A left posterior fascicular block is a very rare abnormality of the cardiac conduction system and is primarily associated with a myocardial infarction of the inferior wall and coronary artery disease. The clinical presentation significantly depends on the extent of coronary artery vessel occlusion. Electrocardiography is the single most important study for revealing a left posterior fascicular block, showing either intermediate (90° to 120°) or marked (120° and 180°) right axis deviation, alongside several other findings involving the QRS complex.
A left posterior fascicular block is an electrophysiologic abnormality that is rarely encountered in clinical practice, but it is well-known for its occurrence in the setting of coronary artery disease (CAD)    . Insufficient blood flow to the left posterior fascicle, which is supplied by the right coronary artery and the anterior descending branch of the left coronary artery, causes improper conduction of the electrical signal through the heart  . Because of blood vessel occlusion, myocardial infarction may ensue and left posterior fascicular is shown to be present in about 5% of all myocardial infarctions of the inferior wall according to isolated reports . Hence, the clinical presentation of these patients is primarily related to myocardial infarction and CAD. Some reports, however (in which iatrogenic induction of this cardiac abnormality by interleukin-2 was documented) have observed patients in whom the symptoms such as chest pain, syncope, or palpitations weren't present . Atrial flutter and atrial fibrillation, two very important types of arrhythmias, are known to induce the phenomenon of "axis deviation", which may promote the development of left posterior fascicular block .
The diagnostic workup of patients with a left posterior fascicular block starts with a thorough clinical assessment. A detailed patient history is necessary in order to reveal possible risk factors, mainly CAD. If the abrupt clinical presentation of sudden chest pain and other typical signs seen in a myocardial infarction are noted, immediate employment of electrocardiography (ECG) is of great importance. Left posterior fascicular block can appear in association with a right bundle branch block (RBBB) or as an isolated entity, and recently proposed criteria for this abnormality state the following findings are necessary to make the diagnosis    :
As soon as these criteria are met, it is recommended to perform coronary angiography (and left ventricular angiography if possible), mainly because CAD in the case of left posterior fascicular block frequently affects up to 3 coronary vessels .