Typical forms of the functional pathology of the heart include:
• myocardial ischemia (coronary vascular insufficiency);
• cardiac arrhythmia;
• heart failure.
Myocardial ischemia is a condition in which oxygen supply is reduced and oxygen deprivation is accompanied by the inadequate removal of metabolites consequent to reduced perfusion.
There are four main groups of causes resulting in ischemia.
1. Partial or total coronary obstruction.
2. An increase in myocardial metabolic demands which outpaces the supply of blood.
3. Acute falling of systemic arterial pressure leading to reduction of myocardial perfusion.
4. A reduction in the oxygen-carrying capacity of the blood. Additional factors of the cardiac ischemia are mentioned in fig. 24.
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Fig. 24
1. Partial or total coronary obstruction
Atherosclerosis and thrombosis
The main cause of myocardial ischemia is fixed coronary obstruction by the atherosclerotic plaque. More than 90% of patients with ischemic heart disease have advanced coronary atherosclerosis. The risk of developing ischemic heart disease depends on the number of affected vessels, distribution of atheromatous plaques, and degree of vessel narrowing. However the clinical manifestations of ischemic heart disease are not strongly predicted by these anatomic observations.
When stenosis reduces the cross-section area by approximately 75%, a full range of increases in flow to meet increased myocardial demand is not possible. When the luminal area is reduced by more than approximately 80%, blood flow at rest may be reduced, and minor decreases in the vessel diameter can cause myocardial ischemia.
The atherosclerotic plaque is a subject to acute changes, such as hemorrhage and thrombosis. Hemorrhage into the atheroma from intraplaque vessels may expand the volume of plaque, while fissuring and erosion are accompanied by an exposure of collagen resulting thrombosis.