
localization of the site of the stenosis or obstruction of the culprit artey. The more leads
exhibit ST changes, the bigger the territory at risk. A sum total of ST depression or
elevation greater than 12 mm in the different leads implies widespread ischemia. The
most severe stage of cardiac ischemia is necrosis since there is cellular death with
cessation of electrical activity. Neither the action potential nor the resting membrane
potential exists anymore and the conduction capability has ceased. The start of
depolarisation (QRS) is modified with the apparition of an "electrical hole" (Q waves),
which could progress as far as the total disappearance of the positive forces (R waves)
and a QS morphology; the necrosis is transmural affecting therefore the full thickness of
the myocardium. Acute coronary syndrome includes STEMI and non-STEMI. STEMI
(ST Segment Elevation Myocardial Infarction) is the acute coronary syndrome with ST
segment elevation and non-STEMI is associated with other ST segment changes
(negative T waves or ST segment depression) but not ST segment elevation.
Electrocardiographically, the electrical changes recorded in the different territories
differ according to the coronary artery involved. There is a good correlation between the
ischemic zone and the coronary artery affected. Ischemia is recorded by the electrode
"exploring" the territory implicated. Involvement of the right coronary artery gives rise
to inferior wall ischemia and this is characterized on the ECG as changes in leads II, III
and aVF. Involvement of the left coronary artery gives rise to anterior wall ischemia and
this is characterized on the ECG as changes in precordial leads.
Keywords: Myocardial infarction, myocardial ischemia, ST segment Elevation
Myocardial Ischemia, non ST segment Elevation Myocardial Ischemia, subendocardial
myocardial ischemia, sub-epicardial myocardial ischemia, myocardial
necrosis, Q wave, ST segment elevation, ST segment depression.
