Vector of ischaemia

The injury vector has approximately the same direction as that of the vector of ischaemia and infarction but opposite sense (see p. 35, 60 and 131 and Figures 3.6, 4.8 and 5.3). Therefore, most probably, in case of injury of the lateral wall, an ST-segment depression will be especially recorded in V1-V2, and in case of injury of the inferobasal wall, the ST-segment depression will be recorded especially in V2-V3. However, further perfusion studies, with imaging techniques in the acute phase have to be done to validate this hypothesis. 

In theory the presence of subendocardial or transmural injury in completely opposite areas of the heart may decrease or even conceal the two injury vectors (Madias, 2006). However, in practice, this does not occur usually, because the ischaemia is usually due to occlusion of only one vessel and this does not generate equal and opposed injured areas (Rautaharju, 2006). Furthermore, with the same amount of injury in two opposite areas, it is more visible in the surface ECG of the injury area that is more close to subepicardium. In the chronic phase it is more often seen that a new vector of infarction in opposed area may cancel the Q-wave pattern of a previous infarction (see Figure 5.38). 

Both the clinical ST-segment depression pattern and the experimental subendocardial injury pattern have a common explanation the sum of subendocardium TAP, which is of worse quality than in the subepicardium (Figure 4.5), or the generation of an injury vector (Figure 4.8). In NSTE-ACS the presence of new ST-segment depression is related with the presence of evident active ischaemia predominantly in the subendocardium. On the other hand, the presence of flat or negative T wave is related with previous ischaemia (often is a reperfusion pattern) without subendocardial predominance (Table 2.1 and Figure 3.9). 

Figure 4.9 (A) In case of diffuse subendocardial circumferential injury due to incomplete occlusion of left main trunk (LMT) in a heart with previous important subendocardial ischaemia, the injury vector that points circumferential subendocardial area is directed from the apex towards the base, from forward to backwards and from left to right. This explains the typical morphology of...

A new infarcted area suddenly masks totally or partially previous Q waves (Madias and Win, 2000) (Figure 5.39). The ECG may seem even normal or nearly normal due to cancellation of vectors. It should be ruled out that the disappearance or decrease of the Q wave is not secondary to the development of a new intraventricular block. Also ischaemia induced by exercise may mask transiently, due to ischaemia in the opposite sites, the Q wave of necrosis (Madias et al, 1997). 

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