What is The critical narrowing of coronary vessels to cause angina is - and explain in detail?
Ans. is 'c' i.e., More than 70%Pathogenesis of Ischemic hea diseaseo More than 90% of patients with IHD have atherosclerosis of one or more coronary aeries.The type of Ischemic hea disease depends on the type of changes that develop in this atheromatous plaque.1. Fixed atherosclerosis with no change in structureo A fixed obstruction of 75% or greater generally causes symptomatic ischemia induced by excercise --> classical angina.o With this degree of obustruction, the augmented coronary flow provided by compensatory vasodilatation is no longer sufficient to meet even moderate increase in myocardial demand.o A 90% stenosis can lead to inadequate blood flow even at rest.2. Disruption of plaque and acute coronary syndromeo In most patients with MI, unstable angina, and sudden cardiac death, the attack is precipitated by abrupt plaque change (plaque disruption) followed by thrombosis.o Acute plague change (plaque disruption) may be any of the following types ?(a) Rupture/fissuring, exposing highly thrombogenic plaque constituents to blood.(b) Erosion/ulceration, exposing the thrombogenic subendothelial basement membrane to blood.(c) Hemorrhage into the plaque, expanding its volume.o All these changes lead to exposure of the thrombogenic constituents of plaque or vessel wall, this in turn activates platelets and there is platelets aggregation with formation of thrombus.o This can precipitate following acute coronary syndrome -(i) If thrombus occludes the vessel completely --> Transmural myocardial infarction.(ii) If thrombus occludes the vessel paially (incomplete) --> Unstable angina or acute subendocardial infarction.(iii) Paial occlusion by thrombus with possible embolus led to regional myocardial ischemia that induces a fatal ventricular arrhythmia --> sudden cardiac death.3. Vasoconstriction at sites of atheromatous plaqueo Vasoconstriction at sites of atheromatous plaque causes stenosis above a critical level and pain at rest. o This is the pathogenic mechanism in Prinzmetal variant angina.