Posterior myocardial infraction related scar
This patient had gone through a inferior infarction with posterior extension in conjunction with a thrombosis of a dominant circumflex requiring urgent angioplasty six hours after onset of chest pain. The posterobasal localization (also called posterior), anatomically close to the posteroinferior localization, is differentiated electrically due to the orientation of the necrosis vector pointing forward and no longer upward. The result is that the pathological images preferentially appear in the precordial leads and not in the limb leads as in an inferior infarction. The posterior infarction can occur in the setting of an inferior infarction (inferobasal) or anterior infarction (laterobasal) because of occlusion of the circumflex artery, the dominant right coronary artery or of one of their branches. An isolated posterior infarction is a rare occurrence.
This infarction type can sometimes be underdiagnosed because the basal wall is not directly explored by a standard 12-lead trace, which only presents indirect signs, since none of the 12 conventional leads are facing the posterior wall. To authenticate the above, it is therefore essential to systematically and precociously record the posterior leads for any suspicion of acute coronary syndrome (V7 at the level of the left posterior axillary line, V8 at the left midscapular line, V9 at the level of left limit of the vertebral column, all 3 being positioned on the same horizontal line as V6).
It is possible to identify direct electrical signs and indirect signs of posterior infarction:
- direct signs: a ST-segment elevation in the posterior chest leads from V7 to V8 attests to the presence of a posterior infarction; it is generally associated with a ST-segment elevation in the inferior territory (inferobasal infarction), in the lateral territory (laterobasal infarction) but can be isolated (true posterior infarction); the direct image of necrosis, i.e. the Q-wave, also appears in the dorsal leads;
- indirect signs: since V1 registers the electrical activity on the opposite side of the posterior wall, a tall R wave and a ST-segment depression correspond to reciprocal images of the Q-wave and the ST-segment elevation, typical signs observed in the acute phase; these indirect signs can also be observed in right precordial leads; remotely, the R wave indicative of the necrosis is generally wider than 40 ms, with an R/S ratio > 1 and a positive concordant T-wave; the slow and slurred nature of the ascending branch of the R wave, the association with a tall and positive T-wave are indirect arguments of necrosis which warrant the search for direct signs in the posterior leads;
There are a certain number of differential diagnoses upon detection of a large R wave in V1:
- right ventricular hypertrophy: there is usually a right axis deviation, possible right atrial enlargement, absence of Q-waves in the posterior leads;
- hypertrophic cardiomyopathy;
- right bundle branch block: the QRS is wide, with rsR' pattern in V1, wide S wave in V6 and absence of Q-waves in the posterior leads;
- left Wolff-Parkinson-White syndrome with delta wave and short PR interval;
- left ventricular pacing;
- normal pattern in the infant.